The Story of Atomic Theory of Matter
The Story of Atomic Theory of Matter
The Story of Atomic Theory of Matter
an eklavya publication
The Story of Atomic Theory of Matter
Text: Sushil Joshi, Uma Sudhir
Editor: Rex D’ Rozario
Illustration: Sonali Biswas
Design and Layout: Ameya Wangalkar
Cover Design: Nishith Mehta
This edition has been developed with support from Parag Initiative of Tata Trusts.
Printer: RK Secuprints Pvt. Ltd, Bhopal (MP) +91 755 268 7589
Contents
The Greeks 11
The Alchemists 15
More insights into matter 21
Atomic theory 30
Calculating atomic weights 35
Epilogue 55
Part 2: Problems in teaching atomic theory 57
This module is part of a series being developed by a high school science group coordinated by Eklavya,
an organisation based in Madhya Pradesh that conducts research in education and prepares curricula
and related material for school education and teacher orientation. The high school science group is a
loose agglomeration of scientists, educationists, college and school faculty, social activists and others
who are concerned about the status of science education in Indian schools and would like to introduce
contemporary ideas and approaches in science pedagogy into the curriculum.
Their work draws inspiration from the Hoshangabad Science Teaching Programme (HSTP), an
innovative science education programme that was conducted for 30 years from 1972 to 2002 in around
800 government and private middle and higher secondary schools across 14 districts of Madhya
Pradesh, reaching out to around 50,000 students annually.
The HSTP sought to improve science teaching in middle schools (classes 6 to 8) by incorporating ideas
and approaches in science pedagogy that were influencing school science teaching across the world in
the 1960s and 1970s. Its basic premise was that rote learning is antithetical to science teaching and
students aged 11 to 14 years should learn science by actually doing experiments in the classroom and
interacting with their immediate environment to 'discover' scientific laws through a process of
observation, analysis, reasoning and discussion.
The HSTP's emphasis on the 'scientific method' saw learning largely structured around experiments, with
many concepts thought to be too abstract for the middle school age group being deliberately excluded.
The curriculum developed for this school stage, while broadly adhering to the state curriculum framework,
focused on concepts and ideas that were considered crucial to developing an understanding of science,
and fostered skills to encourage the self-learning abilities of students.
However, field experience saw the programme's almost exclusive focus on experiment and the
'discovery' approach being tempered in the later stages to incorporate more theoretical explanations
and descriptive details as well as stories about the historical development of ideas and concepts in
science, among other things.
The high school science group takes a similar age-related approach to science teaching at the high
school level (classes 9 and 10), introducing more theoretical content, investigative activities and science
history in keeping with the growing comprehension abilities of the students, but still retaining an
'experiment' core and excluding concepts that can be better treated at the college and university level.
The series of science modules being developed for this stage of schooling broadly covers the syllabus
prescribed by the various examination boards at the national and state levels. These modules are
addressed to both students and teachers and contain additional resource materials that teachers 7
would find useful in classroom instruction.
WHAT IS THE STRUCTURE OF THE MODULE?
This module discusses the subject of 'matter', a concept that is yet to be definitively defined
in science. It is primarily meant as background reading for teachers to strengthen their
understanding of the concepts discussed.
Matter is everything you see around you – trees, birds, air, stones, buildings, cars… your
friends. It's the stuff things are made of and is generally defined as anything that has mass
and volume (which is the space it occupies).
Matter is composed of atoms, and atomic theory is central to chemistry. Natural systems are
fundamentally chemical systems.
Atoms (which combine to form molecules) constitute the elements we see in nature. Each
element consists of identical atoms and elements can be distinguished from one another
because their atomic weights differ. Atoms of different elements combine in chemical
reactions to form compounds. The atom and molecules of the elements/compounds that
take part in reactions form new combinations that constitute the products of the reaction.
These processes are governed by specific laws. It took over 2,000 years for humankind to
develop an understanding of atomic theory and the laws that underlie the way nature
works.
It is this crystallised knowledge that students are expected to understand and internalise in
their five years of study at the middle and high school level. But, given our long experience of
working with school-children, we have been selective in choosing the content of this
module, our judgment governed by what we feel students of this age-group can understand,
internalise and fruitfully relate to.
The module is divided into three parts.
The first part presents a historical overview of how our understanding of atomic theory and
the nature of matter evolved. In essence, it is also a history of the birth and development of
the science of chemistry. Modern-day chemistry can trace its ancestry to the quasi-spiritual
THE STORY OF ATOMIC THEORY OF MATTER
attempts of alchemists to change base metals into gold but subsequently gained a solid
'scientific' foundation through investigative experiments conducted into the structure of
matter. It is this structure that governs the chemical and physical properties and changes
that are the basis of life. But one thing needs to be noted here. Although these investigations
laid the foundation of chemistry as we know it today, they are largely multidisciplinary in
nature, being profoundly influenced by progress in several fields of scientific study.
The second part lists some of the misconceptions students at the middle and high school
level have about the particulate nature of matter. These misconceptions have been
investigated and articulated through various studies conducted across the world among
students of different age groups, beginning with naïve conceptions of matter among young
8 children based on the common sense that 'seeing is believing' and progressing to changing
perceptions, catalysed by classroom instruction, as the students move into the higher
classes. The point of interest is the persistence of some of these naïve beliefs among
students even after they gain insights into the particulate nature of matter with repeated
instruction. It shows how difficult it is for them to take the step from 'seeing is believing' to an
abstract concept about 'invisible' factors that may go against the grain of common sense.
That's why the approach we have taken in this module is influenced by an appreciation of
these misconceptions. At the same time we have tried to point out what students need to
know if they are to internalise the concept of the particulate nature of matter. We have also
tried to identify some of the weak links in the way abstract concepts in atomic theory are
treated in textbooks and in classroom instruction, which compound the comprehension
problems of students.
The third section is a set of experiments that introduces students to facets of chemistry that
are crucial for gaining an understanding of atomic theory. In a way they seek to prepare the
ground for a better understanding of the basic premises of the science of chemistry – its
foundation and framework. Hopefully, these experiments will provide students with the
minimum necessary 'chemical experiences' they need to appreciate why the particulate
model of matter provides the most logical explanation of change in various phenomena,
which are basically chemical in nature. So it would be best if students perform these
experiments before embarking on a study of this module.
And a final word. The module discusses the atom from the point of view of understanding
chemical phenomena. But there is more to the atom, such as its internal structure of a
nucleus with protons and neutrons and orbiting electrons. There is the atomic structure at
the quantum level. There is also the motion of atoms, both intrinsic and extrinsic. All this is
not dealt with here in this module. Some aspects of the motion of particles finds place in
another module 'Heat and Temperature', developed as part of this series. But what should be
noted is their relevance to getting a better understanding of during the chemical change and
behaviour of matter. It goes to show the highly interdisciplinary nature of science where the
distinction between subjects tends to blur. If we go into the subject of organic molecules and
the chemistry of life, the overlap of chemistry, physics and biology becomes even more
evident.
The basic idea of the module is to help the students progress from their common-sense
conceptions about matter through instruction to a better appreciation of the particulate
nature of matter, which is the basis of chemistry. The importance of appreciating the
particulate concept of matter is best summed up in the words of the late Nobel laureate
physicist Richard Feynman:
If, in some cataclysm, all of scientific knowledge were to be destroyed, and only one sentence passed
on to the next generation of creatures, what statement would contain the most information in the
fewest words? I believe it is the atomic hypothesis that all things are made of atoms – little particles
that move around in perpetual motion, attracting each other when they are a little distance apart,
STRUCTURE OF THE MOLECULE
9
PART 1
A HISTORICAL PERSPECTIVE OF
HOW OUR UNDERSTANDING GREW
ore dug out of the soil? Why does this iron rust?
Why do pipes corrode? Why does corrosion All these are basically physical and chemical
occur more rapidly in some seasons, and why is it processes. They are linked to the atomic
a bigger problem in homes that are close to the structure of matter. Atomic theory is considered
sea? Why is it possible for gases to expand so central to understanding everything from heat
much, and how is it that they can be compressed (and temperature) to chemical reactions and
to such an extent? What happens to the air we stoichiometry. It is a model that helps us make
breathe and the food we eat? Why do we grow sense of and make prediction about changes
older? taking place around us.
10
The Greeks
Is the world permanent or in a state of constant flux? Is matter continuous or particulate? Aristotle's
five elements battle it out with the atoms of Democritus.
These kinds of questions about what makes the believed there are four 'root' elements — earth,
world tick have been raised by people from water, air and fire — that possess different
ancient times. Natural philosophers in most early properties and attract or repulse one another to
civilisations – Chinese, Indian, Greek, Egyptian – varying degrees. So the variety we see around us
wondered about the nature of matter and its is due to each substance possessing different
structure but their thinking was mostly intuitive amounts of each of these elements. For instance,
and speculative, with no experimental evidence. a feather floats in air because it has more air in it
Nonetheless, they were keen observers of and hence has the property of lightness. Or mud
nature and deduced the causes of many natural is a combination of water and earth, clouds are a
phenomena through a process of logical combination of air and water, and lava a
reasoning. We have only very sketchy reports of combination of earth and fire.
this speculative thinking of the early
philosophers, the best documented record being
of ancient Greek civilisation. So this narrative
begins with the Greeks and traces the influence
of Greek thought in the development of modern
science and chemistry.
13
The birth of modern science and the scientific method
The 'birth' of modern science in the popular This aspect, which he applied in the study of
narrative is usually traced to the British motion, is most clearly enunciated in his
scientist Francis Bacon (1561-1626), who wrote statement: “The book of the universe cannot
about a new method of scientific inquiry and be understood unless one first learns to
logic in his Novum Organtum Scientiarum, a comprehend the language and to read the
philosophical work published in 1620. alphabet in which it is composed. It is
written in the language of mathematics, and
Bacon rejected the Aristotelian approach to
its characters are triangles, circles, and other
understanding nature, because the method
geometric figures, without which it is
was bas e d on intuitive insights and
humanly impossible to understand a single
speculation and could not be used to
word of it.”
reproduce the natural phenomenon being
investigated. He also rejected the method of It is from here that the foundation of the
the alchemists whose experimental scientific method as we know it today was
investigations were ro ote d in myth, laid. Science is a practical method of trying to
mysticism and religion. According to him, understand natural phenomena and predict
knowledge must come from a planned how they will progress. This method
procedure based on sensory experience requires both experiment and theory to
(empiricism), which included discovering build explanations of what happens in the
evidence through experiments. world. It can be conceptualised as a series of
steps beginning with looking at nature
Bacon wrote that investigating a natural (obs erving a phenomenon), asking a
phenomenon is a process of breaking down question ab out the phenomenon
the phenomenon into its c onstituent (formulating a hypothesis), performing an
components (reductionism), analysing these e x p e r i m e n t t o t e s t t h e hy p o t h e s i s ,
components and then formulating general confirming or rejecting the hypothesis after
laws governing the phenomenon through a analysing the experimental data, and finally
process of reasoning (inductive logic). subjecting the 'scientific law' distilled from
the hypothesis to further experimental
French philosopher Rene Descartes (1596- tests.
