📖 Theory & Concepts ▼

Principle of Crystallisation

Crystallisation exploits the difference in solubility of a substance at different temperatures. Most solids are more soluble in hot solvents.

Steps:
  1. Dissolve impure solid in hot solvent (saturated solution)
  2. Filter hot to remove insoluble impurities
  3. Cool slowly to allow pure crystals to form
  4. Soluble impurities remain in mother liquor

Common Compounds

Compound Solvent Crystal Color
CuSO₄·5H₂O Water Blue
Potash Alum Water Colorless
Benzoic Acid Hot water White needles
Did you know? Slow cooling produces larger, more beautiful crystals. Rapid cooling gives smaller, less pure crystals!

🎯 Aim

To purify an impure sample of alum, copper sulphate, or benzoic acid by crystallisation from a hot saturated solution.

🧪 Procedure Guide

1. Select an impure sample to begin.

Prepare a saturated solution by heating.

Filter the hot solution to remove insoluble dust/sand.

Cool the filtrate slowly to allow pure crystals to form.

Precaution: Do not disturb the solution while cooling to get large, well-defined crystals.
⚠️ Safety
  • Hot solvent & burner — the beaker and solution are near boiling; handle with tongs, keep face back, no naked flame near flammable solvents.
  • Copper sulphate is harmful if swallowed and an irritant — wash hands, never taste.
  • Benzoic acid dust/vapour irritates eyes and airways; if using ethanol as solvent, it is flammable — heat on a water bath, not a naked flame.
  • Hot filtration — support the funnel, pour slowly; hot glass looks the same as cold glass.
  • Wear goggles; collect copper-containing waste as heavy-metal waste.
Action Required

Compound Choice

🔮 Predict First

Experiment Log
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