Explain distillation under reduced pressure.

(N/A) $(i)$ This method is used to purify liquids having very high boiling points.
$(ii)$ It is used for liquids that decompose at or below their boiling points.
Such liquids are made to boil at a temperature lower than their normal boiling points by reducing the pressure on their surface.
$A$ liquid boils at a temperature at which its vapour pressure is equal to the external pressure.
The pressure is reduced with the help of a water pump or vacuum pump.
Use: Glycerol can be separated from spent-lye in the soap industry by using this technique.