1650) also outlined the scientific method in his
Discours de la method, published in 1637. Basically, the approach is to let reality speak
THE STORY OF ATOMIC THEORY OF MATTER
14
The Alchemists
Combining the 'yellowness' of sulphur and the 'shininess' of mercury to get gold… The journey from
transmutation of substances to the birth of modern chemistry
The overwhelming influence of speculative fashion artefacts and ornaments. Chemicals like
thinkers in the early history of science does not phosphorus were isolated and many ancient
mean that there were no experimentally systems of medicine, such as Ayurveda, learnt to
oriented developments taking place. There were extract or synthesise chemicals of medicinal
many doers – practical-minded artisans and value from natural sources.
craftsmen who looked for solutions to everyday
problems through a trial-and-error process. Such
people were to be found in all civilisations across
7 the globe from ancient times.
how to manufacture paper ( 105 AD ) and causes to explain changes in the forms of
gunpowder (11th century), while blacksmiths substances they experimented with. They were
across the world learned the art of alloying influenced by Aristotelian ideas that everything
metals to alter properties such as hardness and is made up of combinations of five elements in
malleability. Among them were the early Gond different proportions that produce different
tribals of Bastar region in India, who mastered qualities or characteristics. So a substance
the craft of smelting and bell metal alloying to containing more of the elements of air and fire
15
would be light and shiny and if you could people thought whatever they got from nature
somehow remove the element air from the was pure and we add impurities to these pure
substance and substitute it with the element things in chemical reactions.
earth, you would get a new substance that was
heavy and shiny. This notion persisted because it was never
challenged by factual data. Alchemists never
This is exactly what the alchemists sought to do. thought of measuring how much metal was
They believed that one substance could be obtained from a weighed amount of calx. Their
changed (transmuted) into another. They tried to primary interest lay in the properties of the
extract the 'yellowness' of sulphur and the products they could obtain, so they seldom
'shininess' of mercury and combine them to get measured or paid much attention to how much
gold that is both yellow and shiny. This kind of of a substance they used in their reactions or
reasoning also led them to think that gold had how much of a new substance was formed. They
more of the element fire in it compared to iron, had no concept of atoms and, like Aristotle, saw
because gold is not only shinier but maintains its matter as continuous, so they believed one
lustre longer. substance could combine with another in any
proportion whatsoever. They used arcane
When it came to metallurgy, they explained the symbols to depict metals and common
process as obtaining a compound – the metal – compounds, using them as a kind of shorthand in
from the elemental ore – calx. We now know that their diagrams and procedures.
metals are elements obtained from their ores
(compounds) – most commonly oxides, sulphides But there was also another side to alchemy that
or carbonates. Why did they get it all wrong? It is was more in line with practices in modern
because this idea had its basis in the notion we chemistry (see box: ANOTHER SIDE TO ALCHEMY).
still cherish that everything in nature is pure. So
THE STORY OFATOMIC THEORY OF MATTER
16
Another side to alchemy
In pursuing their metaphysical quests the alchemists came up with many useful
discoveries and perfected many experimental techniques that helped the later
development of modern chemistry. Among them was the Arab Abu Musa Jabir ibn
Hayyan (considered by many to be the father of chemistry) who developed an early
experimental method around 770 AD and isolated several acids including hydrochloric,
nitric, citric, acetic and tartaric acid.
There were also the Siddhas of Tamil Nadu in South India, whose philosophy was
developed during the period from 200 BC to 1200 AD. These yogic-poets were adept in
the science of healing and alchemy and preserved their insights into material
substances and healing practices in verse. In addition, Rasaratnakara, a treatise
attributed to Nagarjuna in the 10th century AD, dealt with preparations of mercury
(rasa) compounds but also talks of extraction of metals from their ores.
Spanish alchemist Pseudo Geber was one of the first to describe nitric acid around 1310
AD and proposed the theory that all metals are composed of various proportions of
sulphur and mercury, while Paracelsus is said to have laid the foundations of
pharmaceutical chemistry around 1530 AD.
17
Another scientist who played a key role in experiment to obtain mercuric oxide by heating
refuting the claims of alchemists was the French mercury. In those days, a known amount of
chemist Antoine-Laurent Lavoisier (1743-1795), mercury would be placed in a closed vessel and
who, incidentally, is also considered by many to heated from outside using lenses. The mercuric
be the founder of modern chemistry. oxide obtained was then weighed. The weight of
the mercuric oxide formed was generally found
The alchemists claimed that water could be to be less than the initial amount of mercury
transmuted into soil by removing its wetness – used in the experiments.
water was seen to have the properties of
wetness and heaviness whereas soil had the The alchemists explained this apparent loss of
properties of dryness and heaviness. They weight as the result of impurities being removed
distilled water to a high level of purity to get from the impure metal to give the pure,
dryness that would result in the formation of elemental calx! The real explanation is far
'soil'. And it appeared as if they were successful. simpler. When mercury is heated with a lens, the
high temperature generated causes some of the
Lavoisier repeated their experiment to mercury to evaporate and deposit on the walls.
conclusively prove that no transmutation had This deposit was never weighed.
taken place, tracing the 'experimental error' to
the variety of glass used in the apparatus – which Lavoisier repeated the experiment to show that
was slightly soluble in water. The French chemist if the deposit is heated gently, we get back the
accurately weighed the apparatus before and original mercury. He concluded that the deposit
after the distillation to show that the amount of is a compound of metal and oxygen, weighing
'sand' produced was accounted for by the loss in the metal and deposit to show that their
weight of the glass, thus refuting the alchemists' combined weight is greater than that of the
claim. original metal. The experiment also led him to
conclude that air itself is a mixture and not an
Another key alchemical conclusion that element, with one-fifth (oxygen) combining with
Lavoisier proved to be erroneous was in an the metal while four-fifths did not.
THE STORY OF ATOMIC THEORY OF MATTER
18
Lavoisier lays the ground for quantification and the discipline of
stoichiometry
One practical reason for the absence of meticulously measuring all the matter he started
quantification in chemical experiments in those out with and all the matter he ended up with,
days was the lack of equipment to accurately including 'invisible' matter that floats away in
measure weights. There was also no way of gaseous form (probably the first person to do so)
collecting or measuring the gases involved or to show that the sum total of matter is the same.
formed in a reaction. In fact, gases didn't even
The sum total of matter was also shown to remain
figure in experimental observations because
the same in the case of physical changes, such as
they were not considered to be part or product of
when water boils and turns to steam or cools to
a reaction. Even after people started measuring
become ice. The particles of water continue to
weights of reactants and products in a
exist in the vapour or in ice and return to liquid
rudimentary manner (only solids or liquids,
form when cooled or heated.
because there was still no way of collecting
gases), these ideas continued to go unchallenged This important principle – that matter cannot be
largely because of the kinds of reactions that created or destroyed in a chemical reaction or
were being studied by the alchemists. physical change – is today enshrined in
Lavoisier's law of conservation of mass, which he
It was Lavoisier who revolutionised the science
formulated in 1774 in his Traite elementaire de
of chemistry with his experimental techniques
chemie (For another important contribution to
and his quantitative chemical analysis, which laid
chemistry see box: L AV O I S I E R O N F I R E A N D
the ground for the discipline of stoichiometry. He
PHLOGISTON). The law states that during a chemical
perfected apparatus to collect all the products of
reaction the weights of the reactants and
a reaction, including gases, and devised
products are equal (However, today we know
instruments that could weigh substances with a
that nuclear reactions can turn matter into
high degree of accuracy.
HISTORICAL PERSPECTIVE
19
Lavoisier on fire and phlogiston
20
More insights into matter
As astronomers discover that everything in the universe is made of matter, scientists work out new
laws to show how matter behaves in chemical reactions
Our understanding of matter had begun to Building on the Greek idea of atoms he suggested
change from the time of Newton, who first came (in 1738) that the compression/expansion of
up with the idea that matter resisted change in its gases as well as the change in volume when a
motion, and the degree to which an object liquid turns into a gas was the result of particles of
resisted (its inertia) being its mass. In the fourth matter moving apart or closer together,
edition of his Optics, published posthumously in increasing or decreasing the empty space
1730, he talks about matter being formed “in between them.
solid, massy, hard, impenetrable, moveable
particles, of such sizes and figures, and with such The Swiss physicist David Bernoulli (1700-1782)
other properties, and in such proportion to also used the idea of particles in motion to explain
space… (that) may compose bodies of one and the the behaviour of pressurised gas, relating the
same nature and texture”. Further insights into heat of the gas to the accelerating motion of the
matter in the 17th century came from particles.
astronomers who began to realise that the same
Incidentally, Boyle was also the first scientist to
laws govern matter on earth as well as in space,
give us a working definition of elements, which
showing that everything everywhere is made up
was later refined by Lavoisier into a very practical
of matter.
definition (see box: THE EVOLVING DEFINITION OF
Equally important for the evolution of ideas and ELEMENTS).
atmospheric pressure from day to day caused twentieth of the bulk of the phlogisticated air
variations in the height of mercury in a tube. (nitrogen)”. However, he did not proceed
further to investigate this inert bubble.
The invention of the vacuum pump was the
next step in gaining more insights into Not much attention was paid to this little
vacuum, with Otto von Guericke (1602-1686) nugget of information until more than a
conducting his famous Magdeburg hundred years later. In 1892, English physicist
hemispheres experiment to show that teams John William Strutt (1842-1919), who was
of horses could not separate two hemispheres investigating the atomic weights of oxygen
from which air had been pumped out. and nitrogen, announced that oxygen is
22
always 15.882 times denser than hydrogen, dioxide and nitrogen. He then went a step
regardless of how it is prepared, basing his further because he had an advantage over
observation on the experiments he had C ave n d i s h – a c c e s s t o s p e c t ro s c o p i c
conducted. However, when he tried to find techniques that were not available during
the atomic weight of nitrogen he found that Cavendish's time. He heated the remaining
nitrogen in air is always denser by about 0.5 bubble of gas and studied the spectral lines
times than nitrogen formed in chemical emitted and found that they did not fit any
reactions. He could not find an explanation known element. The new gaseous element,
for this discrepancy, asking for suggestions in which was completely inert and made up
a letter sent to the journal Nature. about 1% of the atmosphere, was named
'argon' in a joint paper he wrote with Strutt in
The challenge was taken up by British 1895 to announce the discovery, for which
chemist William Ramsay ( 1852-1916 ) who they received the Nobel Prize in 1904. Argon
repeated Cavendish's experiment, removing comes from the Greek 'argos', which means
all the components of air – oxygen, carbon inactive or lazy.
