A phase change is a physical change that occurs when a substance changes from one state of matter to another. The most common phase changes are those between solid, liquid, and gas states. Heating or cooling can cause a phase change, as can the application of pressure. Some substances can exist in more than one solid state; for example, there are numerous allotropes of carbon, including diamond and graphite.
There are three main types of phase changes: melting, vaporization (or boiling), and sublimation. Melting is when a solid turns into a liquid when it is heated. Vaporization (or boiling) is when a liquid turns into a gas when it is heated. Sublimation is when a solid goes directly to a gas without first becoming a liquid; this only happens at high temperatures for certain solids, such as iodine and carbon dioxide.
The terms “melting” and “freezing” refer to the process of changing from one state to another at constant pressure; these processes are reversible provided the temperature does not exceed the melting point or fall below the freezing point for the given substance. The terms “evaporation” and “condensation” refer to changes in state that occur at constant temperature but varying pressure; these processes are also reversible provided that sufficient time is available for them to take place slowly enough that equilibrium conditions prevail throughout the system (i.e., so long as there are no gradients in either temperature or pressure). All other changes in state involve irreversible processes since they require either heating or cooling (and thus do not occur at constant temperature) or else they require pressures above or below those corresponding to the melting point or boiling point (and thus do not occur at constant pressure).
The enthalpy of fusion ($Delta H_f$) is the heat required to melt one mole of a substance at its melting point; this quantity is always positive because heat must be added in order for melting to occur. The molar enthalpy of vaporization ($Delta H_v$)is the heat required to vaporize one mole of a substance at its boiling point; again, this quantity is always positive because heat must be added for vaporization to take place (with typical values ranging from 40 kJ/mol up to 400 kJ/mol). The standard enthalpies of sublimation ($Delta_{sub}H^o$) have values near zero since sublimation generally occurs only at very high temperatures where thermal energies are already large relative to binding energies within molecules…