Water of crystallisation and plaster of Paris

Water in a crystal

Look at the crystals of copper sulphate. They are blue, and they look dry. Heat them in a dry test tube. Drops of water collect on the cold part of the tube, and the crystals turn white.

The blue crystals had five molecules of water for each formula unit, held in the crystal. This is the water of crystallisation. Without it, the salt is white and is called anhydrous.

If water is now added to the white powder, the blue colour comes back. This shows that the water is part of the crystal.

Examples

Common salt has no water of crystallisation, and it is white and dry. But the common salt that is not purified takes water from the air, and it gets damp, because of magnesium chloride in it.

Plaster of Paris

When gypsum is heated to $373\ \text{K}$ ($100\ ^{\circ}\text{C}$), it loses three quarters of its water, and plaster of Paris is made.

Plaster of Paris is a white powder. When it is mixed with water, it changes back to gypsum, and it sets into a hard solid.

Uses of plaster of Paris:

• Doctors use it to make plaster casts that hold a broken bone in place.
• It is used to make toys, decorative materials and statues.
• It is used to make the surface of walls smooth.

It must be stored in a moisture-proof container. If it takes in water from the air, it sets and cannot be used. Heating gypsum above $373\ \text{K}$ is avoided, because then all the water is lost, and the powder does not set.

You should find that the oven must be hot enough, but not too hot: only a narrow range gives plaster that sets.

Practise with new numbers

Try it in Python

This program finds the molar mass of three hydrated salts, and the percentage of water in each.

Remember these

• Water of crystallisation is the fixed number of water molecules in a crystal. Without it the salt is anhydrous.
• Blue $\text{CuSO}_4 \cdot 5\text{H}_2\text{O}$ turns white on heating, and blue again with water.
• Gypsum heated to $373\ \text{K}$ gives plaster of Paris, which sets hard with water. It is used for plaster casts.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. How many molecules of water of crystallisation are there in one unit of blue copper sulphate?

2. What happens when a few drops of water fall on white anhydrous copper sulphate?

3. Why is gypsum not heated far above $373\ \text{K}$ to make plaster of Paris?

4. Give two uses of plaster of Paris.