Question

Define the following terms:
(a) Efflorescence
(b) Hygroscopy
(c) Water of crystallisation.

Answer 1

(a) Efflorescence is defined as the loss of water of crystallisation from a hydrated salt when it is exposed to dry air. As a result, the hydrated salt gets crushed to powder.
Example: Blue crystalline copper sulphate loses its water molecules and becomes white powder due to efflorescence.

$$\begin{gathered} {\mathrm{CuSO}}_4.5{\mathrm{H}}_2\mathrm{O}\stackrel{\mathrm{Dry}\mathrm{air}}{\to }{\mathrm{CuSO}}_4+5{\mathrm{H}}_2\mathrm{O} \\ \mathrm{Blue}\mathrm{crystals}\mathrm{White}\mathrm{powder} \end{gathered}$$

(b) Hygroscopy is defined as the absorption of moisture from air by substances that become wet by themselves and do not dissolve in the moisture.
Example: Concentrated sulphuric acid (${\mathrm{H}}_2{\mathrm{SO}}_4$) and quicklime (CaO) are used to remove moisture from gases.

(c) Water of crystallisation is defined as a definite quantity of water molecules associated with crystals as their fundamental part of composition.
Example: Blue crystalline copper sulphate pentahydrate (${\mathrm{CuSO}}_4.5{\mathrm{H}}_2\mathrm{O}$), which is a hydrated salt, has five molecules of water of crystallisation.