A student is investigating how temperature affects the rate of reaction between sodium thiosulfate solution and dilute hydrochloric acid. The reaction produces a yellow precipitate of sulfur, which makes the solution go cloudy. The student places a conical flask on a piece of paper with a black cross drawn on it and measures the time taken for the cross to disappear when viewed from above. Explain how the student should carry out this investigation to make it a fair test, and describe one safety precaution they should take.

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).

The student heats the sodium thiosulfate solution to different temperatures before adding a fixed volume of hydrochloric acid each time. Sulfur dioxide gas is also produced during this reaction.

Model answer (5 marks)

1. Use the same concentration and volume of sodium thiosulfate in every trial.
2. Use the same concentration and volume of hydrochloric acid in every trial.
3. Vary only the temperature of the sodium thiosulfate solution – this is the independent variable.
4. Measure the time from the start of the reaction until the black cross on the paper is no longer visible – this is the dependent variable.
5. View the cross from a fixed distance, height and angle each time so that the judgement of when it disappears is consistent.
6. Perform the experiment in a well‑ventilated area or fume cupboard to minimise inhalation of the toxic sulfur dioxide gas.

Examiner tips

  • State each variable clearly – concentration, volume, temperature, time. Use bullet points or numbering to show the sequence. Mention the need for a controlled environment for safety.

Common mistakes

  • Changing more than one variable at a time. Not specifying the fixed viewing position. Ignoring the need for ventilation when SO₂ is produced.

Mark scheme (5 marks)

  1. Keep the concentration / volume of sodium thiosulfate the same each time
  2. Keep the concentration / volume of hydrochloric acid the same each time
  3. Only change the temperature (independent variable) and measure the time for the cross to disappear (dependent variable)
  4. View the cross from the same distance / height / angle each time to ensure consistent judgement of when the cross disappears
  5. Carry out the investigation in a well-ventilated area / fume cupboard because sulfur dioxide is a toxic gas

Key terms in this question

fair test · precipitate

Related

More Practical skills assessed questions

▶ Try answering this question with AI marking (free) →