Suggest suitable separation and purification techniques for mixtures when…

The idea

When a question supplies information about a mixture, you must choose a technique that matches the physical properties given. If the mixture is a solid and a liquid and the solid is insoluble, filtration separates them. If the solid is dissolved, evaporating the solvent or crystallising the solution recovers it. If a pure liquid is wanted from a solution, simple distillation is suitable because the solvent boils and condenses while the solute stays behind. If two miscible liquids have different boiling points, fractional distillation separates them. If substances are dissolved and coloured or otherwise detectable, chromatography can separate and identify them. Purity can be checked by melting point or boiling point: a pure substance melts or boils sharply at one temperature, while an impure sample melts or boils over a range.

What earns the marks

• Select filtration when the mixture contains an insoluble solid in a liquid, and identify the solid as residue and the liquid as filtrate.
• Select crystallisation or evaporation when a dissolved solid must be recovered from its solution, choosing crystallisation when crystals are wanted.
• Select simple distillation when a solvent must be separated from a solution and collected as a pure liquid.
• Select fractional distillation when two or more miscible liquids have different boiling points and both components are needed.
• Select chromatography when dissolved substances need separating for identification, for example dyes or sugars in a mixture.
• Use melting point or boiling point data to judge purity, recognising that a sharp value indicates a pure substance and a range indicates impurities.
• Justify the choice by referring to the property given, such as insoluble, dissolved, different boiling points or different solubilities.

Common mistakes and how to put them right

• Choosing filtration for a solution of salt in water.

  Put it right: Use crystallisation or evaporation to recover the salt.

• Choosing simple distillation to separate ethanol and water completely.

  Put it right: Use fractional distillation with a fractionating column.

• Treating a boiling point range as evidence of purity.

  Put it right: State that a pure substance has a sharp, single melting or boiling point.

• Ignoring the information given and naming a favourite technique.

  Put it right: Quote the relevant property before naming the method.

Exam tips

• Underline the key property in the question, such as insoluble, dissolved or different boiling points, before choosing a technique.
• Name the technique and then justify it in one sentence using the property, for example filtration works because the solid is insoluble.
• If the question asks for purification as well as separation, mention how purity could be checked, such as by melting point.
• Avoid listing several techniques without choosing: a clear decision with a reason scores better than a list of possibilities.