Organic Chemistry
Edexcel AS Chemistry — 3.5 Alcohols — Edexcel AS Chemistry Revision Notes & Topical Questions
5 min read 3 parts Lesson 42 of 42
Lesson summary
Purification: describe how to obtain crystals from a solution and how a liquid's boiling point can provide evidence of purity. Crystallisation is useful when a dissolved solid is more soluble in hot solvent than in cold solvent.
- cool slowly
- filtration
- cold distilled water
Part 1 of 3Make a saturated solution
Crystallisation
Crystallisation is useful when a dissolved solid is more soluble in hot solvent than in cold solvent. Heat the solution gently to evaporate some solvent and make it nearly saturated.
A drop placed on a clean, cold glass rod can help check for saturation: crystals forming on the rod show that enough solvent has been removed.
Part 2 of 3Grow and collect crystals
Cool slowly
Allow the concentrated solution to cool slowly. Solubility falls and solid comes out of solution as crystals; slow cooling can allow larger crystals to grow.
Filtration
Collect the crystals by filtration, rinse with a small amount of cold distilled water to remove soluble impurities, and allow them to dry. Cold water limits dissolution of the product.
Part 3 of 3Check liquid purity
Boiling point
Measure a liquid product's boiling point and compare it with the accepted value for the pure compound under the same pressure. A pure substance normally boils over a narrow temperature range.
Impurities often change or broaden the observed boiling range. A boiling point alone supports a purity judgement but does not identify every possible impurity.
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