The reaction between an acid and a base to give a salt and water is known as:

When an acid reacts with a base, the two cancel out each other's effects, and a salt and water are formed. This is called a neutralisation reaction. The general equation is: acid + base → salt + water.

For example, when hydrochloric acid reacts with sodium hydroxide, sodium chloride (common salt) and water are formed: HCl + NaOH → NaCl + H2O. In this reaction, the hydrogen ions (H+) from the acid combine with the hydroxide ions (OH−) from the base to form water. Neutralisation reactions release heat, so they are exothermic.

Neutralisation has many uses in everyday life:

Acidity or indigestion: excess hydrochloric acid in the stomach is neutralised by antacids, which are mild bases, such as milk of magnesia (magnesium hydroxide) and baking soda (sodium bicarbonate).

Ant sting or bee sting: the sting injects formic acid, which can be neutralised by rubbing a mild base, such as baking soda solution or calamine solution (which contains zinc carbonate). A wasp sting, which is alkaline, can be treated with a mild acid such as vinegar.

Soil treatment: if the soil is too acidic, farmers add slaked lime (calcium hydroxide), quicklime (calcium oxide) or chalk (calcium carbonate). If it is too basic, organic matter such as compost is added.

Factory waste: acidic waste from factories is neutralised with bases before it is released into water bodies.

Tooth decay: bacteria in the mouth produce acids from food, which can damage the tooth enamel when the pH falls below 5.5. Toothpastes, which are basic, help neutralise this acid.

Why not the other options?

• ✗Displacement reaction is one in which a more reactive element displaces a less reactive element from its compound.
• ✗Decomposition reaction is one in which a compound breaks into simpler substances.
• ✗Combustion reaction is the burning of a substance in oxygen, releasing heat and light.