Acids, Bases and Salts · Lesson 11 of 12
Are The Crystals Of Salts Really Dry?
“Those perfectly dry crystals have been hiding water inside the whole time.”
• Explain water of crystallisation. • Describe the effect of heating hydrated copper sulphate. • Distinguish hydrated and anhydrous salts. • Compare gypsum with Plaster of Paris. • Explain the setting reaction and uses of Plaster of Paris.
A crystal may look perfectly dry while still containing water as part of its crystalline structure. This water is not simply moisture sitting on the surface. It occurs in a fixed chemical proportion.
Heating Copper Sulphate Crystals
Blue copper sulphate crystals become white when heated. Water droplets may appear on the cooler part of the tube because water contained within the crystals has been released. Adding water back to the white material restores the blue colour.
Water Of Crystallisation
Water Of Crystallisation is the fixed number of water molecules chemically associated with one formula unit of a crystalline salt.
A Hydrated Salt contains a fixed number of water molecules as part of its crystal structure.
An Anhydrous Salt is the form of a salt from which water of crystallisation has been removed.
The water in a hydrated salt is present in a fixed structural ratio. The crystals may feel dry because this water is part of the crystal structure rather than free liquid on the surface.
Gypsum
Plaster Of Paris
When gypsum is heated under suitable conditions, it loses part of its water of crystallisation and becomes calcium sulphate hemihydrate, commonly called Plaster of Paris.
Why Does The Formula Contain Half A Water Molecule?
The formula does not mean that an isolated half molecule of water exists. It expresses a ratio. Two formula units of calcium sulphate share one water molecule in the crystal structure, so each formula unit is represented as associated with one-half of a water molecule.
Setting Of Plaster Of Paris
When Plaster of Paris is mixed with water, it takes water back into its structure and reforms gypsum. The process produces a hard solid mass.
Uses Of Plaster Of Paris
| Use | Reason |
|---|---|
| Supporting fractured bones | Sets into a hard mass around the injured part |
| Making toys | Can be moulded before setting |
| Decorative materials | Can reproduce detailed shapes |
| Smoothing surfaces | Forms a fine hard coating |
Quiz
What happens when blue hydrated copper sulphate crystals are heated?
What is the formula of hydrated copper sulphate?
What is the formula of gypsum?
What happens when Plaster of Paris is mixed with water?
Practice Problems
- Define water of crystallisation in your own words.
- Explain why hydrated copper sulphate crystals can appear dry even though their formula contains water.
- Compare hydrated copper sulphate with its anhydrous form after heating.
- Compare the formulae of gypsum and Plaster of Paris.
- Explain why the formula CaSO₄·½H₂O does not mean that an isolated half molecule of water exists.
Key Takeaways
• Water of crystallisation is a fixed number of water molecules associated with one formula unit of a crystalline salt. • Hydrated copper sulphate is blue and becomes white when its water of crystallisation is removed. • Washing soda contains water of crystallisation and has the formula Na₂CO₃·10H₂O. • Gypsum has the formula CaSO₄·2H₂O. • Plaster of Paris has the formula CaSO₄·½H₂O. • Plaster of Paris sets into a hard mass by taking up water and reforming gypsum.