Explain how the properties of water resulting from hydrogen bonding between water molecules contribute to the survival of aquatic organisms during periods of freezing temperatures.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Water has a lower density as ice than as liquid water because the hydrogen bonds in ice are fixed in a regular crystalline lattice that keeps the molecules further apart. This makes ice float, forming an insulating layer on the surface of lakes and ponds. The layer of ice prevents the water below from freezing by blocking heat loss. Additionally, water’s high specific heat capacity and latent heat of fusion mean a large amount of energy must be removed before it freezes, buffering temperature changes and keeping the water below liquid for longer periods.
Examiner tips
- Use the phrase ‘lower density as ice’ and explain the lattice structure; mention ‘floating ice forms an insulating layer’; include the high specific heat/latent heat point; keep answer concise and directly linked to survival of aquatic life.
Common mistakes
- Confusing density of ice with density of liquid water; not mentioning the insulating effect of the ice layer; omitting the role of high specific heat/latent heat in delaying freezing.
Mark scheme (4 marks)
- Water has a lower density as ice than as liquid water / ice is less dense than liquid water
- In ice, hydrogen bonds are fixed in a lattice / regular crystalline arrangement that holds molecules further apart than in liquid water, explaining the lower density
- Ice floats and forms an insulating layer on the surface of water bodies, preventing the water below from freezing
- Water's high specific heat capacity / latent heat of fusion means a large amount of energy must be removed before water freezes, buffering temperature change and maintaining liquid conditions for longer
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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