A student is investigating how the mass of copper sulfate crystals produced changes when different volumes of water are evaporated from a copper sulfate solution. Describe how the student could make this a fair test and explain why controlling variables is important when drawing conclusions from the results.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A student dissolves copper sulfate powder in hot water and then evaporates different volumes of the solution to see how much crystalline copper sulfate forms. She wants to compare her results across several trials.
Model answer (4 marks)
1. Keep the initial concentration and volume of the copper sulfate solution the same for each trial.
2. Use the same amount of copper sulfate powder and dissolve it in the same volume of hot water each time.
3. Evaporate the solution using the same method, for the same period of time, and at the same temperature.
4. By controlling these variables, any change in the mass of crystals can be attributed only to the volume of water evaporated.
Controlling variables is important because it ensures that the results are reliable and that the conclusions drawn are valid. If variables are not controlled, it is impossible to determine which factor caused the change in the mass of crystals, making the conclusions meaningless.
2. Use the same amount of copper sulfate powder and dissolve it in the same volume of hot water each time.
3. Evaporate the solution using the same method, for the same period of time, and at the same temperature.
4. By controlling these variables, any change in the mass of crystals can be attributed only to the volume of water evaporated.
Controlling variables is important because it ensures that the results are reliable and that the conclusions drawn are valid. If variables are not controlled, it is impossible to determine which factor caused the change in the mass of crystals, making the conclusions meaningless.
Examiner tips
- Use numbered points to show each control clearly. Mention the dependent variable (mass of crystals) and the independent variable (volume of water evaporated). Explain the link between controlling variables and the validity of conclusions.
Common mistakes
- Failing to state that the initial concentration must be kept constant. Using vague language such as "the same conditions" without specifying which variables. Not explaining why controlling variables matters for drawing valid conclusions.
Mark scheme (4 marks)
- Keep one variable the same (e.g. the initial concentration/volume of copper sulfate solution used each time) to make it a fair test
- Identify a specific control variable that must be kept constant (e.g. starting temperature, same copper sulfate solution concentration, same evaporation method/time)
- If only one variable is changed at a time, any change in the dependent variable (mass of crystals) can be attributed to that variable
- If variables are not controlled, you cannot tell which variable caused the change in results / conclusions would not be valid
Key terms in this question
Related
- All OCR GCSE Chemistry A: Gateway Science (J248) revision notes →
- How to answer a "Describe" question →
- Decode the mark scheme abbreviations →
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