Particles on the move

Pick up a pebble, pour a glass of water, breathe out on a cold window. A solid, a liquid and a gas. They look nothing alike, but scientists explain all three with one simple idea: everything is made of particles far too small to see, and those particles are always moving.

Three states, one model

Between the particles there is nothing at all, just empty space. The particles themselves never change size. When a solid melts, the particles do not melt; they simply move more and pull away from their neighbours.

What heat does

When you heat something, its particles gain energy and move faster. Faster particles push each other a little further apart, so most things expand (get bigger) when they are heated and contract (shrink) when they cool. That is why power lines sag on a hot day and why a stuck jar lid loosens under warm water. Add enough heat and the particles break free of each other: a solid melts, then the liquid boils. Take heat away and it happens in reverse: condensation and freezing. The mass stays the same the whole time, because no particles are lost.

Water, the rule breaker

Most substances are denser as a solid than as a liquid, so a lump of solid wax sinks in melted wax. Water is unusual. As it freezes, its particles lock into an open pattern that takes up more room. Ice is less dense than liquid water, so it floats. That is why icebergs float in Tasman Glacier Lake, why frozen pipes can burst, and why lakes freeze from the top down, leaving liquid water underneath where fish can survive the winter.