The Kinetic Model of Solids, Liquids and Gases

The kinetic model explains the properties of the three states by describing how the particles are arranged, how strongly they are held, and how they move.

Solid — particles are packed closely in a regular arrangement, held by strong forces of attraction. They vibrate about fixed positions but cannot move past one another, which is why a solid keeps both its shape and its volume.

Liquid — particles are still close together, so the volume is fixed, but the arrangement is irregular and the forces are weaker. Particles can slide past one another, so a liquid flows and takes the shape of its container.

Gas — particles are far apart with negligible forces between them except during collisions. They move rapidly in random directions, so a gas has neither a fixed shape nor a fixed volume and will fill any container it is put in.

Brownian motion as evidence — smoke particles viewed under a microscope are seen to jitter along random, jerky paths. The smoke particles are far too large to be moving on their own; they are being struck unevenly by air molecules that are too small to see. This is direct evidence that a gas consists of very many small particles in rapid random motion.

Example: the jerkiness matters. A steady drift would suggest a current of air. The random changes of direction show the particle is being hit from all sides by individually random impacts, and only occasionally more from one side than another.

What changes at a change of state — melting and boiling break or weaken the bonds between particles. The energy supplied goes into increasing the potential energy of the particles by pulling them apart, not into increasing their kinetic energy. Because temperature measures mean kinetic energy, the temperature stays constant during the change.

Why boiling takes so much more energy than melting — melting only needs to loosen the regular arrangement enough for particles to slide past each other. Boiling has to separate the particles completely against the attractive forces, and push back the atmosphere to make room for the vapour. For water the latent heat of vaporisation is roughly seven times the latent heat of fusion.