Just like all other chemical reactions, acid-base reactions can occur in both the forward and reverse direction. The extent to which these reactions occur varies, but they all still technically have a forward reaction and a reverse reaction.

For instance, take the ionisation of hydrochloric acid in water. In the forward direction, hydrochloric acid will act as an acid, donating its hydrogen ion (proton) to the water (the base). This results in the formation of a hydronium cation and a chloride anion...

If we then look in the reverse direction, the hydronium cation will act as an acid, donating its hydrogen ion (proton) to the base chloride and re-forming the hydrochloric acid and water. As a result, in every acid-base reaction, there will always be two acids and two bases.

In chemistry, we can recognise this continual transfer of protons in both the forward and reverse directions using the concept of acid-base pairs. In every acid-base reaction, the acid will react to produce its 'conjugate base', a substance that can accept a proton to re-form into the original acid. In the same way, the base will react to produce its 'conjugate acid', which can donate a proton to re-form the original base.

We can therefore define a 'conjugate acid-base pair' as a pair of one acid and one base that differ by one hydrogen ion (proton). For example, the conjugate base of hydrochloric acid is the chloride anion and the conjugate acid of water is the hydronium cation.

Conjugate acid-base pairs are a very important recognition system, allowing us to acknowledge that all acid-base reactions have a forward and reverse direction. However, the fact that both reactions exist does not necessarily mean that they are both occuring.

Whether an acid-base reaction takes place in one or both directions is dependent upon the extent to which the reaction occurs: complete, partial or negligible. As we know from the previous section on acid and base strength, acidity/basicity constants are used to measure the extent to which acidic/basic reactions occur.