Thermal expansion

On a hot summer day, power lines visibly sag, bridges get a little longer and the Eiffel Tower stands about 15 cm taller than on a cold winter day. Nobody adds anything to it, the heat does all that on its own. Almost all materials expand when they get warmer and contract again as they cool down. This effect is called thermal expansion, and it is so reliable that engineers have to plan for it in every bridge and every railway track.

Jiggling particles need room

Everything is made of tiny particles that tremble and wobble all the time. The warmer a material gets, the more vigorously they jiggle and the more room they claim around themselves on average. The particles themselves do not grow one bit, they just keep more distance from each other, and that makes the whole material a tiny bit bigger. In a short rod this is hard to spot, but in large structures it adds up: a steel track 1 km long that is free to expand grows by about half a metre when it warms up by 40 °C.

Engineers' tricks

Because steel and concrete cannot be stopped, they are given room. Between older rail sections small gaps were left, which is where the famous clickety-clack of old train rides came from. Today rails are welded together instead and anchored so firmly that they can hardly move. Bridges get expansion joints, often comb-like metal fingers you drive over at the start of a bridge.

Sometimes the expansion is the trick itself. The bimetallic strip is especially clever: two metals that expand by different amounts, bonded firmly together. When it warms up, it bends to one side and can open a contact as it does. This is how irons and kettles switch themselves off. And a stuck jam jar lid comes off under hot water because the metal lid expands more than the glass underneath.

One substance breaks the rule, and of all things it is the most important one: water. If you cool water down, it contracts as expected, but only until about 4 °C. If it gets colder still, it expands again, and on freezing the ice needs about 9 % more room than liquid water, enough to burst full bottles and water pipes. That is why ice floats on top, and why a lake freezes over from the surface: the densest water, at about 4 °C, sinks to the bottom, where fish spend the winter under a protective sheet of ice. Without this anomaly of water, lakes would freeze solid from the bottom up, and life in water would have a much harder time.