A student has two blocks of the same material. Block A has a mass of 80 g and a volume of 40 cm³. Block B has a mass of 160 g and a volume of 80 cm³. The student claims that Block B has a greater density than Block A because it has a greater mass. Explain why the student is incorrect.

Cambridge International IGCSE Physics (0625) — 1.4 Density · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

Density is mass per unit volume, not mass alone.

Block A: 80 g ÷ 40 cm³ = 2 g cm⁻³.

Block B: 160 g ÷ 80 cm³ = 2 g cm⁻³.

Both blocks have the same density of 2 g cm⁻³, so the student’s claim is wrong.

Examiner tips

  • Show the density formula and calculate each block’s density.
  • State the numerical result for each block.
  • Explain that equal densities mean the blocks are identical in density.
  • Use the word ‘same’ to emphasise equality.

Common mistakes

  • Calculating density incorrectly (e.g., 80÷40=2 but 160÷80=2, but then claiming different densities).
  • Forgetting to divide by volume.
  • Using the wrong units or mixing up g and kg.

Mark scheme (4 marks)

  1. Density is mass per unit volume (not mass alone)
  2. Correct density of Block A stated or clearly implied as 2 g/cm³
  3. Correct density of Block B stated or clearly implied as 2 g/cm³
  4. Both blocks have the same density, so the student is incorrect

Key terms in this question

density · mass · volume

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