# Explain why ice is less dense than liquid water 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. In liquid water, hydrogen bonds are constantly breaking and reforming, allowing molecules to pack relatively closely together.
2. In ice, water molecules form a regular/fixed lattice in which each molecule is held by four hydrogen bonds, forcing the molecules further apart than in liquid water.
3. Because the molecules in ice are further apart, ice has a lower density than liquid water, so ice floats on water.
4. One biological significance: ice forming on the surface of a body of water insulates the liquid water below, preventing it from freezing solid, allowing aquatic organisms to survive in winter.

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