Why matter expands
When a substance is heated, its particles gain kinetic energy and vibrate (or move) faster. This makes them push a little further apart, so the substance takes up more space. The particles themselves do not get bigger and the number of particles does not change, so the mass stays the same but the volume increases. Because the same mass now fills a larger volume, the density decreases.
Key idea
Heating → particles vibrate faster → particles move apart → volume increases. For the same rise in temperature, gases expand most, liquids less, and solids least, because gas particles are held together most weakly.
Everyday effects
Thermal expansion is put to use and also allowed for in design:
- Bimetallic strip: two metals that expand by different amounts are joined. On heating, the strip bends towards the metal that expands less. This is used in thermostats and fire alarms.
- Liquid-in-glass thermometer: a liquid (such as alcohol or mercury) expands up a thin tube as temperature rises.
- Expansion gaps: small gaps are left between railway rails and in bridges so they do not buckle in hot weather. Overhead power lines are hung slightly loose so they do not snap when they contract in cold weather.
Remember
- Expanding does not change the mass, only the volume (so density falls).
- Water is unusual: between 0 °C and 4 °C it contracts as it warms, so ice floats on water.