Physical Science
Physical Science
Physical Science
may seem that the law of conservation of mass does not apply in 39.7% sodium sodium : chlorine
Sodium chloride
60.3% chlorine 1:1
some cases involving chemical changes. For example, the rusted nail
has a greater mass than the original nail. However, if the rusting is
carried out, including the gas consumed when iron nail rusts, the Dalton's Atomic Theory
mass before and after rusting will be the same.
In 1803, John Dalton (1766-1844)-an English scientist and
schoolteacher- studied the two laws of chemical combination and
other experimental observations and concluded that the behavior of
Law of Definite Proportion
matter could be explained only if matter was composed of atoms or
The second principle was the law of definite proportion, the atomic theory was used. This theory, now known as Dalton's
formulated by another French chemist, Joseph Proust (1754-1826) in atomic theory, marked the beginning of the modern era of chemistry.
1799. The law showed that a given compound always contained the
Dalton's atomic theory may be summarized in three
same elements in the same proportion by mass.
postulates:
The law of definite proportion may be illustrated by
1. All matter is composed of exceedingly small indivisible
examining water. Water is always 88.9% oxygen and 11.1%
particles called atoms.
hydrogen by mass no matter how and where such compound is
prepared or obtained or how much water there is. A drop of water An element is composed of tiny particles called atoms. All
and a bucket of water contain the same elements in the same atoms of a given element show the same chemical properties. Atoms
proportion by mass. of different elements show different properties.
With the law of definite proportion, we can say that if the 2. No two elements or atoms of different kinds have
ratios of the masses of different elements in a given compound are exactly the same properties.
fixed, then the ratios of the atoms of these elements in the compound
Atoms of different elements have different masses and The second postulate supports the law of conservation of
different chemical properties. Atoms are not created or destroyed in mass. If you consider the reaction between zinc and hydrochloric
chemical reactions. acid to form zinc chloride and hydrogen gas, the chemical equation
is as follows:
In an ordinary chemical reaction, atoms transfer from one
substance to another, but no atom of any element just disappears, or Zn(s) + 2HCI ZnCI2(aq) + H2(g)
is changed in an atom of another element.
The atoms simply rearranged themselves to form a new
3. Atoms combine in simple, fixed whole number ratios to form combination of atoms but the zinc (Zn), chlorine (CI), and hydrogen
compounds. Compounds are formed when atoms of two or more (H) atoms before the reaction are the same zinc, chlorine, and
elements combine In a given compound, the relative numbers of hydrogen atoms after the reaction.
atoms of each kind are definite and constant. In general, these
relative numbers can be expressed as integers or simple fractions. How an atom looked like was not the subject of study of
John Dalton and his contemporaries at the time. They just thought
The third postulate supports the law of definite proportion. It that atoms were hard and round like ball bearings. But if no two
also gives rise to the law of multiple proportion which states that if atoms have exactly the same properties according to Dalton's second
two elements can combine to form more than one compound, the postulate, what makes them different from one another? They are all
masses of one element that combined with a fixed mass of the other atoms after all.
element are in ratios of small whole numbers. Take, for example,
carbon and oxygen: The two elements can combine to form two Taking a second look at Dalton's theory, we realize that
stable compounds, carbon monoxide (CO) and carbon dioxide (CO). many of the things we know and understand at the moment can be
In CO, the carbon to oxygen ratio is 1:1 while in CO₂, the ratio is explained in terms of the atomic theory using Dalton's theory. One of
1:2. Another example is shown in Table 1.2. It shows the them is the difference between elements and compounds. Elements
experimental data taken for the reaction of sulfur and oxygen which are composed of only one kind of atom. A sample of the element,
may form two different substances. helium, has only helium atoms in it.