Explain how the thermal properties of water, arising from hydrogen bonding, make it important for the survival of aquatic organisms during freezing conditions.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Water has a high latent heat of fusion, so a large amount of energy must be removed before it changes from liquid to solid.
The latent heat arises because hydrogen bonds between water molecules must be broken or reorganised during freezing, requiring significant energy removal.
Ice is less dense than liquid water because the hydrogen‑bonded crystalline lattice keeps the molecules further apart, so ice floats on the surface.
The floating ice layer acts as an insulator, preventing the water below from freezing and allowing aquatic organisms to survive.
The latent heat arises because hydrogen bonds between water molecules must be broken or reorganised during freezing, requiring significant energy removal.
Ice is less dense than liquid water because the hydrogen‑bonded crystalline lattice keeps the molecules further apart, so ice floats on the surface.
The floating ice layer acts as an insulator, preventing the water below from freezing and allowing aquatic organisms to survive.
Examiner tips
- Use the term ‘latent heat of fusion’ and explain its link to hydrogen bonding.
- Mention that ice floats because of the open lattice structure.
- Show how the ice layer insulates the water below.
Common mistakes
- Confusing latent heat of fusion with specific heat capacity.
- Failing to explain why ice is less dense.
- Omitting the role of the ice layer as an insulator.
Mark scheme (4 marks)
- Water has a high latent heat of fusion, meaning a large amount of energy must be removed before water changes from liquid to solid.
- This is because hydrogen bonds between water molecules must be broken / reorganised during freezing, requiring the removal of significant energy.
- Ice is less dense than liquid water because hydrogen bonds hold water molecules further apart in the crystalline lattice of ice, so ice floats on the surface.
- The floating ice layer acts as an insulator, preventing the water beneath from freezing and allowing aquatic organisms to survive.
Key terms in this question
Related
- All IB DP Biology Higher Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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