(ionisation), pH and buffers
There is a slight tendency
for water molecules to dissociate into ions according to the equation:
2H2O = H3O+
+ OH- water oxonium hydroxide molecule ion (hydroxyl)
ion It is simpler, however, to consider the dissociation as: H2O
= H+ + OH-
water oxonium hydroxide molecule ion (hydroxyl) ion In a litre of
water this dissociation produces I /10 000 000 (10-7)
mole of hydrogen ions. This is equivalent to a pH of 7, which is
neutral. If the concentration of hydrogen ions was greater, say
1/1000 (10-3) mole hydrogen ions per litre, the pH would
be 3 and the solution would be acidic. Any pH below 7 is acidic,
any above is basic. An acid is therefore a substance that donates
hydrogen ions and a base is a hydrogen ion acceptor. Note that the
pH scale is not linear but logarithmic.
very powerful solvent properties and it dissolves
more substances than any other common liquid.
Water as a
substances dissolve in water. A substance that
dissolves in a liquid is said to be soluble.
Sugar, for example, is soluble in water. Water is
the solvent and sugar is known as the solute. The
solvent and the solute together form a solution.
When water is the solvent, the solution is said
to be an aqueous solution. Since there is so much
water in living organisms, water is a very
important solvent. Substances such as glucose
dissolve in the water of the blood, which allows
it to be carried around the human body. In
plants, the most commonly transported substance
is sucrose, which is also soluble in water.
There is a
limit to the amount of solute that can be
dissolved in a solvent. For example, if
increasing amounts of sugar are added to water
there will come a point at which no more sugar
will dissolve in the water. The solution is then
said to be saturated. The point at which
saturation is reached depends on the temperature
of the solvent, because warm water will dissolve
more sugar than cold water. There is an important
exception to this general rule. Gases can
dissolve in liquids. However, gases actually
become less soluble as the temperature rises.
Water will hold more dissolved oxygen at a low
temperature than it will at a high temperature.
This has an important effect on the living
organisms found in water. Fish such as salmon and
trout need a lot of oxygen and are very sensitive
to the amount of oxygen dissolve in the water.
One particular type of water pollution, thermal
pollution, can have a dramatic effect on these
fish. Power stations use water from rivers and
seas as a coolant and return it to the source as
warmer water. However, the warmer water contains
less oxygen, so salmon and trout often e when hot
water is pumped into their river.
is possible to make more of a substance dissolve
in water without raising the temperature. One
method of achieving this is to increase the
pressure being applied to the liquid. If a force
is applied to a liquid, more gas molecules will
dissolve in the liquid. Fizzy drinks are an
example of this technique in action. The bubbles
in drinks are made by the gas carbon dioxide,
which is dissolved into the drink under pressure.
When the can or bottle is opened, the pressure is
released and the excess gas is able to escape in
the form of bubbles. As the gas bubbles to the
surface, the drink loses its fizz. It will
eventually become flat. In some places of the
world water comes out of the ground already
fizzy! The water in the ground is under pressure
and contains dissolved gases, such as carbon
dioxide. As the water leaves the ground the
pressure is reduced, allowing the bubbles of gas
to escape into the atmosphere.