Creating crystals

You will need

• Sugar
• Salt
• Bi-carb soda
• Warm water
• 3 eye-droppers
• 3 spoons
• 3 plastic containers or bowls
• Measuring cup
• 3 small plastic cups
• Marker

What to do

1. Label the containers ‘sugar’, ‘salt’ and ‘bi-carb’.
2. Pour half a cup of warm water into the container labelled ‘sugar’.
3. Add a spoonful of sugar to the water and stir until dissolved. Keep adding sugar until no more will dissolve.
4. Repeat Steps 2 and 3, but with the salt instead of sugar.
5. Again repeat Steps 2 and 3, but this time with bi-carb soda instead of sugar or salt.
6. Label the small plastic cups ‘sugar’, ‘salt’ and ‘bi-carb’.
7. Use separate eye-droppers to put a few drops of each container’s solution into the matching cup.
8. Place the cups in a warm, sunny place and leave them until the liquid has evaporated. What do you see?

You can try this activity with other crystalline substances as well.

What's happening?

When a solid (or ‘solute’) is dissolved in the water until no more dissolves, the solution is ‘saturated’. The amount of substance that dissolves in water increases with temperature. As the solution cools back down to room temperature, there is now more solute in the water than would normally be the case – the solution is ‘supersaturated’.

As the water evaporates, the solute precipitates out of solution in the form of crystals. This is an example of crystallisation. You will notice that each precipitate forms slightly different crystals: they might be different in size and shape. The size and shape of a crystal depend on a number of factors including chemical formula, temperature and pressure. In general, crystals that form slowly tend to be larger than crystals that form quickly.

By Patrick Mahony

This activity was featured in Science by Email.