Dry crystals with water inside

The idea

Water of crystallisation is the fixed number of water molecules present in one formula unit of a salt, written after a dot in its formula; salt crystals that seem dry contain it, and heating removes it.

A dry-looking crystal

Blue copper sulphate crystals look dry. Your teacher heats some gently in a dry test tube.

What do you expect to happen?

• The crystals turn white and a little water comes out
• Nothing: they are dry
• They burn with a flame

Heat it, then wet it

The boxes are formula units of copper sulphate. The blue pills are water molecules attached to them. Press the buttons.

Now read the formulae. How many water molecules does each salt hold?

A fixed number of water molecules

Water of crystallisation is the fixed number of water molecules present in one formula unit of a salt. It is written after a dot in the formula. Crystals that seem dry contain it, and heating removes it.

So Na₂CO₃·10H₂O is not wet. The 10 water molecules are part of the crystal.

• Salt | Formula | Water molecules per unit
• Copper sulphate | CuSO₄·5H₂O | 5
• Gypsum | CaSO₄·2H₂O | 2
• Washing soda | Na₂CO₃·10H₂O | 10

Notes

Water of crystallisation is the fixed number of water molecules present in one formula unit of a salt, written after a dot in its formula; salt crystals that seem dry contain it, and heating removes it.

Check yourself

Why did the dry-looking blue crystals turn white on heating?

How many water molecules are in one formula unit of CuSO₄·5H₂O?

Answer: 5

5H₂O after the dot: 5 water molecules in each formula unit.

Is Na₂CO₃·10H₂O a wet powder?

White copper sulphate powder is moistened with water. What happens?

• Heating removed their water of crystallisation — correct. Yes! The water was inside the crystal all along.
• They burnt to ash. Nothing burnt. Only water was driven out.
• They melted. They did not melt. The water left and the colour changed.
• Yes: it is soaked in water. The water is built into the crystal. The solid is not soaked.
• No: the 10 water molecules are part of each formula unit of the crystal — correct. Yes! It is water of crystallisation, not wetness.
• Yes: 10 drops of water are stuck on it. It is 10 water molecules per formula unit, locked into the crystal.
• It turns black. It does not turn black. It turns blue again.
• Nothing changes. The water attaches again and the blue colour returns.
• The blue colour comes back — correct. Yes! The water molecules join the units again.