A phase transition is a change in the properties of matter at a certain temperature or pressure. The most common type of phase transition is the change from a solid to a liquid, known as melting. Other examples of phase transitions include changes from gas to liquid (condensation), and changes from liquid to solid (freezing).
The physical properties of matter are determined by the types of atoms present and their arrangement. Atoms are constantly moving and vibrating, but they are usually held together by forces that keep them in fixed positions relative to each other. In a solid, these forces are strong enough to overcome the thermal energy of the atoms, so they remain close together in fixed positions. In liquids and gases, however, the thermal energy is too large for the interatomic forces to overcome, so atoms can move freely past each other.
The behavior of matter during a phase transition depends on the strength of the interatomic forces involved. If the interatomic forces are weak, then it takes very little energy to overcome them and cause atoms to change their positions relative to each other. For example, water vapor (a gas) easily condenses into liquid water when cooled because the dipole-dipole interactions between water molecules are relatively weak. On the other hand, if interatomic forces are strong, it takes quite a bit more energy—usually in the form of heat—to cause atoms to rearrange themselves into different phases. An example is carbon dioxide: even though cooling can cause CO2 molecules (a gas) to slow down and stick together temporarily, it requires extremely high pressures—several hundred atmospheres—to force carbon dioxide into its denser liquid state cite{Murnaghan1951}.
In addition to temperature and pressure changes, some substances can also undergo phase transitions in response to light or electric fields cite{Stokes1900}. For example, certain crystals will change color when exposed to light with specific wavelengths; this effect is called photoluminescence cite{Hall1981}. Phase transitions can also be induced by applying an electric field; this process is called electro-optic switching cite{Kuo1993}.