Why Ice Floats

Why it matters

Drop an ice cube into a glass of water and it bobs right on top. Almost every other solid sinks in its own liquid, so why does ice float?

Predict first

Why does a cube of ice float instead of sinking?

When water freezes it expands and becomes less dense than liquid water. Being lighter for its size, the ice floats. This odd behaviour is called the anomalous expansion of water.

• Because ice is denser than water
• Because ice is lighter (less dense) than the water below it — correct
• Because ice is warmer than the water
• Because ice has air pumped into it at the factory

What you do

Decide whether each statement about ice and water is True or False.

Check yourself

What unusual thing happens to water as it freezes into ice?

Water is unusual: it expands as it freezes. The ice takes up more space for the same weight, making it less dense than water, so it floats. This is anomalous expansion.

On a freezing winter day, how does the sheet of ice on top of a pond help the fish below?

The floating ice forms an insulating blanket over the pond. The water beneath stays liquid, so fish and other animals can survive the winter.

At what temperature is water at its heaviest (most dense)?

Water is at its most dense at 4°C. As it cools further towards 0°C it actually expands again, which is why ice ends up lighter than water.

• It expands, so ice takes up more space and is less dense than water — correct
• It shrinks, so ice is denser than water
• It stays exactly the same size
• It disappears completely
• It acts like a blanket, keeping the water underneath from freezing so fish can live — correct
• It sinks and traps the fish at the bottom
• It heats the water until it boils
• It has no effect on the fish at all
• At 4°C — correct
• At 0°C
• At 100°C
• At -10°C