Insoluble salts and water of crystallisation
Making an insoluble salt
- An insoluble salt is made by precipitation:
- mix two solutions that each contain one of the needed ions,
- the insoluble salt forms as a solid,
- then filter, wash and dry it.
Practice
An insoluble salt is made by:
Mixing two solutions each carrying a needed ion forms the insoluble salt, which is filtered, washed and dried.
Water in salts
- A hydrated substance is chemically joined with water.
- An anhydrous substance contains no water.
- The water inside hydrated crystals is the water of crystallisation — e.g. $\text{CuSO}_4 \cdot 5\text{H}_2\text{O}$ has five waters per formula unit.
Practice
A hydrated substance is one that is:
Hydrated means joined with water of crystallisation; anhydrous means no water.
Practice
How many water molecules are in one formula unit of CuSO4·5H2O?
The ·5H2O shows five water molecules of crystallisation per formula unit.
You've got it
Key idea
- insoluble salt → precipitation: mix two solutions, then filter, wash, dry
- hydrated = joined with water; anhydrous = no water
- water of crystallisation: $\text{CuSO}_4 \cdot 5\text{H}_2\text{O}$ (five waters)