Evaporation

In Ahmedabad, Malli and Valli visit Sabarmati Ashram with their father's friend Ghanshyam bhai, and learn about the Namak Satyagrah (the Dandi March). Malli asks where namak, common salt, comes from. "From seawater," says Ghanshyam bhai.

Seawater is a mixture: salts and some other substances are dissolved in water. To get salt, seawater is kept in shallow pits, open to sunlight and air. In a few days the water evaporates completely and a solid mixture is left. Common salt is then obtained from it by further purification.

Evaporation is the process in which a liquid changes into its vapour. We can use it to separate a solid dissolved in a liquid: the liquid goes into the air as vapour, and the solid stays behind. (In salt water, the salt is called the solute, the thing dissolved, and the water is the solvent, the thing it is dissolved in.)

Two activities from the book:

• Spread a few drops of salt solution on thick black paper and let it dry. The water goes; salt is left, and you can feel it on the paper.
• Heat salt solution in a china dish until all the water boils away. The salt is left in the dish.

Evaporation is simple, but it needs heat (or sunshine) and time.