Separation and purification

O Level / IGCSE Chemistry · Experimental techniques and chemical analysis. A short explanation of the idea, the rules to remember, the mistake to avoid, a worked example and practice questions with answers.

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The idea

To separate a mixture, find a property in which the substances differ, then choose the method that uses it. If one solid dissolves in a solvent and the other does not, add the solvent and filter. The insoluble solid is the residue and the solution is the filtrate. To get a dissolved solid out of its solution, use crystallisation: heat until the solution is saturated, then leave it to cool so that crystals form.

Distillation uses boiling points. Simple distillation collects the solvent from a solution. Fractional distillation separates liquids that mix and have different boiling points.

A pure substance melts and boils at one sharp, fixed temperature. This is used to check purity.

Rules to remember

Common mistake

Students use filtration to try to remove a dissolved solid. A dissolved solid passes through filter paper with the water, so use crystallisation or distillation.

Worked example

A colourless liquid boils over the range 101–104 °C and leaves a white solid when it is evaporated. Is it pure water, and how can pure water be obtained from it?

  1. Pure water boils sharply at 100 °C. This liquid boils above 100 °C and over a range, so it is impure.
  2. The white solid left behind shows that the impurity is a dissolved solid.
  3. A dissolved solid cannot be filtered off. Simple distillation boils off the water, which is condensed and collected, and the solid stays in the flask.

Answer: It is not pure water. Use simple distillation and collect the distillate.

Practice questions

Try each one, then open the answer.

1. Which method separates a soluble solid from a solution to obtain the pure solid?

  1. A
    decanting
  2. B
    crystallisation
  3. C
    using a separating funnel
  4. D
    filtration
Show answer

Answer: B. Evaporating some solvent and letting the solution cool makes the solid crystallise out. Filtration separates an insoluble solid.

2. Liquid A boils at 56 °C and liquid B boils at 118 °C. They mix completely. How can they be separated, and which liquid distils first?

  1. A
    separating funnel; A forms the lower layer
  2. B
    fractional distillation; B distils first
  3. C
    filtration; A passes through the filter
  4. D
    fractional distillation; A distils first
Show answer

Answer: D. Miscible liquids with different boiling points are separated by fractional distillation. The liquid with the lower boiling point, A, distils first.

3. Which mixture can be separated by filtration?

  1. A
    chalk and water
  2. B
    ethanol and water
  3. C
    salt and water
  4. D
    the dyes in black ink
Show answer

Answer: A. Filtration separates an insoluble solid (chalk) from a liquid. Salt dissolves; ethanol mixes with water; ink dyes need chromatography.

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