Particles and states of matter
Chemistry · Key Stage 3 · The School
One model, three states
Everything is made of particles too small to see. In a SOLID they are packed close in a fixed arrangement and can only vibrate, so a solid holds its shape and volume. In a LIQUID they are still close but can slide past one another, so a liquid keeps its volume but takes the shape of its container. In a GAS they are far apart and move fast in all directions, so a gas fills whatever it is put in.
Changing state
Heating gives particles more energy to move. Melting and boiling happen when they have enough energy to break free of the arrangement holding them. Cooling reverses it: condensing and freezing. The particles themselves are unchanged throughout — the same particles, arranged and moving differently. That is why a change of state is physical and can be reversed.
Pressure and diffusion
Gas particles constantly collide with the walls of their container, and those collisions are what we measure as PRESSURE. Heat the gas and the particles move faster, hit harder and more often, and the pressure rises. DIFFUSION is the same movement seen differently: perfume spreads across a room because its particles move randomly among the air particles until they are evenly mixed.
Heat ice from −20 °C and plot temperature against time. It rises steadily to 0 °C, then STOPS and stays at 0 while the ice melts, even though heating continues. Then it rises again to 100 °C, stops again during boiling, then rises once more as steam. The flat parts are where the energy is going into separating particles rather than speeding them up — which is why a melting-point plateau exists at all.