Soluble or insoluble?

Ghulan comes home tired, thirsty and sweating after playing. His mother stirs a spoonful of sugar, a pinch of salt and some lemon juice into a tumbler of water and gives him shikanji (lemonade). As she stirs, the sugar and salt disappear.

Materials that disappear completely when mixed in water dissolve in it: they are soluble in water. Sugar and salt are soluble. Materials that do not disappear, even after long stirring, are insoluble. Sawdust (and sand) is insoluble: it stays, however long you stir. Water can dissolve a great many materials, and that is why it plays such an important role in how our body works.

Liquids behave differently too. Some mix completely with water. Others, like oil, do not mix, and form a separate layer when left alone for a while.

Even some gases dissolve in water. Oxygen dissolves in water, and this is very important for the animals and plants that live in water: fish breathe it.

A useful example: ORS (Oral Rehydration Solution) treats dehydration caused by diarrhoea or other illnesses. Ready-made ORS packets are dissolved in a litre of water. With no packet, you can make ORS at home: mix six teaspoons of sugar and half a teaspoon of salt in one litre of boiled and cooled water.