CBSE Class 12-science Answered

(a) Washing soda: Washing soda when exposed to air for a long time loses 9 molecules of its water of crystallisation and forms sodium carbonate monohydrate. The process is called efflorescence.

Na₂CO₃. 10 H₂O Na₂CO₃. H₂O + 9 H₂O

Efflorescence is defined as the process of losing water of crystallisation from a hydrated salt when kept exposed to air for a long time.

It occurs when the aqueous vapour pressure of the hydrate is greater than the partial pressure of the water vapour in the air.

Since the vapour pressures of washing soda (Na₂CO₃·10H₂O) normally exceed that of the water vapour in the atmosphere, these salts effloresce (i.e., lose all or part of their water of hydration), and their surfaces assume a powdery appearance.

(b) Ferric chloride: When ferric chloride is exposed to air, it absorbs misture from the atmosphere and forms a liquid.

Deliquescence is the process by which a substance absorbs moisture from the atmosphere until it dissolves in the absorbed water and forms 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.

Ferric chloride shows deliquescence because it readily combines with water to form hydrates. Firstly a small amount of concentrated solution is formed. Now if the vapour pressure of this solution is n less than that of pressure of water vapour in the atomosphere, the salt keeps on absorbing water till the vapour pressure of the solution becomes equal to the vapour pressure of water in the atmosphere.