23
The law of conservation of mass was one of three The law of constant proportions was derived
laws that added to our understanding of the after analysing many compounds and their
nature of matter during this period. The other compositions. It states that compounds always
two were the law of reciprocal proportions contain elements in a definite proportion. What
articulated by Jeremias Benjamin Richter (1762-
this means is that whatever method you use to
1807) sometime between 1792 and 1794, and the
prepare a compound or whatever the source of
law of constant (or definite) proportions
formulated by the French chemist Joseph Proust your starting materials, you always end up with
(1754-1826) in 1799. Both Richter and Proust the same proportion of elements in that
benefited from the quantitative turn that compound. (Also see boxes: UNDERSTANDING THE LAW OF
chemistry had taken and the vast amounts of CONSTANT PROPORTIONS; THE BURNING CANDLE AND THE
LAW OF CONSTANT PROPORTIONS; AND OUT OF DISPUTE
data now available to them from the carefully COMES KNOWLEDGE).
monitored experiments of many chemists.
25
Understanding the law of constant proportions
Here is an example to help you understand In the second case, 10g of oxygen reacts
what is being said in the law of constant with 125g of mercury, i.e. 10g of oxygen
proportions: requires 125g of mercury. Therefore, 1g of
oxygen requires 125/10g of mercury=12.5g
100 g of mercuric oxide decomposes to of mercury.
g i ve 9 2 . 6 g o f m e r c u r y a n d 7 . 4 g o f
oxygen, 1 0 g of oxygen reacts with In the third case, 10g of mercury reacts
125g of mercury while 10g of mercury with 0.79g of oxygen, i.e. 0.79g of oxygen
reacts with 0.79 g of oxygen to give requires 10g of mercury. Therefore, 1g of
mercuric oxide. Do these values agree with oxygen requires 10/0.79g of mercury = 12.6g
the law of constant proportions? of mercury.
27
Out of dispute comes knowledge
The law of constant proportions was firmly established as a result of a dispute between
Proust, who formulated the law, and French chemist Marcellin Berthollet (1827-1907), who
championed the law of chemical affinity, which says that substances combine in variable
and indefinite proportions according to the relative concentrations of the reactants.
Proust showed that two substances may combine to form more than one compound, but
their proportions are always fixed in each compound. It was fortunate for the progress of
chemistry that a simple rule was first established. This helped chemists to look for regular
patterns in the bewildering variety of reactions around them. However, it is also a fact
that non-stoichiometric compounds have since been identified (for example, rust). They
are called ‘berthollides’ in honour of the man who championed their cause.
The law of reciprocal proportions states that if a given weight of an element A combines with a certain
weight of element B, and the same weight of A combines with a certain weight of element C, then there
should be a definite relationship between the weights of B and C when they combine. They should either
be in the same ratio as they each bear to A, or some integral multiples of those weights.
Let us look at an example that illustrates this proportions, what will be the proportion in
law. which sodium and oxygen combine to give
sodium oxide?
On analysis, sodium hydroxide is seen to
contain 95.8% sodium and 4.2% hydrogen by Both sodium and oxygen react with
weight. On the other hand, when water hydrogen. From the data given, we have to
decomposes, it gives 11.2% hydrogen and find the proportion in which they combine to
88.8% oxygen by weight. On the basis of this give sodium oxide. As in the case of the
data and applying the law of reciprocal example given in the law of constant
28
proportions, we shall simplify matters by Hence, we can see how the law of reciprocal
figuring out how much sodium and oxygen proportions applies in this case. This law has
react separately with 1g of hydrogen. limited applicability because there are only a
few substances that combine with each
From the data given, sodium hydride other and also with a third substance.
contains 95.8% sodium and 4.2% hydrogen by However, it can be applied to a chain of
weight, i.e. 4.2g of hydrogen reacts with 95.8g substances and the weights of different
of sodium. Therefore, 1g of hydrogen reacts elements combining with each other can be
with 95.8/4.2g of sodium = 22.8g of sodium. calculated. These weights are called
equivalent weights.
Water contains 11.2% hydrogen and 88.8%
oxygen (by weight), i.e. 11.2g of hydrogen Some equivalents calculated by Jeremia
reacts with 88.8g of oxygen. Therefore, 1g of Benjamin Richter with 1,000 parts of
hydrogen reacts with 88.8/11.2g of oxygen = sulphuric acid as the standard are given
7.9g of oxygen.
below:
According to the law of reciprocal Bases Acids
proportions, if 22.8g of sodium reacts with 1g Alumina 525 Fluoric 427
of hydrogen to give sodium hydride and 7.9g Magnesia 615 Carbonic 577
of oxygen also reacts with 1g of hydrogen to Ammonia 672 Muriatic (HCl) 712
give water, then if sodium and oxygen react,
Now try the problem given below to verify
the proportion in which they react will be:
the law:
22.8g of sodium to 7.9g of oxygen
Problem 3: Fluorine and oxygen combine to
This compares well with the formula of form a fluoride whose weight-percentage
sodium oxide (Na2O) and the atomic weights composition is 70.5% fluorine and 29.5%
of sodium and oxygen. That is, Na 2 O oxygen. Water decomposes to give 11.2%
contains 44.98 g of sodium and 16.0 g of hydrogen and 88.8% oxygen by weight. Apply
oxygen, or 44.98g of sodium:16.0g of oxygen. the law of reciprocal proportions to get the
According to our calculations: amounts of fluorine and hydrogen that react
to give hydrogen fluoride.
22.8g of sodium:7.9g of oxygen
or
45.6g of sodium:15.8g of oxygen
HISTORICAL PERSPECTIVE
Appendix 2 gives a series of practice problems that should help students develop a better appreciation of
the laws of chemical combination.
29
Atomic theory
Dalton introduces a new kind of logic to work out his seminal ideas about the particulate nature of
matter and usher in a new chapter in the history of chemistry
By the end of the 18th century, we had three laws Writing in his A New System of Chemical
that were universally applicable to chemical Philosophy in 1 8 0 8 , Dalton said: “These
reactions – the laws of conservation of mass, observations have tacitly led to the conclusion
constant proportions and reciprocal which seems universally adopted, that all bodies
proportions. But what made these laws valid? of sensible magnitude, whether liquids or solid,
What accounted for their universal applicability are constituted of vast number of extremely
in chemical reactions? Why did elements small particles, or atoms of matter bound
combine in definite proportions? Why couldn't together by a force of attraction, which is more
they combine in random proportions? What or less powerful according to circumstances.”
made it possible to predict how an element
would react with another? Why was it easier to “…This conclusion, which appears completely
predict the reaction if we knew how the two satisfactory;…we have hitherto made no use of
elements reacted with a third element? it, and that the consequence of the neglect has
been a very obscure view of chemical agency...”
Boyle and Bernouilli had hinted at a possible he added, decrying the fact that Boyle's work
explanation with their idea that matter is made had been overlooked for so long.
up of particles. But it was the English chemist
John Dalton ( 1766-1844 ) who provided a
workable and pragmatic explanation that wove
this idea into a logical framework. The atomic
theory he proposed in the early years of the 19th
century used a different kind of logic to explain
THE STORY OF ATOMIC THEORY OF MATTER
element. “Chemical analysis and synthesis go no indestructible, preserving their identities in all
farther than to the separation of particles one chemical reactions, with each element having its
from another and their reunion. No new creation own specific atom that was different from the
or destruction of matter is within the reach of atoms of other elements. Given this model of
chemical agency. We might as well attempt to matter (see box: DALTON'S ATOMIC THEORY IN A
introduce a new planet into the solar system, or NUTSHELL ), the laws of chemical combination
to annihilate one already in existence, as to become obvious.
31
Dalton's atomic theory in a nutshell
32
Dalton's atomic theory paves the way for the law of multiple
proportions
Dalton's atomic explanation also led to a change This predictive power of the atomic theory arises
in the way chemists reported their data. Earlier, from the postulate that atoms cannot be
the elements or compounds taking part in a subdivided. That is, if two elements A and B react
reaction and their products were given as to form more than one compound, then the
percentages. This caused some problems. Take molecules of the compounds must be made up of:
the example of the two oxides of copper known at
the time. One of them, red oxide, contains 89% 1 atom of A + 1 atom of B or,
1 atom of A + 2 atoms of B or,
copper and 11% oxygen by weight. The other,
2 atoms of A + 1 atom of B or,
black oxide, contains 80% copper and 20% oxygen 1 atom of A + 3 atoms of B or,
by weight. This data gives no hint about any 2 atoms of A + 3 atoms of B and so on.
relationship between these different weights.
So you will definitely not find compounds of A
The situation changes if the data is represented and B having half atoms of A or B, and since the
as mass ratios. The mass ratios of copper and weights of the atoms of each element are fixed,
oxygen in the two oxides are 89:11 and 80:20 the law of multiple proportions is self-evident
respectively (simplified to 9:1 and 8:2 ). It (Also see box: UNDERSTANDING THE LAW OF MULTIPLE
immediately becomes apparent that for a fixed PROPORTIONS).
amount of copper (that is say, 1g), the amount of
oxygen required is in the ratio 1:2 (for 1g of
copper, 0.123 g and 0.25 g respectively). So
reorganising the data can give fresh insights to
discern a new pattern (law).
Let us look at a couple of examples of the The amounts of carbon reacting with 1g of
law to understand it better: oxygen to give carbon dioxide and carbon
monoxide are in the ratio 0.38:0.75, which
Carbon burns in excess oxygen to give can be simplified to 1:2.
carbon dioxide, which is 27.3% carbon by
weight and 72.7% oxygen by weight. In Now calculate the ratio of oxygen
combining with the same amount of
insufficient oxygen, it gives carbon
carbon to give these two compounds.
monoxide, which is 42.9% carbon by weight
and 57.1% oxygen by weight. What is the Also, solve the following problems:
ratio of carbon combining with the same
Problem 4: Fluorine and oxygen combine
amount of oxygen?
t o fo r m a f l u o r i d e w h o s e w e i g h t
We need to calculate the amount of carbon composition is 70.5% fluorine and 29.5%
combining with the same amount of oxygen. These two elements also combine
to produce a second fluoride whose
oxygen (say 1g) to give carbon dioxide and
weight composition is 54.2% fluorine and
carbon monoxide and see if these two
4 5 . 8 % oxygen. Show how this data
amounts are in a simple ratio.
confirms the law of multiple proportions.
