Separation and purification
Filtration, crystallisation, simple and fractional distillation.
Learning objectives
What you need to be able to do
Teacher-mapped phrasing — check against the official Cambridge syllabus for exact wording.
- 12.4.1Describe and explain methods of separation and purification and select the appropriate method for a given mixture.
6 minute read
Separation and purification
Choosing a separation method is really a diagnosis problem: look at what kind of mixture you have — solid-in-liquid, dissolved solid, or liquid-in-liquid — and the correct method follows almost automatically.
Filtration
Separates an insoluble solid from a liquid. The mixture is poured through filter paper in a funnel: the solid (residue) is trapped, the liquid (filtrate) passes through.
Crystallisation
Separates a dissolved solid from a solution, keeping the solid. The solution is gently heated to evaporate some of the solvent until it is saturated (tested by dipping a glass rod in and checking crystals form on cooling), then left to cool slowly so crystals grow. This is preferred over evaporating to dryness, which can decompose the solid or produce impure, poorly-formed crystals.
Simple distillation
Separates a solvent from a solution, keeping the liquid. The solution is heated; the solvent evaporates, then is cooled and condensed back to a liquid in a separate container, leaving dissolved solids behind. Works because the dissolved solute does not evaporate at the solvent's boiling point.
Fractional distillation
Separates two or more miscible liquids with different boiling points (e.g. ethanol and water, or crude oil fractions). A fractionating column allows the lower-boiling-point liquid to evaporate, rise, and be collected first, while the higher-boiling-point liquid stays behind for longer — essentially simple distillation repeated many times over as the vapour rises through the column.
Choosing the method
The decision tree: is there an insoluble solid? → filtration. Is there a dissolved solid you want to keep? → crystallisation. Is there a solution and you want the pure solvent? → simple distillation. Are there two miscible liquids with different boiling points? → fractional distillation.
Think of it like this
Picking a separation method is like picking the right key for a lock: each method is shaped for one specific type of mixture (solid-in-liquid, dissolved-solid, liquid-in-liquid), and using the wrong one just won't separate anything cleanly.
Worked examples
Method, step by step
A student has a mixture of sand and salt dissolved in water. Describe how pure, dry salt crystals could be obtained.
- 1Filter the mixture to remove the insoluble sand, keeping the salt solution as the filtrate.
- 2Heat the filtrate gently to evaporate solvent until the solution is saturated.
- 3Leave the saturated solution to cool slowly so crystals form, then filter and dry the crystals.
Filter out the sand, then evaporate the salt solution until saturated, cool to crystallise, and filter/dry the crystals.
Common misconceptions
- Using simple distillation to separate two miscible liquids — simple distillation only separates one volatile substance (the solvent) from non-volatile dissolved solids; two liquids of different boiling points need fractional distillation.
- Evaporating a solution fully to dryness when asked to "obtain crystals" — this often decomposes the solid or gives poor crystals; the correct method is to evaporate only until saturated, then cool slowly to let crystals grow.
In the exam
- When a question gives you a mixture, first classify it (insoluble solid + liquid / dissolved solid + solvent / two miscible liquids) before naming a method — examiners often penalise a correct method with the wrong justification.
- For crystallisation, mention the saturation test (dipping a glass rod, checking for crystals forming on the cooled rod) as a specific technique point worth a mark.