A block of wood floats on water. A block of iron of the same volume sinks in water. Explain why the block of wood floats whilst the block of iron sinks, using the concept of density.

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).

Water has a density of 1000 kg/m³. Wood has a density of approximately 600 kg/m³. Iron has a density of approximately 7900 kg/m³.

Model answer (4 marks)

Density is mass per unit volume.
Wood has a density of about 600 kg/m³, which is less than the density of water (1000 kg/m³).
Iron has a density of about 7900 kg/m³, which is greater than the density of water.
An object floats when its density is less than that of the fluid it is placed in, and sinks when its density is greater.

Examiner tips

  • Use the definition of density first. State the densities of wood, iron and water. Explain the floating/sinking rule clearly. Keep the answer concise to fit the 4 marks.

Common mistakes

  • Confusing mass with density. Saying the block of iron floats. Using vague terms like "lighter" or "heavier" without reference to density.

Mark scheme (4 marks)

  1. Density is mass per unit volume
  2. Wood has a lower density than water / wood's density is less than 1000 kg/m³
  3. Iron has a higher density than water / iron's density is greater than 1000 kg/m³
  4. An object floats if its density is less than the fluid it is placed in, and sinks if its density is greater than the fluid

Key terms in this question

density · volume

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