Answer the following :

Define the terms efflorescence and deliquescence.

Efflorescence means spontaneous loss of water by a hydrated salt when the aqueous vapour pressure of the hydrate is greater than the partial pressure of the water vapour in the air. For example, washing soda (Na₂CO₃10H₂O) has a higher water content than the water vapour present in the atmosphere, therefore, it loses its hydrated part and become powdery.

Deliquescence is the process by which a substance absorbs moisture from the atmosphere and dissolves in it to make a solution. Deliquescence occurs when the vapour pressure of the solution that is formed is less than the partial pressure of water vapour in the air. All soluble salts will deliquesce if the air is sufficiently humid. A substance that absorbs moisture from the air but not necessarily to the point of dissolution is called hygroscopic. For example magnesium chloride, anhydrous calcium chloride, calcium nitrate etc.