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IGCSE · Practice questions · Separation and purification

Separate, then check.

Six original Cambridge-style questions on separation techniques and purity.

Original questions All questions on this page are original work, written in the Cambridge IGCSE style. They are not from past papers. They test the same concepts and skills the syllabus rewards.
Keep these straight

Find the difference, pick the method.

01
Recall
[3 marks]

Describe how to obtain dry crystals of copper(II) sulfate from copper(II) sulfate solution.

  • Heat the solution to evaporate some water until it is saturated (crystals form on a cold glass rod). ✓
  • Leave it to cool so that crystals form. ✓
  • Filter off the crystals, wash with a little cold water and dry them between filter papers. ✓
02
Recall
[3 marks]

Describe how to separate a mixture of salt and sand to obtain dry salt.

  • Add water and stir so the salt dissolves. ✓
  • Filter: sand is left as the residue and salt solution passes through as the filtrate. ✓
  • Evaporate the filtrate and crystallise the salt (then dry it). ✓
03
Explain
[2 marks]

In simple distillation, explain why the thermometer bulb is placed level with the side arm of the flask.

  • It measures the temperature of the vapour passing into the condenser. ✓
  • This is the boiling point of the substance being collected. ✓
04
Analysis
[2 marks]

A mixture of ethanol (b.p. 78 °C) and water (b.p. 100 °C) is fractionally distilled. Describe what the thermometer shows during the distillation.

  • It stays at about 78 °C while ethanol distils over. ✓
  • It then rises to 100 °C when the ethanol has gone and water starts to distil. ✓
05
Analysis
[2 marks]

Pure compound Q melts at 122 °C. A sample of Q melts between 115 and 119 °C. What does this show? Explain.

  • The sample is impure. ✓
  • Impurities lower the melting point and make the substance melt over a range rather than at a sharp temperature. ✓
06
Explain
[2 marks]

Explain why it is important that the substances in medicines are pure.

  • Impurities could be harmful (toxic) to the patient. ✓
  • The dose of active substance must be known exactly, so the drug works as intended. ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.