In carbon dioxide: 100g of carbon dioxide
Problem 5: Sodium combines with oxygen
contains 27.3g carbon and 72.7g oxygen, i.e. in two distinct chemical combinations.
72.7g of oxygen reacts with 27.3g of carbon. The products of these two reactions are
Therefore, 1 g of oxygen reacts with given below:
27.3/72.7g of carbon = 0.38g of carbon.
· So dium oxide: 74.2% s o dium, 25.8%
In carbon monoxide: 100 g of carbon oxygen
THE STORY OF ATOMIC THEORY OF MATTER
monoxide contains 42.9g carbon and 57.1g · Sodium peroxide: 59.0% sodium,
41.0% oxygen
oxygen, i.e. 57.1g of oxygen reacts with 42.9g
of carbon. Therefore, 1g of oxygen reacts How does this data illustrate the law of
with 42.9/57.1g of carbon = 0.75g of carbon. multiple proportions?
34
Calculating atomic weights
Dalton makes some wrong assumptions in his singular obsession that nature works in simple ways
but his work points the way to deciphering chemical reactions
In addition to explaining the laws of chemical no idea about the number of atoms contained in
reactivity, Dalton's theory also helped further these amounts of each element. Once again, take
the concept of atomic weights. In the third the example of copper and oxygen, which react to
postulate of his theory, he says that atoms are produce two different compounds. What was
matter and so have weight, and the atoms of the clear to Dalton was that the amount of oxygen in
same element have the same weight. Since one compound was double the amount in the
weight is a measurable quantity, he tried to second compound. But what wasn't obvious was
calculate the atomic weights of different whether this represented one atom of oxygen in
elements on the basis of the composition of one compound and two in the other or two in the
various compounds. He had to make some first and four in the second, or so on. It was also
assumptions in order to do so since he knew it not clear whether the atom/atoms of oxygen
was impossible to weigh individual atoms. combined with one or more atoms of copper.
He first assigned a weight of 1 for hydrogen since So Dalton had to assume the formula of each
it was the lightest element known: when any compound in order to assign atomic weights to its
compound containing hydrogen decomposes the constituent elements. He had faith in simplicity,
lowest yield in terms of its weight percentage is which meant that nature would not complicate
hydrogen. The atomic weights of all other matters and confuse us, so he assumed that
elements were calculated relative to hydrogen. elements would combine in one-to-one ratios.
This meant that if a compound is formed from the
Dalton knew from experimental data that a elements A and B, it would have the formula A1B1
definite amount of an element combines with a — one atom of A with one atom of B. (Also see box:
HISTORICAL PERSPECTIVE
35
The weight of assumptions
There are reasons to believe that Dalton (1778-1829) raised it to 7.5 and, ultimately,
could have solved the enigma of atomic Joseph-Louis Proust (1754-1826) arrived at
weights had he not been so caught up with the ‘correct’ figure of 8.
his idea of simplicity. At times he even
tended to reject experimental data in Dalton then turned to the oxides of carbon
favour of his cardinal faith in simplicity. and nitrogen to calculate the atomic
That's what happened to Joseph Louis Gay- weights of other elements. The possible
Lussac's data, which Dalton rejected on the choices he faced are shown in the table
ground of coincidence and experimental below. One oxide of carbon had a C/O ratio
error. We must remember that Gay-Lussac of 0.75:1 and the other had a ratio of 0.375:1.
(1778-1850) was widely respected for his So if one oxide was CO (which Dalton
tremendous experimental skills, unlike assumed) the other would be CO2. Or they
Dalton himself, who was not a very good could be C2O and CO. The first possibility
experimentalist. gives carbon an atomic weight of 6. If the
. second possibility is accepted, the atomic
We have seen that Dalton's assumption weight would be 3. Dalton found the first
about simplicity did not hold water, oxide was more stable on decomposition,
ironically in the case of water itself. so he assumed its formula as CO, which
'Simplicity' led him to assign the formula was the correct choice in hindsight.
HO to water and he calculated the atomic
weight of oxygen as 6.5, which he later Similarly with the oxides of nitrogen,
raised to 7. Continuing to work on the Dalton ruled out possibilities 1 and 3
'simplicity' assumption, Humphry Davy below, because a molecule made up of five
37
If two compounds of A and B are formed, then one (generally the more abundant) would be A1B1 and the
second would be either A2B1 (two atoms of A with one atom of B) or A1B2 (one atom of A with two atoms of
B), depending on the relative weight percentages of A and B obtained on decomposition of these
compounds.
Dalton rounded off the figures he got for the weights of atoms of elements to ensure that they were
mostly whole numbers.
Let's take a specific example that illustrates the procedure he followed to calculate the atomic weight of
oxygen. Water was the only compound of hydrogen and oxygen known at the time, so Dalton assumed a
formula of HO for water on the basis of his rule of simplicity and calculated the atomic weight of oxygen
from the available decomposition data of water. The data showed that water contained 87.5% oxygen
and 12.5% hydrogen by weight. So the atomic weight he calculated for oxygen was 7 :
This is less than half the accepted weight of oxygen, which is to be expected since Dalton assigned HO as
the formula for water instead of H2O. But nature is not always so simple and elements often combine in
ratios other than one-to-one. That's why many of Dalton's weights turned out to be incorrect as seen in
the table below. However, his work still represented a big step forward.
(For some practice problems in calculating atomic weights, see box: CALCULATING ATOMIC WEIGHTS).
Hydrogen 1
Carbon 5.4
Phosphorus 9
Iron 50
Copper 56
Azote (Nitrogen) 5
Oxygen 7
THE STORY OF ATOMIC THEORY OF MATTER
Sulphur 13
Zinc 56
Lead 95
38
Calculating atomic weights
You can also try your hand at calculating atomic weights. Note the importance of assuming a
formula and the difficulties faced by Dalton in this regard while you do these problems.
Problem 7: On analysis, sodium hydride is seen to contain 95.8% sodium and 4.2% hydrogen by
weight. What is the atomic weight of sodium if the formula of sodium hydroxide is NaH and
hydrogen is taken as the standard with a weight of 1?
Problem 8: Water is 11.2% hydrogen and 88.8% oxygen. Calculate the atomic weight of oxygen
assuming hydrogen as the standard with an atomic weight of 1. First, calculate the atomic
weight of oxygen assuming the formula of water is HO, as Dalton did. Then calculate the
atomic weight with the correct formula of water (H2O) and compare your answer with the
atomic weight of oxygen given in the modern periodic table.
The early 19th century was also the time when while weights appeared to increase by simple
many chemists began studying gases. As pointed addition on combination, volumes could increase
out earlier, this component of chemical reactions or even decrease at times.
was generally overlooked because of the lack of
Gay-Lussac formulated a law in 1809 based on his
equipment to collect the gases generated. As the
equipment became available, investigations into observation that gases combine in simple
gases gathered steam, providing new insights proportions by volume at a given pressure and
into atomic theory as well as our understanding temperature and, if the products are gaseous,
of chemical formulae. they also bear a simple whole number ratio to the
gaseous reactants. Some volume and weight
Among those who conducted many experiments ratios of reacting gases are on the next page.
to study how gases reacted and combined was
the French chemist Joseph Louis Gay-Lussac. Two years later in 1811, Italian physicist Amedeo
HISTORICAL PERSPECTIVE
Unlike Dalton, who noted the weights of the Avagadro (1776-1856) proposed a hypothesis that
reactants and products in a chemical reaction, he pointed to a distinction between gases composed
studied the volumes of reacting gases. For of simple units (atoms) and complex units
example, he would say three litres of hydrogen (molecules). Unfortunately, Avagadro's work was
react with one litre of nitrogen to give two litres ignored for 40 years for two reasons. First, he
of ammonia. In the same case Dalton would say lived in Italy while the centres of activity in
3g of hydrogen react with 14g of nitrogen to give chemistry at the time were France, Germany,
17g of ammonia. The point of interest is that Britain and Sweden. The second reason was that 39
Volume and Weight ratios of some reacting gases
most chemists of the time were not prepared to then a given volume of say, 1,000 hydrogen
accept the concept of molecules because they atoms would combine with 1,000 atoms of
could not imagine that a molecule could contain chlorine. Since this amount of chlorine occupies
atoms with similar ‘affinities’ (scientists were not the same volume as hydrogen, it follows that
talking about electric charges in atoms at that equal volumes of any gas would contain the
point of time). same number of atoms under the same
conditions of temperature and pressure. So, if
It was left to Jöns Jacob Berzelius (1779-1848) to
one litre of hydrogen reacts with one litre of
bring some order to the confusing and
chlorine, this volume of each gas would contain
sometimes contradictory quantitative
an equal number of atoms, otherwise they
experimental data. The Swedish chemist, who
would not react completely.
did a lot of work on calculating atomic weights of
various elements between 1808 and 1826, was His reasoning pointed to a link between integral
familiar with the work of both Dalton and ratios of volumes and integral ratios of weights.
