8.

4 Water
8.4.1 Water is distributed on Earth as a solid, liquid and gas

 Define the terms solute, solvent and solution

Solution – a homogeneous mixture of two or more substances
Solute – a substance that is dissolved in another substance or the component of a solution
present in a lower amount.
Solvent – a substance which can dissolve another substance or the component of a solution
present in a greater amount.
 Identify the importance of water as a solvent
Water is essential as a reactant and a solvent in the cycling of C, O, N, P and S in nature. It
also allows biological processes to occur in aqueous solutions and serves as a transport
system for nutrients and waste products in living organisms.
 Compare the state, percentage and distribution of water in the biosphere, lithosphere,
hydrosphere and atmosphere

Biosphere Lithosphere Hydrosphere Atmosphere

Percentage of 70% Variable 96-100% 0-5%
water
State of water Liquid Liquid Liquid gas
Water of Solid ice
crystallisation
Solid ice

 Outline the significance of the different states of water on Earth in terms of water as:
• A constituent of cells and its role as both a solvent and a raw material in
metabolism
Water is the predominant constituent of cells, functioning as:
 A solvent for biochemical reactions that sustain life
 Photosynthesis : 6CO2 (g) + 6H2O (l) sunlight
→
C6H12O6 (aq) + 6O2 (g)
 Respiration : C6H12O6 (aq) + 6O2 (g) → 6CO2 (g) + 6H2O (l)
 A raw material for metabolism (e.g. in plants)
 A transport medium for nutrients and wastes
 A thermal buffer that resists large temperature fluctuations
• A habitat in which temperature extremes are less than nearby terrestrial habitats
 Water serves as a natural habitat for many organisms
 Major advantage: temperatures vary much less in water than on land
 Therefore marine animals are protected from experiencing temperature
extremes
• An agent of weathering of rocks
 As water freezes and thaws repeatedly, the stress due to expansion and
contraction can cause rocks to fragment
 Rain, rivers and glaciers erode loose material, carving through mountains
and shaping the landscape into its present form.
  Weathering is the physical and chemical breakdown and decay of rocks.
 Erosion refers to the processes by which rock fragments are transported by
rivers, oceans and wind.
• Both as liquid and solid a natural resource for humans and other organisms
 Water is critical to the survival of humans and other organisms.
 Humans use water for drinking, bathing, washing clothes, cleaning,
agriculture and in industrial processes
 Water also serves as a source of entertainment and enjoyment for many
people (eg. fishing, swimming, sailing and other water sports)

8.4.2 The wide distribution and importance of water on Earth is a consequence of its molecular
structure and hydrogen bonding

 Construct Lewis electron dot structures of water, ammonia and hydrogen sulfide to
identify the distribution of electrons

Water Ammonia Hydrogen sulfide

S

Total number Arrangement Bonding Lone pairs Shape of Examples

of electron pairs molecule
pairs
2 Linear 2 0 Linear BeCl2, HCN,
BeF2
3 Trigonal planar 3 0 Trigonal BCl2, CH2O
4 Tetrahedral 4 0 Tetrahedral CH4, SiF4
3 1 Pyramidal NH3, PCl3
2 2 Bent H2O, H2S
5 Trigonal 5 0 Trigonal PCl3
bipyramidal bipyramidal
6 octahedral 6 0 Octahedral SF6
 Compare the molecular structure of water, ammonia and hydrogen sulfide, the differences
in their molecular shapes and in their melting and boiling points

