Explain why ice floats on liquid water and discuss the biological significance of this property for aquatic organisms living in cold climates.

IB DP Biology Higher Level (2023 syllabus) — A1.1 Water · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

Ice forms a regular, tetrahedral lattice in which each water molecule is hydrogen‑bonded to four others, creating an open structure that is less dense than liquid water, so ice floats.

The ice layer on the surface acts as an insulating blanket, reducing heat loss and preventing the water below from freezing.

This keeps a liquid layer beneath the ice, allowing aquatic organisms to survive, feed, reproduce and carry out metabolic processes during winter.

Examiner tips

  • Use the term ‘tetrahedral lattice’ and ‘less dense’ to hit the key points. Show that ice floats because of the open structure. Explain the insulating effect and its biological consequence. Keep each point short and to the point.

Common mistakes

  • Confusing the reason for floating with the reason for ice forming. Forgetting to mention the insulating effect. Using vague terms like ‘open structure’ without specifying tetrahedral hydrogen bonding.

Mark scheme (4 marks)

  1. Water molecules in ice form a regular/crystalline lattice in which each molecule is hydrogen-bonded to four others in a fixed, tetrahedral arrangement.
  2. This open lattice structure means ice is less dense than liquid water, so it floats.
  3. Ice forming on the surface of a body of water acts as an insulating layer, slowing further freezing of the water below.
  4. Liquid water is maintained beneath the ice, allowing aquatic organisms to survive, feed, reproduce, and carry out metabolic processes through winter.

Key terms in this question

aquatic organisms

Related

More Water questions

▶ Try answering this question with AI marking (free) →