A solid is composed of molecules that are tightly packed together, thereby giving the object a rigid structure that is resistant to change. As temperature rises, the kinetic energy of the molecules within the solid begin to vibrate which decreases the attraction of these molecules. There is a temperature threshold, referred to as the melting point, at which the vibration becomes sufficient enough to cause the solid to change to liquid. The melting point, in turn, also identifies the temperature at which the liquid will change back to the solid identifying it as the freezing point.
A liquid has molecules that are not as tightly held together as a solid, thereby allowing the molecules to move around. This gives liquid an important property of being able to take the shape of the container in which it is held. As the temperature, and thus the kinetic energy, of a liquid increases, the molecules begin to vibrate more rapidly. They then reach a threshold at which the vibration becomes so great that the molecules escape into the atmosphere converting the liquid into a gas. This temperature threshold is called the boiling point, if the change is from liquid to gas as the temperature increases, and the condensation point, if the change is from gas to liquid as the temperature falls below this point.
Gases have the highest kinetic energy of any state of matter and thus occur at the highest temperatures. Increasing the temperature of a gas in an open system will not further change the state of matter because the gas molecules will only become infinitely further apart. In a closed system, however, increasing the temperature of gases will result in an increase in pressure due to the molecules moving faster and the increased frequency of the molecules hitting the sides of the container.
Pressure is also a factor when examining the effects of temperature on the various states of matter. According to Boyle's Law, temperature and pressure are directly related, meaning that an increase in temperature results in a corresponding increase in pressure. This is again caused by the increase in kinetic energy associated with increasing temperature. At sufficiently high pressures and temperatures, a gas may bypass the liquid phase and be converted directly to a solid through a process called sublimation.