A student is investigating how the mass of a reactant affects the amount of product formed. She dissolves copper sulfate solution in a beaker and adds iron filings to displace the copper. She records the mass of the beaker and its contents at the start and at the end of the reaction. Explain how the student should carry out this investigation fairly, and describe one way she could improve the reliability of her results.

OCR A-Level Chemistry B: Salters (H433) — 1.1 Practical skills assessed · Explain · 5 marks · View as Markdown

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

Iron displaces copper from copper sulfate solution. The student wants to find out whether using a greater mass of iron filings produces a greater mass of copper. She has access to a balance, a beaker, iron filings, copper sulfate solution, and standard laboratory equipment.

Model answer (5 marks)

1. Only one variable is changed – the mass of iron filings – while all other factors are kept constant.
2. A control variable that must be kept constant is the volume (or concentration) of the copper‑sulphate solution, the temperature, and the size/type of iron filings.
3. The dependent variable is the mass of copper produced, i.e. the change in mass of the beaker and its contents.
4. The balance should be zeroed (tare) before each weighing, or the mass of the beaker with solution should be recorded before the reaction and again after the reaction to calculate the change.
5. To improve reliability the student could repeat the experiment at least three times and calculate a mean value for the mass of copper produced.

Examiner tips

  • Identify the independent, dependent and control variables clearly.
  • Explain how to use the balance correctly – zero or tare, record before and after.
  • Show that repeating the experiment gives a mean, improving reliability.

Common mistakes

  • Changing more than one variable, e.g. varying the solution volume.
  • Failing to zero or tare the balance before each measurement.
  • Not recognising that the change in mass of the beaker is the dependent variable.

Mark scheme (5 marks)

  1. Only one variable (mass of iron filings) should be changed whilst all other variables are kept constant — this is a fair test / independent variable identified
  2. State a control variable that must be kept constant, e.g. volume/concentration of copper sulfate solution, temperature, or same type/size of iron filings
  3. Identify an appropriate dependent variable — the mass of copper produced / the change in mass of the beaker and contents
  4. Describe how to use the balance correctly — zero/tare the balance before each measurement OR record mass before and after reaction to calculate the change
  5. Describe one way to improve reliability — repeat the experiment (at least three times) and calculate a mean / average result

Key terms in this question

reliability

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