How do we separate salt from water by evaporation?

Where does the salt go?

Stir two or three teaspoons of salt into half a bowl of water and the salt seems to vanish. It has dissolved, forming a salt solution. The salt is still there: the water tastes salty. Because the salt has dissolved, sieving, decantation and filtration cannot separate it; its particles pass through any filter along with the water. These methods only work for solids that do not dissolve, as explained in sieving, sedimentation, decantation and filtration.

Salt art: watching evaporation

Spread a few drops of the salt solution on a piece of black or dark, thick paper, or paint a picture with it, and let it dry. White patches appear where the drops were. Touch them and you can feel the salt. The water has gone into the air as vapour, but the salt could not go with it.

The same thing happens on dark clothes on hot summer days: sweat dries and leaves white, salty patches.

Getting the salt back by heating

Your teacher can show this. Pour some salt solution into a china dish and heat it on a wire gauze over a spirit lamp until the water boils away. Let the dish cool. Salt is left in the dish; you can feel it with your fingers. (Heat the dish carefully, and only with an adult.)

This method is evaporation: the process in which a liquid changes into its vapour. It is used to separate a solid that is dissolved in a liquid.

Salt from the sea

When Malli asks where namak comes from, Ghanshyam bhai says: from seawater. Seawater is a mixture of salts and some other substances dissolved in water. To obtain salt, seawater is kept in shallow pits and left in the sunlight and air. In a few days the water evaporates completely, leaving behind a solid mixture, and common salt is obtained from it by further purification. Some lakes, such as Sambhar Lake in Rajasthan, also contain common salt.

Salt has a special place in India’s history too. At the Sabarmati Ashram in Ahmedabad, Malli and Valli learn about the Namak Satyagraha, the Dandi March.

Can we get back both salt and water?

Evaporation sends the water into the air. To collect the water as well, the vapour must be cooled so that it turns back into a liquid, which is condensation. If the vapour from heated salt water is led into a cooled container, drops of water collect there, and they are not salty. The salt stays behind in the dish.

Evaporation in daily life

In Ayurveda, India’s traditional system of health and medicine, roots, leaves, flowers or seeds of medicinal plants are often dried in the shade. The extra water evaporates, leaving behind the useful part of the medicine.

Mixture	Method	Why
Salt dissolved in water	Evaporation	Water turns into vapour, salt does not
Sand in water	Sedimentation, decantation or filtration	Sand does not dissolve
Salt and water, both needed	Evaporation, then condensation	The vapour is cooled back into water