Neutralisation

A chemical reaction happens if you mix together an acid and a base. The reaction is called neutralisation. A neutral solution is made if you add just the right amount of acid and base together. Neutralisation is an exothermic reaction, so the reaction mixture warms up during the reaction.

Metal oxides

Metal oxides act as bases. Here is the general word equation for what happens in their neutralisation reactions with acids:

metal oxide + acid → a salt + water

The salt made depends on the metal oxide and the acid used. For example, copper chloride is made if copper oxide and hydrochloric acid are used:

copper oxide + hydrochloric acid → copper chloride + water

CuO + 2HCl → CuCl2 + H2O

Metal hydroxides

Metal hydroxides act as bases. Some of them dissolve in water, so they form alkaline solutions. Here is the general word equation for what happens in their neutralisation reactions with acids:

metal hydroxide + acid → a salt + water

As with metal oxides, the salt made depends on the metal hydroxide and the acid used. For example, sodium sulfate is made if sodium hydroxide and sulfuric acid are used:

sodium hydroxide + sulfuric acid → sodium sulfate + water

2NaOH + H2SO4 → Na2SO4 + 2H2O

Notice that a salt plus water are always produced when metal oxides or metal hydroxides react with acids.

Metal carbonates

Most carbonates are usually insoluble (they do not dissolve in water). They also neutralise acids, making a salt and water, but this time we get carbon dioxide gas too.

Here is the general word equation for what happens:

metal carbonate + acid → a salt + water + carbon dioxide

The reaction fizzes as bubbles of carbon dioxide are given off. This is easy to remember because we see the word 'carbonate' in the chemical names. For example, copper carbonate reacts with nitric acid:

copper carbonate + nitric acid → copper nitrate + water + carbon dioxide

CuCO3 + 2HNO3 → Cu(NO3)2 + H2O + CO2

Using neutralisation

Here are some ways neutralisation is used:

  • Farmers use lime (calcium oxide) to neutralise acid soils.
  • Your stomach contains hydrochloric acid, and too much of this causes indigestion. Antacid tablets contain bases such as magnesium hydroxide and magnesium carbonate to neutralise the extra acid.
  • Bee stings are acidic. They can be neutralised using baking powder, which contains sodium hydrogen carbonate.
A tractor ploughing lime
Farmers use lime (shown in white) to fertilise acidic soil