Neutrality = pH 7 only at 25oC


Home | Chemistry | Chemistry GCSE

The pH of an aqueous solution is a measure of its hydrogen ion concentration; it depends on temperature because the degree of dissociation of an acid does, i.e. Ka changes. Before you go on, write down the definition of pH.

It is:                          pH = - lg{[H+]/mol dm-3}

Note that the concentration of the hydrogen ion has to be divided by its unit to get a number. You can only take logarithms of numbers, not of physical quantities.

A neutral solution can be at pH 8. How? By being cold. If you look up the values of Kw at 0oC, 25oC and 100oC you will be able to calculate the pH of water at these three temperatures; they are roughly 8, 7 and 6. All of the solutions are, however, neutral. The definition of this is pH-independent:

a neutral solution is one where [H+] = [OH-] .

This is at pH 7 only at 25oC.


Chemistry contents    Home Page

© JRG Beavon 2016