A student wants to find the density of an irregularly shaped rock. Describe how the student should measure the volume of the rock and explain how errors in the experiment should be avoided.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
The student has access to a measuring cylinder, water, a balance, and a piece of string.
Model answer (4 marks)
1. Fill the measuring cylinder with water to a marked level and record the initial volume.
2. Attach the rock to a piece of string and carefully lower it into the water until it is fully submerged.
3. Record the new volume reading; the increase in volume equals the rock’s volume.
4. Read the volume at eye level, looking straight on at the meniscus to avoid parallax error.
5. Weigh the empty cylinder on the balance, tare it (or note its mass) before adding water so that the mass of the rock can be measured without zero‑error.
6. Calculate the density using ρ = mass / volume, ensuring units are kg m⁻³.
2. Attach the rock to a piece of string and carefully lower it into the water until it is fully submerged.
3. Record the new volume reading; the increase in volume equals the rock’s volume.
4. Read the volume at eye level, looking straight on at the meniscus to avoid parallax error.
5. Weigh the empty cylinder on the balance, tare it (or note its mass) before adding water so that the mass of the rock can be measured without zero‑error.
6. Calculate the density using ρ = mass / volume, ensuring units are kg m⁻³.
Examiner tips
- Use a straight‑on eye level reading to avoid parallax; this earns the full point for accuracy.
- Tare the balance before weighing the rock to eliminate zero error; examiners reward correct mass handling.
Common mistakes
- Reading the meniscus from an angle, giving a parallax error.
- Forgetting to tare the balance, leading to an incorrect mass measurement.
Mark scheme (4 marks)
- Partially fill the measuring cylinder with water and record the initial volume reading
- Immerse / submerge the rock fully in the water (using string) and record the new volume reading; the difference between the two readings is the volume of the rock
- Read the volume from the measuring cylinder at eye level (straight-on) to avoid parallax error
- Place the empty measuring cylinder on the balance and press tare (or record its mass and subtract) before adding liquid, to avoid zero error when measuring mass
Key terms in this question
Related
- All Pearson Edexcel International GCSE Physics (4PH1) revision notes →
- How to answer a "Describe" question →
- Decode the mark scheme abbreviations →
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