A large block of ice floats partially submerged in seawater. The density of ice is approximately 917 kg/m³ and the density of seawater is approximately 1025 kg/m³. Explain, using the concept of density, why ice floats in seawater rather than sinking.

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.
The density of ice (917 kg m⁻³) is less than that of seawater (1025 kg m⁻³).
A given volume of ice therefore has less mass than the same volume of seawater.
Because the ice is less dense than the seawater, the seawater exerts an upward force (upthrust) sufficient to support the ice, so it floats.

Examiner tips

  • Define density first, then compare the two values; link the lower density to buoyancy.
  • Show the logical chain: lower density → less mass for same volume → upthrust exceeds weight → floating.

Common mistakes

  • Using the wrong density values or mixing up units.
  • Failing to explain the link between density and buoyant force; simply stating that ice is lighter is insufficient.

Mark scheme (4 marks)

  1. Density is defined as mass per unit volume
  2. The density of ice (917 kg/m³) is less than the density of seawater (1025 kg/m³)
  3. A given volume of ice has less mass than the same volume of seawater
  4. Because the ice is less dense than the seawater, the seawater exerts an upward force (upthrust) sufficient to support the ice, so it floats

Key terms in this question

density

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