A student is investigating how the concentration of hydrochloric acid affects the pH of the solution. She measures the pH of five different concentrations of hydrochloric acid using a pH meter and records her results. Describe how the student should carry out this investigation to ensure her results are valid and reproducible, and explain one safety precaution she should take.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A student wants to find out whether increasing the concentration of hydrochloric acid produces a lower pH reading. She has access to a pH meter, a range of hydrochloric acid solutions of different concentrations, distilled water, and standard laboratory glassware.
Model answer (5 marks)
1. Use the same volume of hydrochloric acid solution each time.
2. Rinse the pH meter probe with distilled water between each measurement to avoid cross‑contamination.
3. Take at least three readings for each concentration and calculate a mean.
4. Change only the concentration of hydrochloric acid (independent variable) and keep all other variables constant.
5. Wear eye protection (safety goggles) because hydrochloric acid is corrosive and can damage the eyes.
2. Rinse the pH meter probe with distilled water between each measurement to avoid cross‑contamination.
3. Take at least three readings for each concentration and calculate a mean.
4. Change only the concentration of hydrochloric acid (independent variable) and keep all other variables constant.
5. Wear eye protection (safety goggles) because hydrochloric acid is corrosive and can damage the eyes.
Examiner tips
- Use bullet points or numbering to show each step clearly. Mention the mean calculation to demonstrate data handling. Include the safety precaution to show awareness of lab safety.
Common mistakes
- Using different volumes of acid, leading to variable concentrations. Failing to rinse the probe, causing cross‑contamination. Not taking repeat readings, giving unreliable data. Ignoring eye protection, risking eye injury.
Mark scheme (5 marks)
- Use the same volume of hydrochloric acid solution each time (control variable)
- Rinse the pH meter (or probe) with distilled water between each measurement to avoid contamination / cross-contamination
- Take repeat readings (at least three) for each concentration and calculate a mean
- Change only the concentration of hydrochloric acid (independent variable) and keep all other variables constant
- Wear eye protection (safety goggles) because hydrochloric acid is corrosive / irritant and could damage eyes
Key terms in this question
Related
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