States of matter
Most substances exist in three phases: solid (often crystalline), liquid and vapour. The difference between these states of matter is the degree of order in the material, which is directly related to the surrounding temperature and pressure. At low temperatures, when the material is in its solid state, the constituents (atoms, ions or molecules) cannot move about freely. Their only movements are thermal vibrations about an equilibrium position (see figure 1a).
If the temperature is raised, more energy is put into the system, leading to stronger and stronger vibrations. Finally, at the transition temperature between the solid and liquid states, the long range positional order is broken and the constituents may move about in a random fashion (figure 1b), constantly bumping into one another and abruptly changing direction of motion. The thermal energy is still not, however, high enough to completely overcome the attractive forces between the constituents, so there is still some positional order at short range. Because of the remaining cohesion, the density of the liquid is constant even though, as opposed to the solid, the liquid takes the shape of its container. The liquid and solid phases are called condensed phases. If we keep on raising the temperature until the next phase change, the substance enters its gas (or vapour) state and its constituents are no longer bound to each other (figure 1c).