# Explain why ice floats on liquid water and discuss the biological significance of this property for aquatic organisms living in cold climates.

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

## Mark scheme (4 marks)

1. Water molecules in ice form a regular/crystalline lattice in which each molecule is hydrogen-bonded to four others in a fixed, tetrahedral arrangement.
2. This open lattice structure means ice is less dense than liquid water, so it floats.
3. Ice forming on the surface of a body of water acts as an insulating layer, slowing further freezing of the water below.
4. Liquid water is maintained beneath the ice, allowing aquatic organisms to survive, feed, reproduce, and carry out metabolic processes through winter.

## Key terms

- [aquatic organisms](https://www.gradenine.co.uk/glossary/aquatic-organisms)

## Related

- [Revision notes for IB DP Biology Higher 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-why-ice-floats-on-liquid-c41c8429) · Published by Druglandscape Ltd.