Question

What is meant by ‘anhydrous’ salts? Explain with examples.

Answer 1

Salts that have lost their water of crystallisation are called anhydrous salts. Consequently, the anhydrous salts lack water of crystallisation. When water is added to an anhydrous salt, it rehydrates and restores its original colour.

Copper sulphate crystals (CuSO₄.5H₂O) are blue in colour. When copper sulphate crystals are heated strongly, they lose all the water of crystallisation and form anhydrous copper sulphate (which is white).

\[\ce{\underset{(Blue)}{\underset{Hydrated copper sulphate}{CuSO4 * 5H2O}} ->[Heat] \underset{(White)}{\underset{Anhydrous copper sulphate}{CuSO4}} + \underset{(Goes away)}{\underset{Water}{5H2O}}}\]

Thus, on strong heating, blue copper sulphate crystals turn white (due to the loss of water of crystallisation).

The dehydration of copper sulphate crystals is a reversible reaction. Upon the addition of water to anhydrous copper sulphate, it hydrates and turns blue due to the formation of hydrated copper sulphate.

\[\ce{\underset{(White)}{\underset{Anhydrous copper sulphate}{CuSO4}} + \underset{Water}{5H2O} -> \underset{(Blue)}{\underset{Hydrated copper sulphate}{CuSO4 * 5H2O}}}\]

Hence, anhydrous copper sulphate turns blue when water is added.