Explain how the property of surface tension in water arises from hydrogen bonding 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)
Water molecules form hydrogen bonds with neighbouring molecules in all directions within the bulk liquid.
At the surface, molecules have no neighbours above them, so their hydrogen bonds are directed only inward or sideways, producing a net inward force that minimises the surface area.
This inward force makes the surface behave like a thin elastic film, giving water a high surface tension.
Biologically, this property allows small organisms such as pond skaters to move across the surface without sinking, or it supports the function of lung surfactant in alveoli, or it helps form the meniscus that assists capillary action in plants.
At the surface, molecules have no neighbours above them, so their hydrogen bonds are directed only inward or sideways, producing a net inward force that minimises the surface area.
This inward force makes the surface behave like a thin elastic film, giving water a high surface tension.
Biologically, this property allows small organisms such as pond skaters to move across the surface without sinking, or it supports the function of lung surfactant in alveoli, or it helps form the meniscus that assists capillary action in plants.
Examiner tips
- Use the phrase "hydrogen bonds" and "net inward force" to show understanding of the mechanism. Mention the surface as a film or elastic layer. Give one clear biological example (e.g. water strider, lung surfactant, or plant capillarity).
Common mistakes
- Confusing surface tension with viscosity. Failing to explain the role of hydrogen bonding. Giving a non‑biological example or a vague statement such as "water is important".
Mark scheme (4 marks)
- Water molecules form hydrogen bonds with neighbouring water molecules in all directions within the bulk liquid.
- Molecules at the surface only have hydrogen bonds directed inward/sideways (not upward into air), creating a net inward force that minimises the surface area.
- This inward force causes the surface to behave like a film/elastic layer, giving water a high surface tension.
- Biological significance: surface tension allows small organisms (e.g. pond skaters / water striders) to move across the water surface without sinking, or supports the lung surfactant / alveolar function, or enables the meniscus formation that assists capillary action in plants.
Key terms in this question
surface tension · hydrogen bonding
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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