 Describe hydrogen bonding between molecules

• Hydrogen bonding is a special type of dipole-dipole bond only occurring between
H with N, O, or F of a neighbouring molecule
• Hydrogen bonding is the strongest out of the intermolecular forces
• As the electron from the hydrogen is drawn towards and N, O or F atom within the
molecule, the positive hydrogen nucleus is left. This enables the slightly positive
hydrogen to be attracted to the slightly negative N, O or F of a neighbouring
molecule.
• The size of N, O and F is small enough (i.e. has only 2 electron shells) for the bare
positive hydrogen nucleus to interact with the lone electron pairs of a neighbouring
molecule contain N, O or F.
• Hydrogen bond is the electrostatic attraction between a hydrogen proton and lone
electron pairs.
 Identify the water molecule as a polar molecule
• In H2O, there are 2 bonded electron pairs and 2 lone electron pairs
• There are dipole bonds between the oxygen and hydrogen atoms
• This is due to oxygen being more electronegative than hydrogen
• Hence the electrons being shared between the oxygen and hydrogens will be pulled
more towards the nucleus of oxygen.
• This makes the oxygen slightly negative (δ -) while the hydrogens are slightly positive
(δ +)
• Hence the water molecule is a polar molecule with a net dipole.

 Describe the attractive forces between polar molecules as dipole-dipole forces

 • Covalent bonds in which the electrons are unequally shared are called polar covalent
bonds and is due to the differing electronegativities of different atoms.
• A dipole is a polar molecule which have a net dipole (imbalance)
• As polar molecules have a slightly positive and negative end, they are able to line up
so that the positive end of one molecule attracts the negative end of another
molecule.
• This electrostatic attraction of polar molecules is called dipole-dipole forces.
 Explain the following properties of water in terms of its intermolecular forces:
• Surface tension
 In water, intermolecular forces exert different effects on a molecule at the
surface compared to one in the interior.
 Interior molecules are attracted equally by other molecules on all sides
(isotropic).
 Surface molecules are only attracted to others below and to the sides
(anisotropic).
 As a result, surface molecules experience a net attraction downward
 This pulls the molecules inward and closer together, making the liquid
surface behave like an elastic skin under tension (with minimised surface
area).
 To increase surface area, molecules must move to the surface by breaking
some interior attraction and this requires energy.
 Surface tension is a measure of energy needed to increase the surface area
of a liquid by a unit amount (units=j/m 2).
 Water has high surface tension due to strong intermolecular forces
(hydrogen bonds).
 In general, the stronger the intermolecular forces, the greater the liquid’s
surface tension.
• Viscosity
 It is the resistance to flow
 A liquid’s viscosity depends on the size/shape of the molecules and strength
of its intermolecular forces.
 As water has small and compact molecules that flow readily over each other,
it has low viscosity as compared to highly viscous liquids like motor oil.
 The stronger the intermolecular forces between molecules the more
resistance there is to flow.
 Water with its strong hydrogen bonding has a much higher resistance to
flow than its small molecular size might suggest.
• Boiling and melting points
Due to the strong hydrogen bonds in between water molecules, greater energy
input is required to break these bonds, hence accounting for its relatively higher
melting and boiling points as compared to other liquids and solvents.

8.4.3 Water is an important solvent

 Explain changes, if any, to particles and account for those changes when the following
types of chemicals interact with water:
o A soluble ionic compound such as sodium chloride
If the attractive forces between water and the ions are stronger than the attractive
forces between the positive and negative ions, then the ionic solid dissolves in
water.
o A soluble molecular compound such as sucrose
Most molecular substances such as hexane, kerosene, paraffin wax and chloroform
are insoluble in water. However, some molecular substances like sucrose are
soluble. When soluble molecular compounds are dissolved in water, the crystals of
the solid break up and disperse throughout the solvent, breaking right down to the
molecular level. So a solution of sucrose in water consists of individual sucrose
molecules dispersed throughout the solvent.
A molecular substance only dissolves in water if water can form stronger
attachments to the molecules than the intermolecular forces in the molecular
substance. Generally the only molecular substances which dissolve in water are ones
that have very polar molecules or ones that can form hydrogen bonds with water.
o A soluble or partially soluble molecular element or compound such as iodine,
oxygen or hydrogen chloride
Some non-polar molecular substance such as oxygen and nitrogen gases and iodine
are slightly soluble in water. The solvent-solute interactions are weak dispersion
forces. As these interactions are weak, the solubilities of such substances are quite
low.
o A covalent network structure substance such as silicon dioxide
Covalent lattices are insoluble in water as the hydrogen bonds in water are not
strong enough to break strong covalent bonds between the atoms in these covalent
lattices.
o A substance with large molecules, such as cellulose or polyethylene
Some molecules of substances are so large and held to one another in such orderly
fashions by hydrogen bonds that water is unable to separate them from one
another. (e.g. cellulose, polyethylene)
However, there are many proteins (including enzymes) and some carbohydrates
(amylase, glycogen) are soluble in water despite having large molecular weights.
These molecules are very complex structures and do not pack together neatly into
crystals, so water is able to separate molecules and form solutions.
 Analyse the relationship between the solubility of substances in water and the polar
nature of the water molecule
The most important factors for causing solubility in water are the highly polar nature of the
water molecule and the ability of water to form hydrogen bonds with other molecules. Polar
substances dissolve in polar solvents and not in non-polar solvents while non-polar
substances dissolve in non-polar solvents and don’t dissolve polar ones. As water is a polar
solvent, it will dissolve substances which have polar molecules.
8.4.4 The concentration of salts in water will vary according to their solubility, and precipitation
can occur when the ions of an insoluble salt are in solution together

