Define the following and give an example each.
(a) Efflorescent substances
(b) Deliquescent salts
(c) Hygroscopic salts

Explanation

(i) Efflorescent substances: The salts which lose water of crystallization either completely of partially on exposure to atmospheric air are called efflorescent substances.
`Ns_(2)CO_(3)*10H_(2)O` loses nine water molecules on exposure to air and the remaining one water molecule on heating.
(ii) Deliquescent substance: The substances which absorb moisture from the atmosphere on exposure to air. By absorbing moisture they may dissolve in the water absorbed and change into solution. Such substances are called deliquescent substances.
Examples: Calcium chloride.
(iii) Hygroscopic substances: The substances which do not undergo any change in physical state on the absorption of moisture from the atmospheric air. These are called hygroscopic substance.
Examples: Calcium oxide, cobalt chloride etc.