Gay-Lussac. So if the densities (g/L) of different gases under
identical conditions of pressure and
Taking the example of chlorine, which reacts with temperature are measured, they would be
hydrogen to form hydrogen chloride, he applied proportional to their atomic weights. For
Dalton's logic of simplicity to the combination – a example, if one litre of gaseous element A
binary comprising one atom of hydrogen and one weighs 'x'g and one litre of gaseous element B
atom of chlorine. In accordance with Gay-Lussac, weighs 'y'g, the ratio x:y will be the ratio of their
the gas volume ratio of hydrogen and chlorine in atomic weights because one litre of each gas will
combination would be 1:1. contain the same number of atoms. The table
below gives the atomic weights of some gaseous
Since Dalton also proposed that atoms combine elements calculated on the basis of their
in simple whole numbers to give compounds, densities, using hydrogen as the standard:
THE STORY OF ATOMIC THEORY OF MATTER
Atomic Weight
Nitrogen 5 13.9
Oxygen 7 15.9
Chemists thought the idea of atoms to be not just confusing,
but of no help in explaining phenomena
As we have seen, determining atomic weight chemistry: “I established its (chlorine's) atomic
depended on two things – the number of atoms weight by the following experiments: (1) From
of combining elements in compounds and their the dry distillation of 100 parts of anhydrous
weights. For example, one atom of zinc combines potassium chlorate, 38.15 parts of oxygen are
with one atom of oxygen to give zinc oxide. By given off and 60.85 parts of potassium chloride
measuring the relative weights of zinc and remain behind (Good agreement between the
oxygen in the compound, one gets the relative results of four measurements). (2) From 100 parts
weights of the atoms of each element. (The of potassium chloride 192.4 parts of silver
combining ratio is the weight of the oxide minus chloride can be obtained. (3) From 100 parts of
the weight of the metal divided by the weight of silver 132.175 parts of silver chloride can be
the metal. So if the oxide has two atoms of oxygen obtained. If we assume that chloric acid is
HISTORICAL PERSPECTIVE
to one of the metal, then the ratio must be composed of 2 Cl and 5 O, then according to these
divided by 2.) data 1 atom of chlorine is 221.36. If we calculate
from the density obtained by Lussac, the chlorine
Berzelius standardized the atomic weights of all
atom is 220 (relative to the atomic weight of
gaseous elements to that of oxygen, assigning a
oxygen). If it is calculated on the basis of
value of 100 to it. He described the steps to
hydrogen, then it is 17.735 .” (Also see box:
determine the atomic weight of chlorine relative CHANGING STANDARDS FOR CALCULATING ATOMIC
to oxygen and to hydrogen in his Treatise on WEIGHTS). 41
Changing standards for calculating atomic weights
Atomic weight is the ratio of the average mass of an atom of an element compared to a
standard unit. Since atoms are too small to measure, chemists in the 19th century (beginning
with Dalton) determined the atomic weights of different elements in relative terms – as
multiples of the atomic weight of hydrogen, the lightest element, which was assigned a value
of 1. The atoms of other elements were compared against this standard to calculate their
atomic weights.
But there was a problem in using hydrogen as the standard. If refinements in measurements
through improved techniques led to a better value for the weight of hydrogen, even a small
change would cause the atomic weights of other elements to change by a large factor.
Berzelius suggested using oxygen as the standard assigning it with an atomic weight of 100.
But this unit never really caught on.
Oxygen was, however, a good choice for a standard since it reacts with most other elements
and so the relative weights can be calculated directly for many elements. So it was used as the
standard from around 1900 to 1961 with an assigned value of 16. So the unit of atomic weight
was defined as 1/16 the weight of an oxygen atom.
However, when isotopes were later discovered, it was found that the relative atomic weights
of elements also reflected the percentage composition of their isotopes. Two other isotopes of
oxygen were discovered – O-17 and O-18, making it unsuited to serve as a standard.
Eventually in 1961, a new scale was established based on the carbon-12 atom, the most
abundant isotope. The definition of atomic weight then became 1/12 the weight of the carbon-
12 atom. This is the standard that is currently in use.
The result is half the modern values. The reason The lack of an understanding of molecules and
lies in the distinction between atoms and atoms gave rise to another problem. If Berzelius'
molecules. We must remember that the concept conclusion that equal volumes of gases
THE STORY OF ATOMIC THEORY OF MATTER
of 'molecule' was unknown at the time and 'atom' contained equal numbers of atoms was correct,
was used to describe the ultimate particle of it meant the atoms were splitting during some
both elements and compounds, with Dalton chemical reactions. Let's again consider the
using the term 'compound atom' to describe the example of hydrogen chloride. One litre of
smallest particle of a compound made up of hydrogen and one litre of chlorine combine to
atoms of two or more elements. Hydrogen exists give two litres of hydrogen chloride gas.
as a molecule made of two atoms of hydrogen. So According to Berzelius, this would mean that
the standard would be 2H = 1, which is what one atom of hydrogen and one atom of chlorine
Berzelius understood, giving an atomic weight give two atoms of hydrogen chloride.
42 for chlorine of 17.735 relative to hydrogen.
1 litre of hydrogen + 1 litre of chlorine → 2 litres of hydrogen chloride
or (according to Berzelius)
One atom of hydrogen chloride having half an elements to form compounds. As we have seen,
atom each of hydrogen and chlorine! That was Dalton used hydrogen as a standard, calculating
something Dalton could never accept since he atomic weights of elements as multiples of its
was convinced that atoms cannot be split. So he a t o m i c w e i g h t . H o w e v e r, B e r z e l i u s '
rejected the work done by Gay-Lussac, claiming investigations ended up revealing that the atomic
that the observation that gases reacted in simple
weight of some elements falls between two
volume ratios was either sheer coincidence or
multiples of the hydrogen weight. (Today we
due to experimental error.
know this is because of isotopes of these
There is an irony in the situation. Berzelius is elements. See the earlier box on changing
HISTORICAL PERSPECTIVE
known today for having taken measurements of standards for calculating atomic weight.) He
gaseous reactants to a new level of accuracy and published a table of atomic weights in 1826 that
precision. He had a reason for doing so. He mostly shows good agreement with modern
wanted his data to fit the requirements of values (given in the next page). He analysed over
Dalton's atomic theory, which placed strict 2,000 compounds, and also calculated the weights
conditions on the combination of different of 43 elements.
43
Incidentally, Berzelius also developed a system only difference being that Berzelius
of chemical notation in which each element was superscripted the numbers showing the
assigned a simple written label – O for oxygen, Fe number of atoms of each element (H²O) while
for iron, etc – with proportions written in we subscript them (H2O).
numbers. This is the system we use today, the
44
Specific heat provides another way to calculate atomic weights and
determine formulae
Since the specific heat could be determined If 1.074g of silver oxide is formed from 1.000g of
experimentally, this law could be used to find the silver, what is the formula of the compound
approximate atomic weights of metals. Some formed between silver and oxygen, and what is
values obtained by Dulong and Petit are given in the atomic weight of silver, if the approximate
the table below. They were also able to spot some atomic weight of silver, calculated from its
errors in Berzelius' atomic weights, as in the case specific heat, is 113.3 and the atomic weight of
of silver. oxygen is 16.
HISTORICAL PERSPECTIVE
(*The unit for specific heat used was cal/gºC, but The weight of silver in 1.074g of silver oxide is
modern SI system uses J/gK. If the SI units are 1.000g.
used, the constant will be 25 instead of 6.4.) The weight of oxygen in 1.074g of silver oxide is
0.074g.
The following example shows how this law can be
used to decide between different formulae of Therefore, the mass ratio of silver and oxygen is:
compounds: 1.000/0.074 = 13.51.
45
Assuming the formula of silver oxide to be AgO, atomic weight of silver is 216.16/2 = 108.08,
this means an atom of silver is 13.51 times heavier which compares well with the accepted value
than an oxygen atom. Since the atomic weight of for the atomic weight of silver.
oxygen is 16, the relative atomic mass of silver in
The following table compares atomic weights of
silver oxide is 16× 13.51 = 216.16
some elements obtained by the three methods
discussed above. But there was no way of
From the specific heat data, we have the
deciding between the values!
approximate atomic weight of silver as 113.3,
which is roughly half the weight obtained above. Atomic weights are given relative to hydrogen,
Hence, the formula of silver oxide is Ag2O and the although Berzelius used oxygen as the standard.
Hydrogen 1* 1*
Iron 50 58.2
Copper 56 67.4
Nitrogen 5 13.9
Oxygen 7 15.9
THE STORY OF ATOMIC THEORY OF MATTER
46
Cannizzaro revives Avogadro's ideas to conclude the long story of
atomic weights
Unfortunately, the Dulong and Petit method was But then atomic weights are a useful measure
not adopted until the 1850s because the two because they give us a way to interpret our
scientists gave no explanation for the regularity macroscopic measurement in terms of the
they found. So chemists felt it was just as number of atoms taking part in reactions.
speculative as Dalton's rule of simplicity. Ultimately, resolving the problem of determining
atomic weights was a triumph of logic and one of
the foundations of modern chemistry. Things
were finally sorted out due to the contributions
of Avogadro and the Italian chemist Stanislao
Cannizzaro (1826-1916).
If we have one crate of apples weighing 10 kg and another of Let us take a closer look at Avogadro's work,
bananas weighing 15 kg, and if we count and find that the which was overlooked during the time Berzelius
number of apples and bananas is the same, we can find out the
weight of apples relative to the weight of bananas conducted his investigations. The Italian
physicist presented a totally new model of gases
The basic problem in determining atomic weights while trying to reconcile Dalton's atomic theory
lay in extrapolating macroscopic measurements with Gay-Lussac's experimental results. There
of some amount of an element, comparing it with was no clear concept of molecules at the time, so
an amount of another element and then drawing confusion reigned when it came to deciding how
conclusions about the relative masses of many atoms of each element combined in a
individual atoms. Such comparisons are valid compound.
only if we are dealing with the same number of
Now take a second look at Gay-Lussac's data.
atoms of each of these elements. For example, if
When we worked with weights, we saw that it
we have a box of bananas weighing 15 kg and a
was simple arithmetic to say that the total weight
box of apples weighing 10 kg, it would only be
of the reactants was equal to the total weight of
valid for us to say that each banana is one and a
the products. However, gases do not behave in
half times as heavy as an apple if we know that
such a simple manner when their volumes are
each of them contains a dozen of each fruit. With
measured. For example, if two litres of hydrogen
bananas and apples, it would be simple enough to
and one litre of oxygen are mixed without
open the box and count. But while dealing with
bringing them in contact with a spark, the volume
atoms, the problem is that we cannot count them.
of the mixture would equal the combined volume
of the two gases – three litres. So physical mixing
HISTORICAL PERSPECTIVE
49
Calculating molecular weights from vapour densities
Avogadro's hypothesis suggests a method to after which its volume is measured. This
determine molecular weights. If equal equals the volume of the vapourised liquid.
volumes of different gases always contain an The weight of the liquid used, divided by the
e qual numb er of mole cules, then the volume of the vapour in litres, gives the
molecular weights are proportional to the vapour density. The molecular weight is
densities of the gases. The German chemist obtained by multiplying the vapour density
Victor Meyer (1848-1897) devised a method in of the liquid by the molecular weight of air.
1878 to determine the vapour densities of Meyer took the molecular weight of air to be
liquids. A weighed quantity of the liquid is 28.73, assuming that air is 14.367 times heavier
evaporated in a tube at a constant high than hydrogen. He added that the number
temperature. The air displaced by the vapour 28.95 should be used instead of 28.73, if the
is collected and cooled to room temperature molecular weight of oxygen is taken as 16.
Displaced
Victor Meyer Air
Tube
Hot
Vapours
Copper
Jacket
Glass wool
Liquid Hoffman
Bottle
THE STORY OF ATOMIC THEORY OF MATTER
50
Cannizzaro outlined his method in a pamphlet That made 'molecules of elements' a more
‘Sketch of a course of chemical philosophy’, which palatable concept even though he could not
was distributed at the first international explain how similar atoms are linked in a
congress of chemists that was held in Karlsruhe compound. It was only in the 20th century that
(Germany) in 1860 to resolve issues related to Linus Pauling eventually explained the nature of
determining atomic weights and the difference 'covalent' bonds underlying this phenomenon.
between 'atom' and ‘molecule’.
