Solubility and Crystallisation
Investigate solubility and grow pure crystals from solution.
Crystallisation produces pure solid crystals from a solution.
- Define solubility and describe how it changes with temperature.
- Explain how crystallisation purifies a solid.
- Describe a crystal-growing procedure.
Solubility
Solubility is the mass of solute that dissolves in 100 g of solvent at a given temperature. Most solids become more soluble as water warms; gases become less soluble.
Crystallisation
A saturated solution is warmed to evaporate some solvent. As it cools, the solution becomes supersaturated and pure crystals form. Impurities stay in the remaining solution.
Procedure
- Dissolve the impure solid in the minimum hot solvent.
- Filter while hot to remove insoluble impurities.
- Cool slowly to form crystals.
- Filter the crystals and dry them gently.
Dissolve 50 g of sugar in 100 ml of hot water.
Tie a string to a pencil and suspend it in the solution.
Leave it for a week and watch sugar crystals grow.
Practice
- Define solubility.
- Explain why cooling a hot saturated solution forms crystals.
- Why is slow cooling better than fast cooling for large crystals?
Recap
Solubility depends on temperature. Crystallisation uses cooling and evaporation to produce pure solid crystals.
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Separation Techniques
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