Question

Taking sodium carbonate as an example, give the meaning of the following terms:
(i) water of crystallisation
(ii) anhydrous

Answer 1

(i) Water of crystallisation is a definite quantity of water molecules associated with crystals as their fundamental part of composition. All hydrated salts have water of crystallisation. For example, sodium carbonate salt (washing soda), which is also called sodium carbonate decahydrate (${\mathrm{Na}}_2{\mathrm{CO}}_3.10{\mathrm{H}}_2\mathrm{O}$), has 10 molecules of water in it responsible for its crystalline structure.

(ii) Hydrated salts lose their water of crystallisation on heating and become amorphous powder. These dehydrated salts are called anhydrous salts. For example, sodium carbonate decahydrate (${\mathrm{Na}}_2{\mathrm{CO}}_3.10{\mathrm{H}}_2\mathrm{O}$) loses its water molecules on heating and forms amorphous powder​.

$$\begin{gathered} {\mathrm{Na}}_2{\mathrm{CO}}_3.10{\mathrm{H}}_2\mathrm{O}\stackrel{\mathrm{Heat}}{\to }{\mathrm{Na}}_2{\mathrm{CO}}_3+10{\mathrm{H}}_2\mathrm{O} \\ (\mathrm{Crystalline}\mathrm{salt})(\mathrm{White}\mathrm{amorphous}) \end{gathered}$$