 Identify some combinations of solutions which will produce precipitates, using solubility
data
Ba(NO3)2 (aq)+ ZnSO4 (aq) → ZN(NO3)2 (aq) +BaSO4 (S)
Ba2+ (aq) + SO4 2- (aq) → BaSO4 (s) (BaSO4 (s) is a white precipitate)
AgNO3 (aq) + NaCl (aq) → NaNO3 (aq) + AgCl (s)
Ag+ (aq) + Cl- (aq) → AgCl (S) (AgCl (S) is a white precipitate)
CuSO4 (aq) + 2NaOH (aq) → Na2SO4 (aq) + Cu(OH)2 (s)
Cu2+ (aq) + (OH)- (aq) → Cu(OH)2 (s) (Cu(OH)2 (s) is a blue precipitate)
Pb(NO3)2 (aq) + 2KI (aq) → 2KNO3 (aq) + PbI2 (s)
Pb2+ (aq) + 2I- (aq) → PBI2 (s) (PBI2 (s) is a yellow precipitate)
Na2So4 (aq) + Ba(NO3)2 (aq) → 2NaNO3 (aq) + BaSO4 (s)
Ba2+ (aq) + SO42- (aq) ) → BaSO4 (s) (BaSO4 (s) is a white precipitate)
 Describe a model that traces the movement of ions when solution and precipitation occur

Before the two solutions are mixed, they remain as free moving electrons where the
electrostatic attraction between the water molecules and the ions is greater than the
electrostatic attraction between the ions. When mixed, a precipitate may form. A
precipitation reaction only occurs when the force of attraction between two ions is stronger
than the electrostatic attraction between the water molecules and the ions. In the above
diagram, the electrostatic attraction between Ag + (aq) and Cl- (aq) ions is stronger than the
electrostatic pull of the polar water molecules. Hence the precipitate AgCl is formed.
 Identify the dynamic nature of ion movement in a saturated dissolution

Saturated solution: solution in which no more of a particular solute will dissolve in a

particular solvent
In a saturated solution, a dynamic equilibrium exists between dissolution and precipitation.
As 2 ions break off the ionic crystal and dissolve in the solution, another 2 ions will
precipitate out. These processes occur at the same rate such that there is no net change in
concentration.
n
 Describe the molarity of a solution as the moles of solute per litre of solution using c=
V
Molarity is one type of measurement for concentration. The molarity of a solution is equal to
the number of moles of solute per litre of solution.

