Almost every liquid becomes denser as it cools and turns into a solid, so the solid sinks in its own liquid. Water breaks this pattern: ice floats on liquid water instead of sinking. What causes water to behave this way?
- Ice contains tiny pockets of trapped air bubbles that make it buoyant, in the same way a boat floats despite being made of dense metal
- Water is lightest exactly at its freezing point of 0°C, and steadily gets denser again as it turns into ice
- Gravity affects frozen water differently from liquid water because the molecules are no longer able to move at all
- When water freezes, its molecules lock into an open hexagonal lattice held apart by hydrogen bonds, which takes up more space than the same molecules do when moving freely as a liquid, making ice about 8-9% less dense
Why D? And why not the others?
Correct answer: D. When water freezes, its molecules lock into an open hexagonal lattice held apart by hydrogen bonds, which takes up more space than the same molecules do when moving freely as a liquid, making ice about 8-9% less dense
When water freezes, its molecules arrange themselves into an open, hexagonal lattice held apart by hydrogen bonds, and this structure takes up more space than the same molecules do when they are free to move closely together as a liquid; the result is that ice ends up roughly 8-9% less dense than liquid water, which is why it floats. The option about trapped air bubbles is wrong because the density difference comes from the arrangement of the water molecules themselves, not from air pockets, and ice would still float even without any dissolved air at all. The option claiming water is lightest at 0°C and gets denser again as ice is wrong because it has the pattern backwards; liquid water is actually densest at 4°C and becomes less dense both as it cools further and once it freezes. The option about gravity acting differently on frozen water is wrong because gravity treats ice and liquid water identically; buoyancy, not gravity behaving differently, is what determines whether ice floats.
Source: BBC Bitesize GCSE Physics / GCSE Chemistry: Density and states of matter