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Science · Matter and heat

Thermal expansion in everyday life

Why are gaps left in bridges, and why does a stuck jar lid open easily under hot water? Find out that substances expand when heated and contract when cooled, and how much solids, liquids and gases expand.

  • 18 min
  • 1 simulation
  • Mini quiz

Updated:

Preview

The question

When walking across a bridge, have you ever noticed a strip in the road where metal teeth fit into each other? Or that power lines sag more in summer than in winter? Engineers leave these gaps and this slack on purpose. Why? Because as the temperature changes, even a huge steel bridge changes length.

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Ordering

Which expands more?

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Prediction

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If equal volumes of a solid, a liquid and a gas are heated by the same amount, which do you think expands the most?

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Frequently asked

4 questions

Does a substance's mass increase when it gets hotter?

No. In expansion the number of particles stays the same; the particles just move further apart. Because the volume grows while the mass stays the same, the substance becomes less dense. That is why warm air rises and hot-air balloons fly.

Why does water expand when it freezes?

When water freezes, its particles arrange themselves in an orderly structure with gaps in it, so ice takes up more space than the same mass of liquid water. That is why ice floats on water and why pipes with water left in them can crack in winter.

When I hold a jar lid under hot water, doesn't the glass expand too?

It does, but metal expands more than glass. The lid grows slightly more than the glass, so the grip between them loosens and the lid opens. Running the hot water only over the lid also keeps the glass from warming up as much.

Why do lakes freeze from the top?

Water is densest at 4 °C. As the air cools, surface water cools and sinks; once the water at the bottom is about 4 °C, water that cools further stays at the surface and freezes. The ice layer insulates the water beneath it, so the lake does not freeze from the bottom and living things survive.