Which of the following salts are the main causes of temporary hardness?

(a) Calcium sulphate

(b) Magnesium sulphate

(c) Magnesium chloride

(d) Magnesium carbonate

(d) Magnesium carbonate

Temporary hardness in water is caused by the presence of dissolved bicarbonate minerals, specifically calcium bicarbonate and magnesium bicarbonate. While magnesium carbonate itself is not very soluble, it exists in equilibrium with the highly soluble magnesium bicarbonate, which is the direct cause.

This type of hardness is called "temporary" because it can be removed by boiling the water. Boiling converts the soluble bicarbonates into insoluble carbonates, which then precipitate out of the water.

The other options are incorrect because they cause permanent hardness:

• Calcium sulphate and Magnesium sulphate
• Magnesium chloride