Week 11, Lesson 2
Chapter 10 - Water: Essential to
Life
A Molecule Essential for Life
• Water is the most abundant liquid on Earth.
• It covers 70% of our planet.
• It moderates our weather, shapes our land
and is essential to the existence of life.
• The Earth’s position in the solar system means
that it is not close enough for the water to
boil, nor is it too far away for it to freeze.
The World’s Water
The Water Cycle
• Water on Earth exists in solid, liquid and gaseous states and
can be readily changed from one state into another.
• The water cycle could be said to begin with evaporation
from the oceans, solar energy being the main energy
source.
• Evaporation also occurs from lakes and rivers.
• Human activities such as the combustion of fossil fuels,
which produces steam, also contribute to the water cycle.
• Water vapour in the air is transported around the globe
until it condenses to form clouds.
• It precipitates as rain water, or occasionally, as ice crystals
in hail or snow and fall to the ground.
The Water Cycle cont…
Water and Living Things
• Water is involved in the reactions of life:
photosynthesis and respiration.
• The overall process of photosynthesis can be
represented by:
sunlight
6H2O(l) + 6CO2(g) 
chlorphyll
C6H12O6(s) + 6O2(g)
glucose
Water and Living Things cont…
• In this essential reaction, water is absorbed into the
cells of plants of algae and in the presence of
chlorophyll and sunlight it is used to make glucose.
• Cells of all oxygen-using life forms use glucose as their
energy source.
• The reaction between glucose and oxygen produce
carbon dioxide and water.
• In this way, water that was removed during
photosynthesis is returned to the cycle.
• The process of respiration is:
C6H12O6(s) + 6O2(g)  6H2O(l) + 6CO2(g)
Functions of Water
• Provides a system to transport nutrients and soluble
wastes: water fulfils a similar transport role in animals and
plants. The ability of water to act as a solvent and dissolve a
wide range of materials is central to its functions in
transporting materials in living things.
• Provides a system to transfer heat: Water has the capacity
to store a large amount of heat energy. Water transfers
heat energy from the cells where it is produced in
respiration, to the body’s surface where it is lost to its
surroundings.
• Provides a system to cool the body: skin is always moist.
When water evaporates from the skin, it absorbs a
relatively large amount of heat energy from the body and
so cools it efficiently. Plants lose water from the surface of
their leaves in a process known as transpiration.
Water and Climate
• Water vapour, from the evaporation of waterways, precipitate to
Earth as rain or snow.
• Water can penetrate the surface and become groundwater, where it
dissolves minerals from rocks and transports them to the oceans.
• The balance of water on the surface runs off and empties back into
rivers, lakes and oceans, where the cycle is completed.
• The evaporation stage produces water free of dissolved substances.
• Water is also a major means of distributing heat around the planet.
• Warm water-laden tropical winds blow towards the poles, where
they warm the rest of the Earth.
• The ocean currents also transfer heat and disruptions in typical
currents, such as the El Nino effect, can have disastrous effects,
causing drought or major floods.
Week 11, Lesson 3
The Properties of Water
• Water is described as a covalent molecular
compound, with a molecular formula of H2O.
• The oxygen-hydrogen bonds are polar, with the
oxygen atom having the larger share of the
bonding electrons, due to its higher
electronegativity.
• The forces between water molecules are
hydrogen bonds, which are relatively strong on
comparison to other types of intermolecular
bonds.
High Melting and Boiling Temperatures
• Water is the only substance which is
commonly found in three states.
• The graph below gives the melting and boiling
temperatures of some other molecular
substances that hand the same number of
electrons.
High Melting and Boiling
Temperatures cont…
• Two observations can be made from the data
in this figure:
– Water exists as a liquid over a temperature range
commonly found on the Earth.
– The melting and boiling temperatures of water are
significantly higher than those of other molecular
substances of similar size.
Using the Bonding and Structure to
explain this property…
• In an ice crystal, the water molecules are arranged in a
structured and regular manner, with each molecule forming
hydrogen bonds directly to four other molecules.
• For ice to melt, energy must be provided to increase kinetic
energy of the water molecules to the point where they break
free of the crystal lattice.
• In the melting process, some of the hydrogen bonds break as
the ice structure collapses to produce liquid water, which is
roughly 10% denser than ice.
• When water is boiled, the water molecules separate from
each other and all hydrogen bonds are broken.
