Soluble and insoluble substances

Disappearing sugar

Ghulan came home tired and thirsty after playing, so his mother made him shikanji: a spoonful of sugar, a pinch of salt and some lemon juice in a tumbler of water. As she stirred, the sugar and salt disappeared. Where did they go?

Testing materials in water

In the chapter’s activity, five tumblers are two-thirds filled with water, and a teaspoon each of sugar, salt, chalk powder, sand and sawdust is stirred into them. After a few minutes:

Material	What happens
Sugar	Disappears: soluble
Salt	Disappears: soluble
Chalk powder	Turns the water milky, then settles: insoluble
Sand	Settles at the bottom: insoluble
Sawdust	Floats on top: insoluble

Materials that disappear completely in water dissolve, or are soluble in water. Materials that do not mix and do not disappear, even after long stirring, are insoluble. The dissolved sugar is not lost: it is spread all through the water, which is why the shikanji tastes sweet.

Liquids and gases in water

• Liquids: some mix completely with water, such as vinegar and honey. Others, such as oil, do not mix and form a separate layer when left alone.
• Gases: some dissolve in water and some do not. Oxygen dissolves in water, and fish and water plants depend on it to survive.

Why it matters

Water dissolves a large number of materials, which is why it plays such an important role in our bodies. That is also how ORS works: Oral Rehydration Solution treats dehydration from diarrhoea or other illnesses. A ready packet is dissolved in a litre of water; at home, ORS can be made by mixing six teaspoons of sugar and half a teaspoon of common salt in one litre of boiled and cooled water. The dissolved sugar and salt help the body take back the water and salts it has lost, which is why ORS must be fully dissolved in clean water before it is drunk.

Solutions in daily life

Many things we use every day depend on dissolving. Tea and coffee taste sweet because sugar dissolves in them, salt dissolves into dal and vegetables as they cook, and a pinch of salt and sugar in lemonade makes shikanji. Detergent powder mixes into water to wash clothes. Insoluble materials matter too: sand and pebbles do not dissolve, so they settle at the bottom of a pond, and sawdust floats, so it can be skimmed off.

Making things dissolve faster (Extra)

Sugar dissolves faster if you stir, if the water is warm, and if the sugar is in small grains rather than a big lump. Try putting a sugar cube in one glass of water and the same amount of powdered sugar in another, and see which disappears first.

Pairs to remember

From vinegar, honey, mustard oil, water, glucose and wheat flour:

• Soluble pairs: honey in water, glucose in water, and vinegar in water.
• Insoluble pairs: mustard oil and water, and wheat flour and water.

To read about other properties of materials, see properties of materials. For the textbook’s questions, see the Chapter 6 NCERT solutions; for the whole chapter, see the Chapter 6 notes.