When an alkali or base reacts with an acid, a neutralisation reaction occurs. In this reaction, the alkali or base donates hydroxide ions (OH-) while the acid donates hydrogen ions (H+). These ions react to form water (H2O) and a salt.

The general chemical equation for a neutralisation reaction is:

Alkali or base(OH-) + acid(H+) --> salt + water(H20)

Examples of neutralisation reactions include:

- Sodium hydroxide (NaOH) + hydrochloric acid (HCl) --> sodium chloride (NaCl) + water (H2O)

- Potassium hydroxide (KOH) + sulfuric acid (H2SO4) --> potassium sulfate (K2SO4) + water (H2O)

- Calcium hydroxide (Ca(OH)2) + nitric acid (HNO3) --> calcium nitrate (Ca(NO3)2) + water (H2O)

Neutralisation reactions are important in many chemical processes, including the production of fertilisers, soaps, and medicines. They are also used to neutralise the effects of acids and bases in the environment.