States of Matter

All matter is made of molecules in constant motion, and adding heat makes them move faster. In a solid they are locked in a regular lattice by strong bonds and can only vibrate about fixed positions, so a solid keeps both its shape and its volume. In a liquid the bonds are weaker: the molecules stay close together but slide over each other, so a liquid keeps its volume but takes the shape of its container. In a gas the bonds are broken altogether and the molecules fly apart, filling whatever space they are given.

For water at normal atmospheric pressure the changes happen at fixed temperatures: it melts at 0 °C and boils at 100 °C. The heating curve shows something that surprises people the first time: at those two temperatures the line goes flat. Heat is still going in, but the temperature does not rise, because the energy is being spent breaking the bonds between molecules rather than speeding them up. That energy is the latent heat, and it is far larger than you would expect — boiling a kilogram of water takes more than five times the heat needed to raise it from freezing to boiling in the first place.

The same energy comes back out on the way down. That is why steam scalds so much worse than hot water, why a de-icing fluid works, and why refrigeration systems carry heat around by boiling a refrigerant in one place and condensing it in another.

• Try this: Cool below 0 °C. How are the molecules arranged now, and what motion is left to them?
• Try this: Set exactly 0 °C, then exactly 100 °C. Where does the marker sit on the heating curve, and what is the heat doing there?
• Try this: Compare the flat step at 0 °C with the one at 100 °C. Why is boiling so much more expensive than melting?