• Water requires a significant amount of energy to overcome
these bonds, thus the high melting and boiling temperatures.
High Latent Heat Values
• Latent heat measure the energy needed to
change the state of a substance at its melting or
boiling temperature.
• The latent heat of fusion of water is the amount
of energy needed to change a fixed amount of
water from solid to liquid at 0 degrees.
• The latent heat of evaporation of water is the
amount of energy needed to change a fixed
amount of water from liquid to gas at 100
degrees.
Latent Heat Values cont…
• These values of water are considerably higher than those of most other
common molecular substances.
• This means much more heat is needed to melt or boil water than for
equivalent amounts of other substances.
• The high latent heat values of water can also be explained in terms of the
hydrogen bonds between molecules.
• A relatively large amount of energy is needed to disrupt the lattice when
some hydrogen bonds are broken.
• An even larger amount is needed to break all of the hydrogen bonds and
separate the water molecules to form a gas.
• The high latent heat of vaporisation makes water a good coolant.
• When you perspire, the water evaporates by absorbing heat energy from
your skin.
• This heat energy, 44kJ/mol of water, is transferred from your body to the
evaporating water and you feel cooler.
High Heat Capacity
• Several uses of water depend on its capacity to store
large amounts of heat energy.
• The specific heat capacity of a substance measures the
amount of heat energy needed to increases the
temperature of a certain amount (usually 1 gram) of that
substance by 1 degree.
• Water has a specific heat capacity of 4.2G/g/°C.
• This means that 4.2 joules of heat energy is needed to
increase the temperature of 1 gram of water by 1°C.
• The higher the specific heat capacity of a substance, the
more effectively it will store heat energy.
Finding Heat Energy
• The equation below can be used to answer
questions on heat energy:
Heat Energy = specific heat capcaity (J/g/°C) x
mass(g) x temperature change (°C)
OR
Heat Energy = SHC x mass x ΔT
Heat energy is measure in Joules and is usually
converted into kJ by dividing by 1000.
Expansion on Freezing
• As liquid water is cooled, the water molecules move
more slowly.
• On approaching the freezing temperature of water, the
molecules take up an arrangement typical of an ice
crystal.
• Each water molecule forms bonds with four other
molecules.
• This is a very open arrangement – the water molecules in
ice are more widely spaced than in liquid water.
• When ice melts, the organised structures are lost, and
the molecules become free to change places, they move
closer together and the density increases.
Density
Density =
mass
volume
Week 11, Lesson 4
Water as a Solvent
• Almost all biological and industrial processes
occur in an aqueous environment, in the
presence of water.
• When substances are dissolved in water, the
particles move freely and so interactions
between reactants are very effective.
• The polar nature of the water molecule
enables water to dissolve a large number of
substances.
Solutions
• Solutions have the following important
characteristics:
– The are homogenous, that is the solute and
solvent cannot be distinguished from one another.
– The dissolved particles are too small to see.
– The proportion of dissolved solute varies from one
solution to another.
Water
• More than half of the naturally occurring
elements are found dissolved in the oceans of
the world.
• Seawater also contains smaller quantities of at
least 40 other elements, including silver and
gold.
• It is estimated that the oceans of the world
contain 10 billion tonnes of dissolved gold.
Why do substances dissolve?
• When one substance dissolves in another, the
following processes occur:
– The particles of the solute are separated from one
another.
– The particles of the solvent are separated from
one another.
– The solute and solvent particles attract each other.
Why do substances dissolve?
Cont…
• A solute will dissolve if the attraction between the
solute and solvent particles is strong enough to
compete with the solute-solute and solvent-solvent
forces of attraction.
• Substances that dissolve readily in water have ionic
or polar covalent bonding. They fall into one of the
following three groups:
– Polar covalent compounds that can form hydrogen bonds
with water.
– Polar covalent molecular compounds that ionise.
– Ionic Compounds
Polar Covalent Compounds that
can form Hydrogen Bonds
• Some covalent molecular substances such as
sugar and ethanol, dissolve easily in water.
• These molecules are polar in character, usually
through one or more –OH groups.
• Ethanol contains a polar O-H bond, so there
are hydrogen bonds between the molecules in
pure ethanol.
Forming Hydrogen Bonds cont…
• When ethanol is added to the water, hydrogen bonds
form between the ethanol and water molecules and
the ethanol dissolves.
