Heat and Expansion

Matter is made of tiny particles that are always vibrating. When you heat a body, you give those particles more energy, so they vibrate faster and push a little further apart. The average spacing grows, and the whole object swells. This is thermal expansion. When the body cools, the particles slow down, settle closer, and the body contracts again. Because solids hold their particles in fixed positions, a heated solid expands in every direction at once.

For a solid we name three kinds of expansion by what is growing. Linear expansion is the increase in length of a rod or wire. Superficial expansion is the increase in surface area of a plate or sheet. Cubical (or volume) expansion is the increase in volume of a block. A heated solid shows all three together. A liquid or a gas has no definite shape, so it has only cubical expansion - only its volume can be talked about, measured inside its container.

The same rise in temperature makes different states expand by very different amounts. Solids expand the least because their particles are tightly bonded. Liquids expand more, because their particles are looser. Gases expand far more than either, because their particles are almost free. That is why a gas thermometer is very sensitive, why railway lines and bridges are built with expansion gaps, and why a tight metal lid loosens when you run it under hot water - the metal expands more than the glass jar.

Almost everything contracts as it cools. Water does this too - until it reaches 4 degrees C. Cool it further, from 4 degrees C down to 0 degrees C, and it does the opposite: it expands. This back-to-front behaviour is called the anomalous expansion of water. Because of it, water has its smallest volume, and therefore its greatest density, at 4 degrees C. The coldest water near freezing is lighter, so it rises to the top. In a freezing pond the surface turns to ice first while 4 degrees C water sinks to the bottom, letting fish survive the winter underneath. The same expansion can burst water pipes in a hard frost.