The pH of pure water
Why does pure water have a pH of 7?
That question is actually misleading! In fact, pure water only has a pH
of 7 at a particular temperature - the temperature at which the K w
value is 1.00 x 10 -14 mol 2 dm -6 .
This is how it comes about:
To find the pH you need first to find the hydrogen ion concentration
(or hydroxonium ion concentration - it's the same thing). Then you
convert it to pH.
In pure water at room temperature the K w value tells you that:
[H + ] [OH - ] = 1.00 x 10 -14
But in pure water, the hydrogen ion (hydroxonium ion) concentration
must be equal to the hydroxide ion concentration. For every hydrogen
ion formed, there is a hydroxide ion formed as well.
That means that you can replace the [OH -] term in the K w expression
by another [H +].
[H + ] 2 = 1.00 x 10 -14
Taking the square root of each side gives:
[H + ] = 1.00 x 10 -7 mol dm-3
Converting that into pH:
pH = - log 10 [H +]
pH = 7
That's where the familiar value of 7 comes from.
The variation of the pH of pure water with temperature
The formation of hydrogen ions (hydroxonium ions) and hydroxide ions
from water is an endothermic process. Using the simpler version of the
equilibrium:
The forward reaction absorbs heat.
According to Le Chatelier's Principle, if you make a change to the
conditions of a reaction in dynamic equilibrium, the position of
equilibrium moves to counter the change you have made.