A student investigates whether the concentration of hydrochloric acid affects the rate of its reaction with zinc powder. Describe how the student could carry out a fair and valid experiment to investigate this.

Cambridge International IGCSE Chemistry (0620) — 12.1 Experimental design · Describe · 4 marks · View as Markdown

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

Model answer (4 marks)

Use the same mass of zinc powder in each trial.
Use the same volume of hydrochloric acid in each trial, but vary only its concentration.
Keep all other variables constant – e.g. temperature, particle size of zinc and the apparatus.
Measure the reaction rate, for example by timing how long it takes to produce a fixed volume of hydrogen gas, or by recording the volume of gas produced in a set time.

Examiner tips

  • State clearly that only acid concentration changes; mention keeping zinc mass, volume, temperature and particle size constant.
  • Explain how you will measure the rate – e.g. timing a fixed gas volume or recording gas volume over time.

Common mistakes

  • Changing zinc mass or volume of acid between trials.
  • Failing to control temperature or particle size, or not specifying how the rate will be measured.

Mark scheme (4 marks)

  1. Use the same mass / amount of zinc powder in each experiment
  2. Change only the concentration of hydrochloric acid between experiments (while keeping volume the same)
  3. Control all other variables — e.g. temperature, volume of acid, or particle size of zinc kept constant
  4. Measure the rate of reaction — e.g. measure the time taken for a given volume of gas to be produced, or time for the reaction to finish, or measure volume of gas produced in a set time

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