 Explain why different measurements of concentration are important

A variety of ways of expressing concentration is used because each method has advantages
for particular situations. In commerce and industry and in shopping where the main concern
is with how much solute is present, then mass per unit volume is very convenient. In
environmental contexts concentrations are usually very low. Masses per unit volume or
percent compositions generally lead to very small numbers so parts per million (ppm) gives
more manageable numbers.
8.4.5 Water has a higher heat capacity than many other liquids

 Explain what is meant by the specific heat capacity of a substance

The specific heat capacity of a substance is the amount of heat required to raise the
temperature of 1 gram of the substance by 1°C (or 1K).
 Compare the specific heat capacity of water with a range of other solvents

Liquid Specific heat capacity (J/°C/g)

Water 4.18
Ethanol 2.44
Ethylene 2.39
glycol
Glycerol 2.38
Acetone 2.17
Chloroform 0.96
Hexane 2.26
Mercury 0.14

 Explain and use the equation ∆ H =−mC ∆ T

This equation is used to measure heat energy changes.
ΔH: change in heat energy, in joules (J)
m: mass of substance, in grams (g)
C: specific heat capacity, in J/°C/g
ΔT: temperature change, in digress Celsius (°C)
o If the temperature goes up (+ΔT), the energy change is considered negative (-ΔH)
o If the temperature drops (-ΔT), the energy change is considered positive (+ΔH)

 Explain how water’s ability to absorb heat is used to measure energy changes in chemical
reactions
Calorimeter: equipment used to measure heat energy.
Since many chemical processes occur in water and due to water’s high
specific heat capacity, it is often used in calorimeters as the ‘working fluid’
or the medium used to absorb the heat energy.
Limitations of calorimeter:
o It is assumed that the calorimeter itself does not absorb a
significant amount of heat energy of the reaction
 o It is assumed that there is no heat lost or gained between the calorimeter and its
surroundings.
o It is assumed that the specific heat capacity of the solution reacting in the
calorimeter is the same as water (i.e. 4.18 J/°C/g)
Heat of solution: energy change that occurs when 1 mole of solute dissolves in water.
ΔHsol is negative if energy is released. (exothermic: the calorimeter temperature rises)
e.g. soluble hydroxides like NaOH, KOH
ΔHsol is positive if energy is absorbed. (endothermic: the calorimeter temperature falls)
e.g. ammonium nitrate NH4NO3 (aq), ammonium chloride NH4Cl (aq)
 Describe dissolutions which release heat as exothermic and give examples
Exothermic reactions are reactions that produce and release energy
When chemicals lose energy, the temperature in the calorimeter rises because the energy
release heats up the water in the calorimeter. When the temperature rises, the energy
quantity is considered negative.
e.g. freezing water, precipitation
 Describe dissolutions which absorb heat as endothermic and give examples
Endothermic reactions are reactions that absorb energy where energy must be supplied in
order to make the reaction occur.
When chemicals absorb energy, the temperature in the calorimeter drops because the
energy absorbed from its surroundings cools down the water in the calorimeter. When the
temperature drops, the energy quantity is considered positive.
e.g. melting ice cubes, evaporating liquid water
 Explain why water’s ability to absorb heat is important to aquatic organisms and to life on
earth generally
Water plays a significant role to weather, climate and life on Earth due to its high specific
heat capacity. Water is able to absorb a large amount of energy from the Sun without much
temperature change, keeping water habitats at a very stable temperature. This means that
aquatic organisms do not require complex temperature control mechanisms because their
habitat remains quite stable. More importantly, the oceans absorb and transport huge
quantities of heat from the tropics towards the poles via ocean currents.
 Explain what is meant by thermal pollution and discuss the implications for life if a body of
water is affected by thermal pollution
o Many aquatic organisms rely on a fairly constant water temperature to aid proper
metabolism, survival and reproduction.
o Thermal pollution is the discharge of large quantities of hot water into a river or lake
sufficient to cause a significant increase in the water’s temperature (2-5°C).
o River or lake water is used in the industry for cooling and when generating
electricity.
o When the cooling water is discharged back into a body of water it has absorbed
substantial amounts of heat energy.
o Thermal pollution has adverse consequences for aquatic life.
o In particular, the solubility of oxygen decreases as water temperature increases. Less
dissolved oxygen causes stress to aquatic organisms.
o Other detrimental effects of rising water temperature:
 Increased metabolic rates  further demand of oxygen
 Fish eggs do to develop properly or hatch with a sudden change in
temperature
 False temperature cues given to aquatic life