Explain why ice is less dense than liquid water and outline one biological significance of this property.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
In liquid water the hydrogen bonds are constantly breaking and reforming, so the molecules can pack relatively closely together. In ice the molecules form a regular lattice in which each molecule is hydrogen‑bonded to four neighbours, forcing the molecules further apart than in the liquid.
Because the molecules in ice are further apart, ice has a lower density than liquid water and therefore floats.
One biological significance is that the ice layer on the surface of a body of water insulates the liquid water below, preventing it from freezing solid and allowing aquatic organisms to survive in winter.
Because the molecules in ice are further apart, ice has a lower density than liquid water and therefore floats.
One biological significance is that the ice layer on the surface of a body of water insulates the liquid water below, preventing it from freezing solid and allowing aquatic organisms to survive in winter.
Examiner tips
- Use the terms ‘hydrogen bonds’, ‘lattice’ and ‘density’ – these are key words.
- Show the cause–effect chain: hydrogen‑bonding → lattice → lower density → floating.
- Mention the biological example of insulation to earn the extra point.
Common mistakes
- Claiming ice is denser than water – the opposite is true.
- Forgetting to explain that the lattice keeps molecules further apart.
- Giving a biological significance that is not related to the density effect, e.g. photosynthesis.
Mark scheme (4 marks)
- In liquid water, hydrogen bonds are constantly breaking and reforming, allowing molecules to pack relatively closely together.
- In ice, water molecules form a regular/fixed lattice in which each molecule is held by four hydrogen bonds, forcing the molecules further apart than in liquid water.
- Because the molecules in ice are further apart, ice has a lower density than liquid water, so ice floats on water.
- One biological significance: ice forming on the surface of a body of water insulates the liquid water below, preventing it from freezing solid, allowing aquatic organisms to survive in winter.
Related
- All IB DP Biology Standard Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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