Question

Give an example of a hygroscopic substance?

Answer 1

Hint: Water vapour is abundantly available in the atmosphere, keeping the air moist. The hygroscopic nature of certain substances is based upon their ability to interact with the moisture present in air to an extent that their physical properties get altered in the process.

Complete answer:
A hygroscopic substance has a high affinity for water and a natural tendency to absorb moisture from its surroundings. The process of attracting water vapour from the environment may result into any one of the following phenomenon:
Absorption of water vapour: The water vapour accepted by the substance goes into the bulk and increases its volume. This may be accompanied by other physical changes like changes in density, boiling point, melting point etc.
Adsorption of water vapour: Since adsorption is a surface phenomenon, the accepted water vapour only accumulates around the surface of the substance preventing its direct contact with the atmosphere but does not affect its physical properties.
Thus hygroscopic substances must be protected under humid conditions (high density of water vapour in air) as they might show a change in their properties.
Examples of substances that show these properties are: sodium hydroxide, silica gel, sulphuric acid and nylon.

Note:
Solids that tend to show catalytic properties and act as heterogeneous catalysts by providing a large surface area for the reactants to get adsorbed onto, must not be hygroscopic in nature. If water vapour covers their surface through adsorption then it might adversely affect their catalytic efficiency. nts is that they have a lifetime of a few seconds and cease to exist for longer periods of time.