The Amazing World of Solutes, Solvents, and Solutions
Heat, pressure, and why ice floats
Find out how heating and cooling change density, and why ice floats.
Usually, heating a substance makes its density go down, and cooling makes it go up. Why? From Particulate Nature of Matter: when particles are heated they move apart and spread out. The volume grows, but the mass stays the same. Same mass in more volume means less density.
That's why hot air rises: it is less dense than the cooler air around it. A hot air balloon works this way.
Pressure affects each state differently:
| State | Effect of more pressure on density |
|---|---|
| gas | big: the particles are pushed closer, so volume falls and density rises |
| liquid | small: liquids hardly squash |
| solid | usually negligible: the particles are already packed close |
Now, why does ice float? Water is unusual: it is densest at 4 °C. As it cools further and freezes at 0 °C, its particles rearrange into a pattern that takes up more space: it expands.
The same amount of water now fills a bigger volume, so its density drops. Ice becomes lighter than liquid water and floats.
That matters hugely for life. Floating ice forms a layer on top of lakes and seas, and the water underneath stays warm enough for fish and other creatures to survive even in very cold weather.
Try it: a raw egg sinks in tap water. What could you change about the water to make it float? (Hint: stir in lots of salt to make the water denser.)
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