What is the difference between hydrated and anhydrous salts?

Hydrated salts contain water molecules in their structure, while anhydrous salts do not.

Hydrated and anhydrous salts are both types of salts, but they differ in their composition. Hydrated salts are salts that have water molecules integrated into their crystal structures. This water is known as water of crystallisation. The number of water molecules per formula unit of the salt can vary and is usually indicated in the chemical formula of the salt. For example, in copper(II) sulphate pentahydrate (CuSO4.5H2O), there are five water molecules for each formula unit of copper(II) sulphate.

On the other hand, anhydrous salts are salts that do not have any water molecules in their structure. The term 'anhydrous' comes from the Greek words 'an' meaning 'without' and 'hydor' meaning 'water'. These salts have had all their water of crystallisation removed, often by heating. When a hydrated salt is heated, it loses its water of crystallisation and becomes an anhydrous salt. For instance, if you heat copper(II) sulphate pentahydrate, it loses its water and turns into anhydrous copper(II) sulphate (CuSO4).

The presence or absence of water in these salts can also affect their physical properties such as colour and density. Hydrated salts often have a different colour compared to their anhydrous counterparts. For example, hydrated copper(II) sulphate is blue, while anhydrous copper(II) sulphate is white. This change in colour can be used as a test for water.

IGCSE Chemistry Tutor Summary: Hydrated salts contain water molecules in their crystal structure, known as water of crystallisation, making them different from anhydrous salts, which lack water. The amount of water in hydrated salts can vary, and removing it through heating turns them anhydrous. This process changes their properties, like colour—hydrated salts often appear different in colour compared to their anhydrous forms.