# Explain how the property of surface tension in water arises from hydrogen bonding and outline one biological significance of this property.

> IB DP Biology Standard Level (2023 syllabus) — A1.1 Water · Explain · 4 marks

## Mark scheme (4 marks)

1. Water molecules form hydrogen bonds with neighbouring water molecules in all directions within the bulk liquid.
2. 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.
3. This inward force causes the surface to behave like a film/elastic layer, giving water a high surface tension.
4. 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

- [surface tension](https://www.gradenine.co.uk/glossary/surface-tension)
- [hydrogen bonding](https://www.gradenine.co.uk/glossary/hydrogen-bonding)

## Related

- [Revision notes for IB DP Biology Standard Level (2023 syllabus)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-how-the-property-of-surface-0a4c4d17) · Published by Druglandscape Ltd.