A student investigates how the density of different liquids compares by carefully lowering small objects of known mass and volume into measuring cylinders containing the liquids. Describe how the student could use measurements of mass and volume to determine the density of one of the liquids, and explain why the density of a gas would be much lower than the density of the same substance in its liquid state.

OCR A-Level Physics A (H556) — 1.1 Practical skills assessed · Describe · 5 marks · View as Markdown

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

A student has access to a measuring cylinder, a balance, and several liquids including water and cooking oil. They wish to compare the densities of the liquids and then consider how density relates to the particle model of matter.

Model answer (5 marks)

1. Measure the mass of a known volume of the liquid using a balance (or weigh the empty cylinder, add a known volume of liquid and subtract to find the liquid mass).
2. Read the volume of liquid from the measuring cylinder.
3. Calculate the density by dividing the mass by the volume (density = mass ÷ volume).
4. In a gas the particles are spread far apart, so the spacing between particles is much greater than in a liquid.
5. Because the gas occupies a much greater volume for the same mass, its density (mass per unit volume) is much lower.

Examiner tips

  • Use the exact formula density = mass ÷ volume; include units. Show the two-step mass determination (direct or subtraction).
  • Explain the particle spacing difference clearly to gain the gas‑density point.

Common mistakes

  • Mixing up mass and volume units or forgetting to subtract the cylinder mass. Failing to mention that gas particles are far apart, or not linking this to lower density.

Mark scheme (5 marks)

  1. Measure the mass of a known volume of the liquid using a balance (or: measure the mass of the empty measuring cylinder then add a known volume of liquid and find the mass of the liquid by subtraction)
  2. Read the volume of liquid from the measuring cylinder
  3. Calculate density by dividing mass by volume (density = mass ÷ volume)
  4. In a gas, the particles are spread far apart / the spacing between particles is much greater than in a liquid
  5. Because gas particles occupy a much greater volume for the same mass, the density (mass per unit volume) is much lower

Key terms in this question

density · mass · volume

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