Cannizzaro's method led to calculations of
His theory was accepted because it appeared to atomic weights that were consistent and gave
be the best way to resolve the confusions arising values close to modern values. The weights of
out of the different methods of calculating molecules (of elements or compounds) were
atomic weights. He was helped by the fact that a o b t a i n e d by a d d i n g t h e w e i g h t s o f t h e
large number of carbon compounds were known constituent atoms.
by that time, with different numbers of carbon (Also see box: MEASURING THE ATOMIC WEIGHT OF
atoms linked to one another in these compounds. OXYGEN).
Knowing the atomic weight was vital for because of technical difficulties, most
finding out the chemical composition of a chemists measure d two of the thre e
compound, calculating reacting quantities in quantities – oxygen, hydrogen, water –
chemical reactions and understanding these involved in the reaction and calculated the
reactions. But the 'wet chemistry' methods third on the basis of the law of conservation
used by chemists in the 19th century were of mass.
laborious and far less precise than later
physical methods – procedures such as Edward Williams Morley (1838-1923) avoided
filtrations, dissolutions, and such assumptions to come up with the most
recrystallisations. By the early 20th century precise 'wet chemical' determination of
atomic weights could be determined easily oxygen's atomic weight in 1895. The American
and pre cis ely using s ophisticate d scientist was a skilled experimentalist who
instruments like the mass spectrometer. was known for his meticulous and precise
measurements of various natural constants –
The most important atomic weight that had chemical compositions, densities, weights,
to be determined was oxygen's. This was lengths, etc. He develop e d elab orate
because the atomic weights of most elements apparatus and methods to remove impurities
had to be calculated by synthesis or analysis like water vapour and other gas vapours from
of their oxides. So the accuracy of the gas samples to weigh them more precisely. So
HISTORICAL PERSPECTIVE
calculations depended on fixing oxygen's he was able to get precise measurements of all
atomic weight as accurately as possible. Any three quantities, calculating oxygen's atomic
small error in this value would be magnified weight at 15.892 by two comparisons – water
in the case of the heavier elements. and oxygen and hydrogen and oxygen. He
then cross-checked his results with physical
Most chemists of the time tried to determine methods – measuring and comparing the
oxygen's atomic weight by synthesising densities and volumes of hydrogen and
water from hydrogen and oxygen. But oxygen.
51
Atomic theory and what happens when solids, liquids and gases
change physically
the reaction? The answer lies in Avogadro's Pressure – SATP). For example, 2g of hydrogen
theory. gas and 32g of oxygen gas both occupy a volume
of 22.4 litres at STP.
We find his theory (or Gay-Lussac's/Berzelius')
gives us another way of looking at the problem – In other words, 22.4 litres of any gas at standard
in terms of numbers. Avogadro said equal pressure and temperature contain the same
volumes of gases at the same temperature and number of molecules. The weight of these
53
molecules is the gram molecular weight By extrapolation, we could say that the gram
(molecular weight expressed in grams). This atomic or molecular weight of any substance
number is called a mole and is abbreviated as mol. (solid, liquid or gas) would contain these many
The actual number was calculated much later particles. If a gas is cooled to its liquid state, and
using several different methods to get its then the solid state, it would still contain the
accepted value — 1 mole = 6.022 × 10²³. This same number of particles and would weigh the
number is called the Avogadro number in honour same (though only the gas would occupy a
of the man who played an important role in volume of 22.4 litres at STP, the solid and liquid
unravelling the mystery about atoms (even states being a lot denser). The mole is a very
though Avogadro himself had no idea of what this useful and powerful tool because, using it, we
number was!) can calculate the number of atoms or molecules
of any substance we weigh, if we know its atomic
or molecular weight.
54
Epilogue
Atomic theory is accepted by the scientific community and moves several steps ahead, although
the end is still not in sight as nature unfolds new mysteries…
We have seen how thinking on the nature of gave a detailed explanation of Brownian motion.
matter has evolved down the centuries, with He reasoned that small particles (such as pollen
each new development building upon previous grains) moving in a liquid are pushed around in
knowledge. In the modern era of science, this every direction by far smaller atoms of the liquid,
process has essentially been carried forward on and their random motions, though unpredictable,
the twin pillars of experimentation and obey certain laws of probability. He proposed
mathematics, backed by intuitive thinking, logic statistical/mathematical formulae for these
and reasoning. motions and even calculated the number of water
molecules per square inch to a high degree of
A new scientific theory is not easily accepted. It accuracy.
goes through a process of churning. Many
scientists look at it. Some examine the logic of its Einstein's work on Brownian motion allowed
argument. Others look at what the theory can French chemist Jean Perrin (1870-1942) and
predict and then see if the predictions are true. others to prove the physical reality of molecules
Still others point out data that may contradict or and atoms.
question the theory.
Austrian scientist Josef Loschmidt (1821-1895)
By the end of the 19th century, chemists had had Earlier been the first to estimate the size of
generated a large amount of data about different molecules that make up air (1865). The value he
elements, how they reacted with each other and arrived at was twice their actual size but his feat
in what proportions. They had empirical laws was remarkable, given the approximations he
derived from this data as well as theories to show had to make. His method made it possible to
why these laws were valid. estimate the number of molecules in a given
volume of gas under standard conditions. Known
The existence of atoms itself was a hotly debated today as the Loschmidt constant, its modern
issue during this time, with several chemists value is 2.69 x 1025 per cubic metre at STP, a
holding the view that chemistry is the art of the number that is sometimes confused with
possible and one should not worry about fanciful
HISTORICAL PERSPECTIVE
that have the same number of protons – the Therefore, the chemical reactivity/properties of
atomic number – are atoms of the same element. an element depend on the number of electrons
in an atom of that element and the way these
The discovery of the neutron in 1932 by English electrons bond with electrons of other
physicist James Chadwick (1891-1974) showed elements. Protons also play a significant role
that the nucleus of an atom contains neutrons in because the tendency for an atom to either lose,
addition to protons. The neutron has no electrical gain or share electrons depends on the charge of
charge but contributes to the stability of the the nucleus. But the chemists say the inner
atom and is slightly more massive than the structure of atoms belongs to the realm of
proton, so it adds to the total weight of the atom, physics and they deal with chemical reactions
56
called the atomic weight. This weight is the sum only under conditions where atoms are still the
of the weights of the protons and neutrons, while ultimate particles!
Part-2
Atomic theory lies at the heart of chemistry. In beyond the primary stage, arise because
India, the idea that matter is particulate is students tend to see solids and liquids as
introduced at the middle school level, before continuous, not particulate. This is a common
students begin studying chemistry as a separate sense idea which they find difficult to forsake
discipline. This is the age when they struggle to because they cannot seem to accept the
make sense of abstract concepts. The problem is existence of empty space – vacuum – between
compounded by the fact that they come to school atoms. They see the space as filled with air, dust,
with preconceived notions about matter that or something else. They also err in estimating the
may not be in consonance with what is taught. space (distance) between particles in gases,
Even with instruction, many of these ideas tend liquids and solids, the separation being grossly
to persist. So many students enter the higher overestimated in the case of liquids.
secondary level with this mental baggage.
This superficial understanding that matter is
So what are these misconceptions that have been continuous makes them assign the bulk
brought out by several studies of students in properties of the substance to individual
different age groups? Let's begin with primary particles, attributing macroscopic changes to
school students. We find that they have a naïve changes in the shape, size or state of the atoms.
view of matter based on the common sense So when a substance expands, they believe that
principle of 'seeing is believing'. So they tend to the atoms swell and increase in size, or when it
put powders and gels in categories separate from melts, they see the atoms melting.
solids and liquids. They do understand that each
substance has a physical state and a solid can Students at the middle school level are simply
melt if heated or a liquid can freeze if cooled. unable to grasp what a particle means or
However, they have a problem with gases conceptualise how it behaves. They cannot grasp
because these are 'invisible' and because they that particles have intrinsic motion, nor believe
often don't see gases as matter. And when sugar that particles move in liquids, and even solids.
dissolves in water, they tend to think it Even for extrinsic motion they may believe
'disappears'. Some may say its 'taste' remains. particles move faster in a heated substance, but
may not equate cooling of a substance with a
Many of these misconceptions, which persist slowing down of particle motion. 57
Atomic theory in the classroom
Students begin to develop atomistic ideas as chemical reactions. With little experience of
they gain a better understanding of the concept seeing the mass relationships between the
of conservation of substance, weight and reactants and products, they find it difficult to
volume. However, they still do not easily make connections between events taking place
substitute 'scientific' ideas for the many at the bulk scale and their description at the
misconceptions they carry in their minds. A lot atomic level in the form of formulae and
depends upon how these abstract concepts that equations.
throw light on the nature of matter are treated in
school textbooks and how teachers put them Symbols:
across to students.
When talking about matter, teachers tend to
Existing textbooks tend to treat these concepts constantly shift from the macroscopic (reacting
in a cursory manner, without going into details or substances) to the sub-microscopic (atoms and
providing evidence of the particulate nature of molecules taking part in the reaction) to the
matter. The diagrams in these textbooks are symbolic (formulae and equations). Textbooks
usually not of much help either and may in fact also move between descriptions of macroscopic
often convey wrong ideas or mislead students. properties, sub-microscopic properties and the
What compounds the problem is the use of symbol system used to denote them. If these
formulae as a symbolic representation of shifts take place without explanation, they tend
atomic/molecular combinations in various to confuse students. They must be told which
substances, without giving proper explanation. aspects are being discussed and how the
different aspects are interrelated.
The result is that atoms remain a mystery for
students and they fail to develop an Take the following example. Students often say
understanding of the implications of atomic that N2O5 cannot be prepared from N2 and O2
theory in describing chemical change or even because you would need three more atoms of
change in state. They also fail to create a mental oxygen to form the product. The confusion
picture of the reality represented by abstract arises because they are unable to realise the
symbols. relation between the element and its depiction
in the form of a symbol/formula. They have no
Let's take a more detailed look at the way atomic appreciation of how a formula is arrived at. So
theory is dealt with in the classroom. This will they tend to merely change the numbers of
help teachers get a better idea of what they need different atoms in the formula in order to
to do if they wish to improve the situation and balance equations. They see the activity as a
THE STORY OF ATOMIC THEORY OF MATTER
ensure that learning of this difficult concept is straightforward mathematical exercise instead
enhanced. of something that reflects the exact quantitative
nature of the reaction taking place.
