Preparing Insoluble Salts & Crystallisation
Chemistry: Cambridge International GCSE Supplement, topic 7.3.4
Preparing Insoluble Salts & Water of Crystallisation
Insoluble salts form via precipitation by mixing two soluble salts. Many salts are hydrated, containing water of crystallisation, which affects colour.
Preparing insoluble salts by precipitation
- Precipitation forms an insoluble salt when two solutions react.
- Mix two soluble salts containing ions that form an insoluble salt.
- The insoluble salt appears as a solid precipitate.
- Filter to separate the solid from the solution.
- Wash the precipitate and dry it.
- This method is common for salts like barium sulfate and lead chloride.
Example of precipitation - Barium sulfate
- Mix barium chloride solution with sodium sulfate solution.
- Barium sulfate (BaSO4) forms as a white solid precipitate.
- Filter and wash the BaSO4 precipitate.
- Barium sulfate is insoluble in water.
- This preparation shows how insoluble salts form by double displacement.
Water of crystallisation explained
- Water of crystallisation is water trapped inside a crystal structure.
- It affects the salt's appearance and mass.
- Hydrated crystals contain water molecules in a fixed ratio.
- The water can be removed by heating.
- Water of crystallisation changes a salt from hydrated to anhydrous form.
Examples of hydrated salts
- Copper(II) sulfate pentahydrate, CuSO4•5H2O, is bright blue.
- Heating CuSO4•5H2O removes water, leaving white anhydrous CuSO4.
- Cobalt(II) chloride hexahydrate, CoCl2\u20226H2O, is pink.
- Heating CoCl2•6H2O removes water, turning it blue.
- These changes help identify water of crystallisation in salts.
Test yourself on Preparing Insoluble Salts & Crystallisation
What is formed when two solutions containing ions that form an insoluble salt are mixed?