A student wants to find out whether the type of metal affects how quickly it reacts with dilute sulfuric acid. The metals available are magnesium, zinc, and iron. Describe how the student should carry out a fair investigation to answer this question.

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 volume and concentration of dilute sulphuric acid for each metal.
Use the same mass (or size) of each metal.
Measure the rate of reaction in a consistent way, for example by timing how long it takes for the metal to disappear or by collecting and measuring the volume of hydrogen gas produced in a fixed time.
Keep all other variables the same, such as temperature (room temperature) and repeat the experiment for each metal under the same conditions.

Examiner tips

  • State each control clearly; examiners look for explicit mention of volume, concentration, mass and temperature.
  • Show how you will keep the method consistent – e.g. the same timing device or gas collection set‑up for every trial.
  • Mention that the experiment should be repeated to ensure reliability.

Common mistakes

  • Failing to specify that the acid concentration must be identical for all trials.
  • Using different masses of metal or not recognising the need to keep temperature constant.
  • Measuring the rate inconsistently, such as timing the disappearance of the metal but not measuring gas volume for another trial.

Mark scheme (4 marks)

  1. Use the same volume (and concentration) of dilute sulfuric acid for each metal
  2. Use the same mass (or size) of each metal
  3. Measure the rate of reaction in a consistent way, e.g. measure the time taken for the metal to disappear / collect and measure the volume of hydrogen gas produced in a given time
  4. Keep all other variables the same, e.g. temperature / room temperature, and repeat the experiment for each metal under the same conditions

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