Page 3 - REMOVING IMPURITIES FROM WATER AND PURIFYING IT

B. Methods to remove soluble impurities

 Soluble impurities are more difficult to remove than insoluble impurities.
 The two methods of removing soluble impurities are evaporation and

distillation.

1. EVAPORATION
 In evaporation, the water containing impurities is heated. After some

time, the water changes into water vapour, leaving the impurities behind.
The water evaporated can be obtained back by cooling.

 For example, when we take some water in a vessel and add some salt to it
and heat the mixture, we will see that all the water has evaporated from
the vessel and the salt which we dissolved before heating is left behind in

the vessel. Thus, evaporation can separate a soluble impurity from water.

2. DISTILLATION

 It is a separation process in which a liquid is first heated and then cooled
to obtain it in pure form.

 On heating, the liquid changes into the vapour state. These vapours then
pass through a condenser. Cold water circulates in the condenser. This

cools the vapours and gives back the liquid in its pure form.
 When a mixture of water and a soluble impurity, for example salt, is

heated, the water starts boiling and evaporates to form water vapour.