How do hydration and dehydration affect salts?

Hydration and dehydration can cause salts to absorb or lose water, changing their physical properties.

Hydration is a process where water molecules are attracted to ions in a salt. This can cause the salt to dissolve, or in some cases, form a new compound known as a hydrate. Hydrates are salts that have water molecules integrated into their crystal structures. For example, copper(II) sulphate pentahydrate (CuSO4.5H2O) is a blue crystal that contains five water molecules for each unit of copper(II) sulphate. When these salts are in a hydrated state, they often have different colours, densities, and other physical properties compared to their anhydrous (water-free) forms.

Dehydration, on the other hand, is the process of removing water from a substance. When a hydrated salt is heated, it can lose its water of crystallisation, becoming anhydrous. This process is often accompanied by a change in colour. For instance, when blue copper(II) sulphate pentahydrate is heated, it loses its water and turns into white anhydrous copper(II) sulphate. This is a common experiment in IGCSE Chemistry to demonstrate the process of dehydration.

In summary, hydration and dehydration can significantly affect the properties of salts. Hydration can cause salts to dissolve or form hydrates with different physical properties, while dehydration can remove the water from these hydrates, often resulting in a change in colour. Understanding these processes is crucial in many areas of chemistry, including the study of reactions, the preparation of certain compounds, and the analysis of chemical samples.