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Dissolving

Radiation and the atom · level 1, lesson 4 of 6

A question to start

Stir a spoonful of sugar into a glass of warm water. The sugar disappears. Has it gone?

The idea

The sugar has dissolved. It has broken up into pieces so tiny that you cannot see them, and they have spread out all through the water. The sugar is still there:

  • Taste the water (only if an adult says it is safe): it is sweet.
  • Leave the water in a warm, sunny place for a few days. The water dries up and leaves sugar crystals behind.

Salt dissolves too. Sand does not; it just sinks to the bottom. So you can separate salt from sand: stir them into water, pour the salty water through a filter to leave the sand behind, then let the water dry up to get the salt back.

sugar goes init dissolveswater dries: sugar

Worked examples

1. Sweet water. You cannot see the sugar, but you can taste it, so it is still there.

2. Warm or cold. Sugar dissolves faster in warm water than in cold.

3. Sea salt. In some hot countries, sea water is let into shallow ponds. The sun dries the water, and salt is left behind to collect.

Try it

  1. Where does the sugar go when it dissolves?
  2. How can you show the sugar is still there?
  3. Does sand dissolve in water?
  4. How could you get salt back out of salty water?
Answers
  1. It breaks into tiny pieces that spread out all through the water.
  2. Taste it, or let the water dry up and see the sugar left behind.
  3. No, it sinks to the bottom.
  4. Let the water dry up; the salt is left behind.

Common mistake

"Dissolved sugar has disappeared." It is still there, in pieces too small to see. Dissolving is a kind of mixing.

Where it leads