Purification: crystallisation, sublimation, extraction3 concepts
Crystallisation
3× asked, last 2023Solvent dissolves the compound well hot, sparingly cold; impurities stay in solution. Charcoal removes coloured impurities.
(°C)g per 100 mL waterO255075100135hot: 5.6 g dissolvescool to 25 °C: 94% crystallises0.34 g stays dissolvedbenzoic acid
- Not benzene for benzoic acid + naphthalene. Both dissolve in it. Hot water dissolves benzoic acid but not naphthalene, so pure benzoic acid crystallises from the filtrate on cooling.
Sublimation
3× asked, last 2026Solid → vapour → solid, with no liquid stage. Separates a sublimable solid (naphthalene, camphor) from non-sublimable impurities.
vapour condenses on the cool funnel: pure crystalscotton plugsublimable solid + non-volatile impurityheat
- Not 'a low-melting solid'. It must pass straight to vapour without melting. Benzoic acid and naphthalene both sublime, so that pair needs crystallisation.
Differential extraction
4× asked, last 2024Uses solubility in two immiscible solvents: shake with a solvent that dissolves the compound better; split in a separating funnel.
organic layer: compound (lighter solvent such as ether on top)water layer: impurities that stay in waterstopcock: run off the lower layer
Also remember
- Choice of method depends on the compound and its impurity. A pure compound: sharp m.p. or b.p., one spot on TLC.
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14 more concepts in 4 topics
Distillation, Chromatography, Detection of elements: Lassaigne's test and Quantitative estimation. Read the first part free. Sign in free with Google to read the rest and download the PDF.
Distillation4 concepts
- Simple distillation4×
- Fractional distillation4×
- Distillation under reduced pressure3×
- Steam distillation4×
Chromatography3 concepts
- Types of chromatography5×
- Retardation factor6×
- Polarity and order of elution6×
Detection of elements: Lassaigne's test2 concepts
- Sodium fusion extract2×
- Tests on the fusion extract2×
Quantitative estimation5 concepts
- Carbon and hydrogen (Liebig)8×
- Nitrogen by Dumas method2×
- Nitrogen by Kjeldahl method3×
- Halogens by Carius method5×
- Sulphur and phosphorus2×
- Also remember1
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