Separation and Purification

Choose by the difference.

Difference	Method	Example
Insoluble solid in liquid	Filtration	Sand from sandy water
Soluble solid in liquid (want solid)	Evaporation	Salt from sea water
Want a pure solid from solution	Crystallisation	Copper sulfate crystals
Want pure liquid from a solution	Simple distillation	Pure water from sea water
Two miscible liquids with different b.p.	Fractional distillation	Ethanol from a wine mixture
Magnetic from non-magnetic	Magnetic separation	Iron from a sand-iron mix
Sublimable from non-sublimable	Sublimation	Iodine from sand-iodine mix
Coloured / dissolved components	Chromatography	Inks, dyes

Sequence of methods. Sometimes need multiple steps:

• Sand + salt + water mixture → FILTER (sand stays as residue) → EVAPORATE filtrate (salt + water) → solid salt remains.

Worked qualitative. A student is given a mixture of sand, sodium chloride, and water. Suggest a method.

1. Filter — sand removed (residue), salt water passes through (filtrate).
2. Evaporate the filtrate — water evaporates, leaving solid sodium chloride.

OR (to keep the water too):

1. Filter to remove sand.
2. Distil the filtrate — pure water collects (distillate); salt remains in flask.

Simple distillation.

• Heat the mixture in a flask.
• Volatile component (lower b.p.) evaporates first.
• Vapour passes through a CONDENSER (cooled by water flowing in opposite direction).
• Condensed liquid (DISTILLATE) collects in a receiver.

Used to separate a LIQUID from a NON-VOLATILE solute. E.g. pure water from sea water — boil, water vapour rises, condenses; salt left behind in flask.

Fractional distillation.

• Same set-up but with a FRACTIONATING COLUMN above the flask.
• Column has temperature gradient (hot at bottom, cool at top).
• Vapour rises, partially condenses on the column packing, re-evaporates, partially condenses again — many cycles.
• This effectively distils MANY TIMES, separating components even with similar b.p.

Used for two MISCIBLE LIQUIDS with different b.p. The ONE WITH LOWER b.p. comes over first.

Worked qualitative. Why use fractional distillation (not simple) for ethanol-water mixture? Their boiling points are not very different ($78\degree\text{C}$ vs $100\degree\text{C}$); simple distillation would give an impure mixture. Fractional column achieves much cleaner separation.

Worked qualitative. Why does the condenser have water flowing in the OPPOSITE direction to the vapour (counterflow)? Counterflow gives the most efficient cooling — vapour entering the condenser meets the warmest water; vapour leaving meets the coldest water. Maximises temperature difference across the condenser.

Procedure.

1. Make a SATURATED solution (heat the solvent, dissolve as much solute as possible).
2. EVAPORATE some water gently in an evaporating basin until crystals start to form.
3. STOP heating; let the basin cool slowly.
4. Pure crystals form as solubility drops with temperature.
5. FILTER off the mother liquor (the remaining solution).
6. PAT crystals dry between filter papers.

Why cool slowly? Slow cooling gives larger, purer crystals (impurities stay in solution).

Use. Recovering a salt in PURE crystalline form, e.g. copper(II) sulfate, sodium chloride.

Difference from evaporation.

• Evaporation: heats away ALL solvent → gets all solute (as a solid lump, possibly impure).
• Crystallisation: stops mid-way → cooling crystallises the salt; impurities stay in solution.

Worked. Make pure copper(II) sulfate crystals from a slightly impure solution.

1. Heat solution gently; evaporate some water.
2. Cool slowly.
3. Blue crystals form.
4. Filter off remaining solution.
5. Pat dry.

Cambridge tip. Always state "cool SLOWLY" and "PAT dry" for the full marks on a crystallisation question.

Pure substance test. A pure substance has:

• Sharp, fixed melting point (e.g. pure water freezes at $0\degree\text{C}$).
• Sharp, fixed boiling point.

Impure substance. Has:

• A RANGE of melting temperatures (starts melting earlier, finishes later).
• LOWER melting point than the pure substance.
• HIGHER boiling point than the pure substance.

Why? Dissolved impurities disrupt the ordered crystal structure (lower m.p.) and bind to escaping molecules (higher b.p.).

Worked. A student measures the m.p. of two solid samples.

• Sample A melts sharply at $122\degree\text{C}$ (matches pure benzoic acid).
• Sample B melts gradually from $115\degree\text{C}$ to $120\degree\text{C}$.

Conclusion: Sample A is pure (or close to it). Sample B contains impurities.

Other purity tests.

• Chromatography: pure = ONE spot.
• Density: pure substances have specific densities.
• Spectroscopy (more advanced — not on IGCSE).

Cambridge tip. When asked "how would you test if a sample is pure?", give TWO options: melting point (sharp/range) AND chromatography (one spot vs many).