Explain how the surface tension of water arises from hydrogen bonding and outline one biological example in which surface tension is important.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Water molecules form hydrogen bonds with neighbouring molecules in all directions within the bulk of the liquid. At the surface, molecules lack bonding partners above them, so they experience a net inward cohesive force, creating surface tension. This high surface tension makes water resistant to breaking at its surface and gives it a high surface energy compared with most liquids. For example, pond skaters (Gerridae) can walk on water because their body weight is insufficient to overcome the surface tension, allowing them to remain on the surface without breaking it.
Examiner tips
- Use the phrase ‘net inward cohesive force’ to show understanding of surface tension.
- Give a clear biological example and explain why surface tension is important in that context.
- Keep the answer concise – 4 marks, so one sentence per point is enough.
- Use UK spelling (e.g. ‘surface tension’).
Common mistakes
- Confusing surface tension with viscosity or surface energy. Failing to mention the lack of hydrogen bonds above surface molecules. Giving an irrelevant example or not explaining the role of surface tension in the example.
Mark scheme (4 marks)
- Water molecules form hydrogen bonds with neighbouring water molecules in all directions within the bulk of the liquid.
- Molecules at the surface lack hydrogen-bonding partners above them, so they experience a net inward cohesive force, creating surface tension.
- This high surface tension makes water resistant to breaking at its surface / gives water a high surface energy compared with most liquids.
- A named biological example explained correctly — e.g. small arthropods such as pond skaters (Gerridae) can walk on water because their body weight is insufficient to break the surface tension; or pulmonary surfactant reduces surface tension in alveoli to prevent their collapse during exhalation; or the water surface in a xylem vessel resists breakage, maintaining the water column under tension during transpiration.
Key terms in this question
surface tension · hydrogen bonding
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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