Physics
Why Does Ice Float?
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Most solids sink in their own liquid, but ice floats. How open hexagons make ice less dense, and why that keeps lakes alive.
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Key points
- A solid floats if it is less dense than the liquid.
- When water freezes, its molecules lock into open hexagons with empty space between them.
- The mass stays the same but the volume grows about 9%, so density falls to about 0.92 g/cm³.
- Lakes freeze from the top, so water stays liquid below and fish survive.
- A floating object's fraction under the surface = its density ÷ the liquid's density.
Density decides
| Solid | Liquid | Result |
|---|---|---|
| Ice (0.92) | Water (1.00) | Floats, about 92% under |
| Ice (0.92) | Ethanol (0.79) | Sinks |
| Wax (0.90) | Water (1.00) | Floats, 90% under |
| Iron (7.87) | Water (1.00) | Sinks |
| Iron (7.87) | Mercury (13.5) | Floats, about 58% under |
Try it yourself
Goal: Predict whether a solid floats or sinks, and how much of it sits under the surface.
- Choose a solid and a liquid.
- Predict Floats or Sinks.
- Press Drop it in and read the densities.
- Try ice in ethanol, then iron in mercury.
Quick quiz
3 questions. Choose an answer for each, then submit to see your score and the explanations.
