12.4 Separation and purification
- Syllabus
- 0620–2026–2027
- Topic
- 12.4
- Level
- —
Learning objectives
12.4.1Methods of separation and purification• Describe/explain methods of separation and purification using: (a) a suitable solvent (b) filtration (c) crystallisation (d) simple distillation (e) fractional distillation12.4.2Suggest suitable separation and• Suggest suitable separation and purification techniques, given information about the substances involved12.4.3Substances and assess their purity• Identify substances and assess their purity using melting point and boiling point information
Explain five separation and purification methods
| Method | Property used | Essential process | Product obtained |
|---|---|---|---|
| suitable solvent | difference in solubility | dissolve the soluble component, then separate the undissolved material | selected soluble or insoluble component |
| filtration | insoluble solid particles do not pass through filter paper | pour mixture through filter | residue = insoluble solid; filtrate = liquid/solution |
| crystallisation | solubility usually decreases as a hot saturated solution cools | concentrate to near saturation, cool, filter and dry crystals | dissolved solid as crystals |
| simple distillation | solvent has a lower boiling point than dissolved solute | boil, condense vapour and collect distillate | solvent/purer liquid from a solution |
| fractional distillation | miscible liquids have different boiling points | heat through a fractionating column and collect fractions at their boiling ranges | separated liquids |
In crystallisation, cooling reduces the amount of solute that can remain dissolved, so excess solute forms crystals. Filter, wash with a little cold solvent and dry the crystals.
Distillation separates by vaporising and condensing. Simple distillation is used for a solvent from a solution; fractional distillation repeatedly separates vapours in the column when several liquids are present.
Evaporation alone may recover a dissolved solid but does not collect the solvent. Filtration cannot remove a dissolved solute because dissolved particles pass through the filter paper.
Choose a separation technique from substance data
Choose the technique from the substances' physical states, solubilities and boiling points, and first decide which component must be collected.
| Information about the mixture | Suitable choice |
|---|---|
| insoluble solid + liquid/solution | filtration |
| dissolved solid wanted from solution | crystallisation |
| solvent wanted from a solution | simple distillation |
| two or more miscible liquids with different boiling points | fractional distillation |
| one solid dissolves in a chosen solvent and another does not | add suitable solvent → filter → recover dissolved solid from filtrate |
| precipitate formed in an aqueous reaction | filter, wash residue and dry |
Nickel(II) sulfate dissolves in water but sand does not: warm with water, filter off sand, then concentrate and cool the filtrate to crystallise nickel(II) sulfate.
A complete choice states both the property difference and the sequence. 'Use filtration because the desired precipitate is insoluble in the liquid' is stronger than naming filtration alone.
Do not choose a technique from the substance names alone. The same technique may be right or wrong depending on solubility, boiling point, physical state and which fraction is wanted.
Identify substances and judge purity from melting and boiling points
A pure substance has characteristic melting and boiling points. Compare measured values with reliable reference data to support identification and assess purity.
| Measurement | Pure sample | Impure sample |
|---|---|---|
| melting | sharp melting point at the characteristic value | usually melts lower and over a range |
| boiling | sharp boiling point at the characteristic value | usually boils higher and over a range |
If a liquid boils sharply at the reference boiling point for substance X, the result supports the identity and purity of X. A different value or a boiling range suggests that the sample is not pure X.
A solid with a reference melting point of 80 °C that melts sharply at 80 °C is consistent with the pure substance. Melting gradually from 74–78 °C indicates impurity.
A temperature match is evidence, not proof by itself. Use the correct property for the sample and compare under the same stated conditions; boiling point also depends on pressure.