Water and neutral solutions
Water molecules can break down into hydrogen ions and hydroxide ions.
\(H_{2}O(l)\rightleftharpoons H^{+}(aq)+OH^{-}(aq)\)
This is a reversible reaction. A small proportion of water molecules break up to form hydrogen ions and hydroxide ions. Some of these hydrogen and hydroxide ions then react together again to form water molecules.
This is called an equilibrium and is present in water and all aqueous solutions.
In water and neutral solutions, the concentration of hydrogen ions is equal to the concentration of hydroxide ions.
All acidic solutions contain more hydrogen ions than hydroxide ions.
All alkaline solutions contain more hydroxide ions than hydrogen ions.
Diluting acids and bases
Adding water to an acid or base will change its pH. Water is mostly water molecules so adding water to an acid or base reduces the concentration of ions in the solution.
When an acidic solution is diluted with water the concentration of H+ ions decreases and the pH of the solution increases towards 7.
To make the pH change by 1, a tenfold dilution is required (eg adding 9 cm3 of water to 1 cm3 acid). The acid is becoming less acidic.
Similarly, when an alkali is diluted with water the concentration of OH- ions decreases. This causes the pH of the alkali to fall towards 7, making the solution less alkaline as more water is added.