Lack of time:
The problem of gases:
This is a serious methodological problem.
Students are not given sufficient time to We have seen that students find it difficult to
assimilate and internalise ideas – many of them recognise gases as matter. So they often do not
counter-intuitive – about the nature of matter. take into account gases that may be used up or
58 They also do not get enough time to conduct produced during a reaction. This gives a
confusing idea about what exactly the chemical
reaction is. For example, if the role of oxygen and
atmospheric moisture is not recognised in the
rusting of iron, students tend to think that iron
solid
turning red and crumbly after some time is a
property of the metal. They also find it difficult to
imagine the vast empty space between particles
in gases.
liquid
Illustrations:
correct, the density of the solid would be at least electrolysis of water occurs when a seemingly
twice that of the liquid form of the same small amount of energy from a battery is passed
substance, and the density of the liquid would be through water. In such a situation students often
only around four times that of the same erroneously conclude that water 'boils' to give
substance in the gaseous state. This is obviously hydrogen and oxygen (both gases) since they
not true of any known substance. cannot judge which categories apply where.
59
Analogies: account of the process. If shown a piece of ice
kept at room temperature, they correctly say it
We often look for analogies to explain various is melting, but have no idea what melting means.
phenomena. So do most students, often
erroneously. For example, students may use It isn't only students. Even adults, including
ageing in humans as an analogy for rusting of teachers, face difficulties. In a small study,
iron. A child named Nirjuli grows up and ages – several teachers were shown the picture from
height, weight, shape all change – yet she the NCERT textbook and asked: “What is
remains Nirjuli to the end. Similarly, iron may present in the space between the molecules in
rust but continues to be iron – because rusting is the case of gases?” Some responded, saying
in the nature of iron, which continues unchanged there is air between the gas molecules. Some felt
within the dusty brown product. So, this wrong there is an intermolecular force between the
analogy indicates that students do not realise molecules. Only a few said there is nothing
rust is not same as iron, but a new product. between the molecules.
If students are to internalise the concept of the under which these changes take place. For this,
particulate nature of matter, they need to they need to be able to perform simple tests to
appreciate how these ideas developed. They find out the properties of substances and how
must also get some exposure to the following the properties of the starting materials and
ideas/concrete experiences before they move on products are different.
to the theoretical issues.
THE STORY OF ATOMIC THEORY OF MATTER
61
Appendix 1
The Cannizzaro method of calculating atomic weights
In 1858, Stanislao Cannizzaro applied Avogadro's hypothesis to select the correct weights for the
atoms of the different elements. Here are his postulates:
Ÿ The atomic theory states that all atoms of any element have a definite weight.
Ÿ Since molecules such as the hydrogen molecule or the water molecule contain definite numbers
of atoms, they must have definite weights, which we refer to as formula weights.
Ÿ These formula weights contain one atomic weight (or a whole number multiple of that atomic
weight) for each element present.
Based on these postulates, he proposed a method to calculate atomic weights, following the steps
given below:
Ÿ If all gases have an equal number of molecules in equal volumes, their densities will be
proportional to their molecular weights, i.e. M D or M = KD, where K is a constant, M is the
molecular weight of the given gas, and D is the density of the given gas.
Ÿ If we know the molecular weight of a gas, we can calculate the constant K from its density. For
example, hydrogen has a molecular weight of 2 and oxygen 32. Therefore:
Molecular
Gas Density K
weight
Ÿ To calculate the atomic weights of, let’s say, carbon and chlorine, we have to find out the molecular
weights of various gaseous compounds of carbon and chlorine from their densities:
THE STORY OF ATOMIC THEORY OF MATTER
Molecular weight
Gaseous Compound Density
(Density x K)
Methane 0.72 16
CARBON
TETRACHLORIDE 7.8 - 92.9 11.01 - 141 CCl4
Ÿ After calculating the molecular weights, we need to find the percentage weight of every
element in each compound experimentally. The probable formula that can be derived from this
information is given in the following table, which also gives the calculation method:
Ÿ Let us see how the above information is derived from the data. Step 4 gives 16 as the molecular
weight of methane. The percentage of carbon in methane (column 2) is 74.8. That is, 100g of
methane has 74.8g of carbon. Therefore, 16g of methane (one mole of methane) contains
(74.8/100) × 16 = 12g of carbon. The other values in the table have been calculated in a similar
manner.
Ÿ We have calculated the amount of every element in one mole of each of the compounds. Next,
we look at the minimum amount of an element present in these compounds. We can see that
one mole of each compound has different amounts of carbon. The minimum amount of carbon
in one mole of its compounds is 12g. From this, we take the atomic weight of carbon as 12
because we assume that these compounds contain at least one atom of carbon. If later studies
give us compounds that contain 6g or 4g of carbon/mol, we will have to revise the atomic weight
of carbon and all the formulas calculated in the above table.
Ÿ What would be the formulas of methane if the atomic weight of carbon was 4 instead of 12?
The formulas of compounds can be calculated from the atomic weights of the elements by
reversing the process.
TEACHING ATOMIC THEORY
63
Appendix 2
More problems related to laws of chemical combination
1. In a set of experiments designed to verify the law of definite proportions, very pure tin metal was
quantitatively combined with elemental bromine, forming tin tetrabromide. Using the data given
below, confirm the law by calculating the percentage of tin in each sample of the tetrabromide:
2. Direct combination of zinc and sulphur yields zinc sulphide. In a number of experiments, the
weights of reacting zinc and sulphur are as follows:
3. Fluorine and oxygen combine to form a fluoride whose weight-percentage composition is 70.5%
fluorine and 29.5% oxygen. Also, water decomposes to give 11.2% hydrogen and 88.8% oxygen by
weight. Apply the law of reciprocal proportions to get the proportion in which fluorine and
hydrogen will react.
4. Fluorine and oxygen combine to form a fluoride whose weight-percentage composition is 70.5%
fluorine and 29.5% oxygen. These same two elements combine to produce a second fluoride
whose weight-percentage composition is 54.2% fluorine and 45.8% oxygen. Show how these data
confirm the law of multiple proportions. Suggest simple formulae for the two fluorides.
5. Determine the atomic weight of copper from the following facts: 63.5g of copper combines with
16.0g of oxygen to form a 1:1 oxide of copper (given that the atomic weight of oxygen is 16).
6. Magnesia refractories such as for sterite are composed of magnesia and silica.
a) Without consulting the periodic table of elements, can you figure out what the relative atomic
THE STORY OF ATOMIC THEORY OF MATTER
weight of magnesium is if the Mg/O2 combining weight ratio in magnesia (MgO) is 1.52:1.00?
Compare your answer with the value given in the periodic table.
b) If the Si/O2 combining weight ratio in silica (SiO2) is 0.878, what is the combining weight of silicon
and what is its relative atomic weight?
7. Kaolinite refractories contain alumina (Al2O3). Given that the atomic weight of aluminium is 26.98,
calculate the Al/O2 ratio and the combining weight of aluminium.
8. The atomic weight of chromium is 52.01 and its combining weight with oxygen is 34.667. What is
64 the chemical formula for the oxide?
9. The combining weight of molybdenum metal in one of its oxides is known to be O2. The specific
heat of molybdenum is known to be 0.250J/gK. What is the atomic weight of molybdenum?
(specific heat [in J/gK] × atomic weight = ~25 J/mol.K)
11. Sodium enters into two distinctly different chemical combinations with oxygen. Here are the
percentage compositions of the products of these two reactions:
12. Here are some data on the analysis of a set of compounds composed of only two elements, carbon
and hydrogen (hydrocarbons). Show how these data can be used to demonstrate the law of
multiple proportions:
13. On the basis of the information below, determine the relative atomic weights for the atoms in
each compound. Assume the formulas are correct and the atomic weight of hydrogen is 1.0.
14. It was found that the ratio of the weights of equal volumes of chlorine and oxygen was 2.22. What
is the apparent molecular weight of chlorine, assuming oxygen to have an atomic weight of
16.0g/mol?
15. At room temperature and atmospheric pressure, a gram of oxygen occupies a volume of 0.764
litre, whereas a gram of an oxide of nitrogen under the same conditions occupies a volume of
TEACHING ATOMIC THEORY
16. The specific heat of lead is 0.13J/gK; its combining weight in lead chloride has been found to be
exactly 103.605/35.453 of chlorine.
a) Using Dulong and Petit's law, determine the approximate atomic weight of lead.
b) Since the atomic weight must be an integral multiple of the combining weight, determine the
correct atomic weight of lead. 65
c) What is the empirical formula for this particular chloride?
17. The specific heat of lead is 0.13J/gK. Its combining weight in a certain lead oxide is found to be
exactly 138.133g. What is the empirical formula of this particular oxide?
18. The combining weight of elemental chromium in one of its common oxides was found to be
17.332g. Its specific heat is 0.510J/gK. What is the atomic weight of chromium?
19. The atomic weight of hydrogen is known to be exactly 1.008 atomic mass units. Nitrogen (N2) can
be combined with hydrogen (H2) to produce ammonia (NH3). Data (obtained experimentally)
shows the weight-percentage of nitrogen in ammonia is 82.25%. Calculate the atomic weight of
nitrogen.
20. When a carefully weighed sample of an unknown metal M reacted completely with oxygen, it
was found that the resulting oxide was exactly 10.30% oxygen by weight. If the empirical formula
is known to be M2O3, what are the atomic weight and specific heat of the metal?
21. Calculate the weight of one chlorine atom and of one hydrogen atom. What is the ratio of the
weight of 1000 chlorine atoms to the weight of 1000 hydrogen atoms? Compare this with the
ratio of their respective atomic weights.
22. A mole of sodium atoms weighs 23.0g and a mole of chlorine atoms weighs 35.5g. What weight of
sodium atoms must you buy in order to get the same number of atoms as there are in a mole of
chlorine atoms?
23. A flask contains 28g each of carbon monoxide (CO), ethylene (C2H4) and nitrogen (N2). How many
molecules are present in the flask?
a) Lithium in LiOH
b) Carbon in SrCO3
c) Oxygen in Mn2O7
d) Water in CuSO4.5H2O
e) Sulphur in H2SO4
25. Alkali metal oxides and hydroxides have been successfully used to scavenge carbon dioxide
(CO2) from the breathing space in closed human environments such as submersible vehicles and
spacecraft. The reactions taking place can be represented by:
THE STORY OF ATOMIC THEORY OF MATTER
Calculate the theoretical removal of CO2 in grams of CO2 per kilogram of the reagent (NaOH or
Na2O).