• Water and ethanol molecules mix freely with each
other, held together in the solution by hydrogen
bonds.
• As the ethanol molecules are now surrounded by
water molecules , the solution forming process can
be represented by:
H2O
C2H5OH(l)  C2H5OH(aq)
Forming Hydrogen Bonds cont…
• In summary, when ethanol bonds with water:
– Hydrogen bonds between water molecules are
broken
– Hydrogen bonds between ethanol molecules are
broken
– Hydrogen bonds between water molecules and
ethanol molecules are formed.
Forming Hydrogen Bonds cont…
• The larger the non polar section of the molecule, the
less soluble it is in water.
• Non polar substances like oil, are insoluble in water
because water molecule form hydrogen bonds with
each other in preference to the weak attraction with
non-polar molecules.
• A way of recalling which substance dissolve is the
rule ‘like dissolves like’.
• Meaning, polar solutes will dissolve polar solvents
and non-polar solutes will dissolve non-polar
solvents.
Polar Covalent Molecular
Compounds that Ionise
• Some compounds have one or more covalent bonds
that are so highly polarised that they break when the
compound is placed in water.
• These bonds break because of the electrostatic forces
of attraction between the solute molecules and the
water molecules.
• For example, the polar hydrogen chloride molecules
attract water molecules.
Compounds that Ionise cont…
• This attraction results in the hydrogen-chlorine bond
breaking.
• Both bonding electrons go with the chlorine atom and an H+
forms a covalent bond with a water molecule.
• In this way two ions are formed – the H3O+ and the Cl- ion
• H3O+ is known a the hydronium ion.
• This hydrolysis process is known as ionisation.
• The HCl molecules have ionised in water.
• The chloride ions and the hydronium ions will be surrounded
by water molecules.
• They are hydrated ions.
• The ionisation process can be represented by the equation on
Compounds the Ionise cont…
• In summary, when a polar covalent molecular
substance that ionises in water:
– Polar covalent bonds within molecules are broken,
producing hydrogen ions and anions.
– A covalent bond forms between each H+ and an
H2O molecule, giving H3O+ ions. Ion-dipole
attractions between the newly formed ions and
the polar water molecules are formed.
Other polar covalent molecular compounds that ionise in
water include the common acids nitric acid (HNO3), sulfuric
acid (H2SO4) and ethanoic acid (CH3COOH).
Ionic Compounds
• Ionic solids contain positive and negative ions held in a 3D
lattice by strong electrostatic forces.
• When an ionic solid such as NaCl is placed in water, the
positive ends of the water molecules are attracted to the
negatively charged chloride ions.
• The negative ends of the water molecules are attracted to the
positively charged sodium ions.
• Because water molecules are in a continuous state of random
motion, they pull the sodium and chloride ions on the outer
part of the crystal out of the lattice and into the surrounding
solution.
Ionic Compounds cont…
Ionic Compounds cont…
• When the sodium chloride has dissolved in the
water, the sodium ions and chloride ions are
surrounded by water molecules.
• The ions are hydrated.
• The attraction between an ion and a polar
molecule such as water is described as an iondipole attraction.
• When an ionic compound dissolved in water,
positive and negative ions in an ionic lattice are
separated from one another.
• This process is known as dissociation.
Dissociation of Sodium Chloride
Ionic Compounds cont…
• When writing equations for the dissociation of
ionic compounds in water, the rules for
balancing equations apply.
• The charges on the right hand side of the
equation must also balance.
• For example, the equation for the dissociation
of potassium carbonate in water is:
H2O
K2CO3(s)  2K+(aq) + CO32-(aq)
Ionic Compounds cont…
• In summary, when an ionic solid dissolves in
water:
– Ionic bonds within the solid are broken
– Hydrogen bonds between water are broken
– Ion-dipole attractions between the ions and the
polar water molecules are formed.
Solubility
• Not all ionic compounds are water soluble.
• For insoluble substances, the energy required to separate the
ions from the lattice is much greater than the energy released
when the ions are hydrated. The ions therefore stay in the
lattice.
• Although substances are described as ‘soluble’ or ‘insoluble’
that is a generalisation.
• Substances described as insoluble tend to dissolve sparingly.
• Those that are described as soluble dissolve to varying
degrees.
• The solubility of a substance also varies based on
temperature.
Solubility Guide
Relationship between Solubility of
Substances in Water and Bond
Type