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📘 Why Ice Floats

Watch an ice cube wobble in your glass, then settle with a small part above the surface. That waterline is a clue about density: ice contains the same water molecules as liquid water, but its solid structure takes up more space. The surroun

3
lessons
~15 min
to learn
Adults
level
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What you’ll learn

  1. The glass and the cubeUse density and buoyancy to explain why an ice cube rests partly above water.The visible waterline records a balance between the cube's weight and the denser water it displaces.
  2. A strange solid structureConnect hydrogen bonds and water's unusual temperature-density curve to the lower density of ice.Freezing organizes water molecules into an open lattice, while water is densest near 4 °C.
  3. The winter life raftExplain why floating ice can help lakes and oceans retain liquid habitats in cold conditions.Surface ice limits heat exchange with cold air, and sea ice and icebergs float because their average densities are lower than seawater.

Questions this course answers

Why does ordinary ice float in liquid water?

Freezing forms an open lattice, so the same water molecules occupy more volume and have lower density.

What happens to water as it cools toward 4 °C?

Water reaches its greatest density near 4 °C before the more open structure becomes increasingly important below that temperature.

How can floating lake ice help aquatic life?

Surface ice limits direct heat exchange with cold air and can leave a liquid habitat beneath it.

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