A student suspects that a white powder could be either sodium chloride or sodium carbonate. Describe how the student could use chemical tests to identify which substance the white powder is. Include the expected observations for each substance.

Eduqas GCSE Chemistry — C8 Chemical analysis · Describe · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

1. Add a few drops of dilute hydrochloric acid to the white powder.
2. If the powder is sodium carbonate, bubbles of gas are produced (effervescence). If it is sodium chloride, no reaction occurs.
3. To confirm the gas, collect it and bubble it through limewater.
4. If the gas is carbon dioxide (from sodium carbonate), the limewater turns milky or cloudy; if no gas or no change, the powder is sodium chloride.

Examiner tips

  • Use the command word ‘describe’ – list the steps in order and give the expected observations for each step.
  • Mention the reaction of Na₂CO₃ with HCl and the test with limewater to confirm CO₂.
  • Show the difference in behaviour between NaCl and Na₂CO₃ clearly.

Mark scheme (4 marks)

  1. Add dilute hydrochloric acid (or dilute acid) to the powder
  2. Sodium carbonate produces bubbles / fizzing / effervescence whereas sodium chloride does not react
  3. To confirm, test the gas produced with limewater
  4. Limewater turns milky / cloudy, confirming the gas is carbon dioxide and therefore the powder is sodium carbonate

Key terms in this question

chemical test

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