Write similar equations for Li2O and LiOH, and determine whether on a weight basis LiOH is
more effective than NaOH or not. Is LiOH more effective than Li2O?
66
26. Aluminium metal can be prepared by reducing aluminium chloride with sodium metal,
producing sodium chloride at the same time. When a charge of 34.5g of sodium (atomic weight
= 23.0g/mol) is used, 13.5g of aluminium (atomic weight = 27.0g/mol) is produced from 66.8g of
aluminium chloride. Determine the simplest chemical equation that describes the process
and agrees with these data.
27. A certain metallic element has an atomic weight of 24g/mol. A certain non-metallic element
has an atomic weight of 80g/mol. When this metal and non-metal are combined chemically,
they do so in a ratio of 1 atom to 2 atoms respectively.
a) Determine the number of grams of the metal that would react with 5.00g of the non-metal.
b) How many grams of the product will be formed?
28. A flask contains 30g of nitrogen oxide (NO) and 30g of diimide (N2H2). How many molecules are
present in the flask?
67
Part-3
Students need to get a feel for chemistry by performing experiments. But they generally don't get
enough opportunities to do experiments. It is important that they get some basic idea about the
chemical properties of matter to lay the ground for understanding an abstract concept like atomic
theory. For this they need to perform more qualitative as well as quantitative experiments.
This series of simple experiments gives them the opportunity to observe and study chemical changes
and interactions between various substances. The first set of experiments helps them to assess the
chemical nature of various substances and gain a qualitative understanding of why one substance is
different from another. Next is a set of quantitative experiments that focuses on the concept of chemical
equivalence, pointing out that equivalence cannot be deduced from the amounts (weights) of
substances used in the reactions. Finally, there are a couple of experiments that will help students get an
inkling of just how small the particles we are talking about are.
Experiment 1:
Chemical properties – acids and bases
You can also experiment with various flower colours to see if they can act as
acid-base indicators. For this, mash the flower in water till the colour mixes
fully or rub the petals on a piece of paper till the colour appears on the paper.
Use the coloured paper the same way as litmus paper.
One thing needs to be kept in mind: most of the tests in this activity need an
aqueous medium.
Experiment 2:
Titration
Experiment 3:
SOME SIMPLE EXPERIMENT
Record the boiling point of water, noting the temperature of the water as well
as the water vapour. The temperature of the vapour can be read by holding
the thermometer a little above the surface of the boiling liquid. What is
important is that you should observe the change in temperature as the water
is heated and also note that the temperature remains constant once the
water starts boiling properly. 69
There is a misconception prevalent among adults as well as children - that the
change of state occurs only at the boiling point. How do you address this? Do
several experiments to get a hang of it. Also correlate your observations with
your everyday experiences. Such as, answer the question: When water
condenses on the outer surface
of a glass containing ice, where
does this water come from?
Relate your answer to where
water goes when clothes dry, and
c o m p a r e a n d c o n t ra s t t h i s
process with the process of
boiling. Is the amount of water
that condenses different on days
when clothes take a long time to
dry compared to days when
clothes dry quickly? In which case
would the amount of water be Measuring the temperature of
boiling water and the steam produced
less or more and why? Draw
conclusions from your
observations.
Experiment 4:
More on evaporation and condensation
Make a dilute solution of copper sulphate in water and pour equal amounts of
this solution in five test tubes. Put one of the following things in each test
tube: an iron nail, a piece of aluminium foil, a match stick (without the masala),
a plastic spoon, or a piece of paper.
Experiment 6:
Rusting of iron
71
Continue your observations as long as the water level
keeps changing. Does it remain constant after some
time? (This might take a few days depending on the
ambient temperature.)
You will need a 30mL injection bottle, a 3-4cm long empty ball pen refill, and a
30cm long piece of cycle valve-tube. Insert the refill through the rubber cap of
the injection bottle. Attach the valve-tube to the exposed end of the refill.
This set-up is air-tight and the reactions can be carried out in the injection
bottle.
Experiment 7:
Preparing oxygen and testing its properties
THE STORY OF ATOMIC THEORY OF MATTER
Carbon dioxide is heavier than air. You can demonstrate this by pouring the
gas from one test tube into another and then testing for the presence of
carbon dioxide in the second test tube.
We generally think that carbon dioxide does not support combustion, so an
interesting experiment is to see its reaction with burning magnesium. For
this, collect carbon dioxide in a gas jar, ignite a strip of magnesium ribbon and
drop the ribbon in the gas jar. You can observe that the magnesium ribbon,
which burns with a bright white light in the atmosphere, now burns with a
yellow flame inside the gas jar. After the reaction is over, you will see black
deposits inside the gas jar.
Magnesium forms a strong bond with oxygen, hence it can displace carbon
from carbon dioxide. The carbon is deposited on the sides of the gas jar as
soot while the magnesium oxide formed is seen as a white powder (which can
be tested with litmus paper after dissolving in water).
Experiment 9:
Preparing hydrogen and testing its properties
Hydrogen can be prepared by reacting zinc with hydrochloric acid, using the
same apparatus as described above. Put some zinc granules (two or three
small pieces) in the injection bottle and add 4-5mL of hydrochloric acid.
Collect the gas produced in the manner described above. Test it with litmus
and check its combustibility by bringing a lighted candle near the mouth of
the test tube containing the gas, whereupon it burns with a characteristic
popping sound.
The fact that hydrogen burns but does not support combustion can be
SOME SIMPLE EXPERIMENT
Experiment 10:
Factors affecting the production of carbon dioxide
The reaction of marble chips with hydrochloric acid can be used to study the
factors that affect the rate of this reaction. You can try out different
variations, such as the size of the marble chips, the concentration of the acid
and the temperature at which the reaction is taking place.
To study the effect of temperature, put equal amounts of marble chips and
acid in both bottles but keep the bottles at different temperatures: you can
put one in hot water and the other in cold water.
Experiment 11:
THE STORY OF ATOMIC THEORY OF MATTER
You can compare the effect of the size of zinc pieces on the rate of reaction by
putting zinc powder in one bottle and zinc granules in the other. The reaction
will be faster with zinc powder.
You can study the reaction between a reactive metal and an acid
(hydrochloric acid) to demonstrate that chemical equivalence is
not the same as mass equivalence. Take equal weights (say 0.1g)
of two metals (magnesium and zinc) and react them separately
with the same amount of hydrochloric acid. Collect the gas
produced in both cases and measure its volume. To make sure
that the volume of the gas produced is measured accurately, the
Set-up for preparing and collecting a gas.
acid can be injected into the reaction bottle (injection bottle) as The acid can be added to the injection
shown in the diagram. bottle using a syringe so that none of the
gas produced escapes
Experiment 13:
Chemical vs mass equivalence - titration
Take equal amounts of tartaric acid and sodium hydroxide. You can do this by
putting two pellets of sodium hydroxide in one pan of a balance and
then slowly adding tartaric acid to the other pan until the two
balance. Now dissolve these substances in equal amounts of water
(distilled if available). You can check whether equal amounts of
these solutions would neutralise each other by actually doing the
experiment.
Experiment 14:
Chemical vs mass equivalence - Colligative properties and equivalence
SOME SIMPLE EXPERIMENTS
Next, take equal quantities of water (100mL) in two beakers. Add some salt to
one beaker and an equal amount of sugar to the other. Heat both the 75
solutions and note the change in boiling point in both
cases. The extent to which the boiling point is raised
will differ. This is another indication that mass
equivalence (roughly) is not the same as chemical
equivalence. The elevation of the boiling point
depends on the number of particles of the solute in
the given volume of the solution.
Experiment 15:
Particle size - dissolving potassium permanganate crystals
Experiment 16:
Molecule size – oil film on water
You can estimate the size of a molecule in this activity by using the
observation that oil spreads on water, the assumption being that the oil film
makes a single molecule thick layer on the water surface. In order to find the
thickness of this layer, all we need to know is the initial volume of the oil drop
and the area of the film formed. The volume/surface area ratio gives us
THE STORY OF ATOMIC THEORY OF MATTER
Since we need a very small volume of oil, dissolve 0.2mL of oil (any
cooking oil or oleic acid) in 10mL of an organic solvent (say, hexane).
You can easily calculate the volume of one drop of this solution. Pour
some water in a large flat plate and allow it to become still. Sprinkle
talcum powder on the surface. This helps to clearly demarcate the area
of the oil film and this should be done just before you add the oil drop. Finding out the thickness of
Otherwise the water wets the talcum powder and stops it from spreading on an oil film to estimate the
size of a molecule
the surface. Now use a syringe to gently add one drop of the oil solution to the
76 water. The hexane (or organic solvent used) evaporates as soon as the film
spreads out, leaving behind a circular oil film whose diameter can be
measured. You can then calculate the approximate size of an oil molecule.
Acknowledgements:
Eklavya has tried out two different approaches to the particulate nature of matter, the
first one was an experimental route which was developed by Dr. Pramod Upadhyay and
Dr. Shashi Saxena and this was tried out while training HSTP resource teachers.
This module takes a historical approach, but nonetheless owes its inspiration to the
earlier attempt. This approach has been tried out in three trainings and has further been
tested with school and college students, and the feedback and comments from all these
trials have gone into making this module more coherent and meaningful.
In addition, many members of our resource group have gone through the module at
different stages and given various helpful suggestions which have been incorporated
into it.
These include: Vijaya S. Varma, Amitabha Mukherjee, Pramod K. Srivastava, N.
Panchapakesan, the late M. M. Kapoor, Patrick Dasgupta, Anand Fadnis, G. S. Holkar,
Kamal Mahendroo, Arvind Sardana and Aamod Karkhanis, Bhas Bapat, Ankush Gupta,
Sumit Tripathi and Laltu (Harjinder Singh). Also, present and past members of the
Science team – Himanshu Srivastava, Shubhra Mishra and Reena Purohit – put in
considerable work into getting diverse details right. Sonali Biswas took a keen interest
in the subject matter in order to get the illustrations just so.
Rex D’Rozario worked on the text to fine-tune it and make it more readable. Sanat
Mohanty went through the final text to check the content for the technical details. The
final editing and layout was done by the Eklavya publications team. We shall make the
usual caveat that any mistakes can be attributed to us.
Sushil Joshi
Uma Sudhir
ACKNOWLEDGEMENTS
77
Index
78
We dedicate this module to Rex
Rex did not just edit this module, he also oversaw most of Eklavya's publications
while they took their baby steps. Likewise, he also encouraged all new people
who joined to give their best the way he always did. He set an almost impossible
standard of perfection, and we shall remember him as we try to live up to it -
with affection, with respect, and with a never-diminishing sense of loss.