There are six phase changes possible. Transition from solid to gas is sublimation, while transition from gas to solid is deposition. Transition from solid to liquid is melting, while transition from liquid to solid is fusion (freezing). Transition from gas to liquid is condensation, while transition from liquid to gas is vaporization (boiling). Each phase transition is considered a physical change, not a chemical change, because the identity of the compound remains unchanged.

Decomposition is a reaction type that involves a reactant being broken down into two or more products. This is a chemical change, since the identity of the reactant is changed.

A heating curve shows the transition of a solid to a liquid to a gas. A solid, liquid, or gas can exist within a range of varying temperatures, but when a solid is turning into a liquid, or a liquid is turning into a gas, the temperature stays constant. This is the point at which there is a mixture of solids and liquids or liquids and gases within the system. Heat is still being applied to the system, but instead of raising the temperature the heat is converting from one phase to another. Temperature will remain constant during a phase transition until all of the substance has been converted to the final phase.

A phase diagram is used to show what phases of a certain compound exist at given temperatures and pressures. Decreasing pressure and increasing temperature generally cause the compound to favor the gaseous phase, while increasing pressure and decreasing temperature generally cause the compound to favor the solid phase.

On a phase diagram, there is a point known as the critical point. This point gives coordinate at which gases and liquids stop being distinguishable from one another. If pressure or temperature is increased above this point, the sample will enter a state that is fluid, but is neither gas nor liquid. Remember that both gases and liquids have fluid properties. This state of matter is known as a supercritical fluid.

Upon reaching , the temperature of the water will stop increasing and stay at for a brief time. At this point, the energy being added to the water is being used to break the intermolecular bonds between the water molecules. Once the bonds are broken, the water will have fully entered the gas phase, and the water vapor will continue to increase in temperature.

The amount of heat needed to break the intermolecular bonds is called the enthalpy of vaporization. During the plateau, heat is being added and immediately used to break bonds. The length of the plateau corresponds to the amount of heat added during this period, which will equal the enthalpy of vaporization.

The triple point of a compound refers to the temperature and pressure conditions under which a substance exists in all three phases of matter simultaneously. If the pressure is below that of the triple point, the compound will only exist as a solid or a gas, depending on the temperature of the compound. As a result, standard pressure is below carbon dioxide's triple point since it cannot exist as a liquid.

A phase diagram is divided into three regions based on temperature and pressure conditions. Solids exist at low temperatures and high pressures. Liquids exist at medium temperatures and relatively high pressures. Gases exist at high temperatures and low pressures. The lines dividing each region show the conditions required to change between phases, such as the boiling point or freezing point of the compound.

The critical point, or critical temperature, refers to the terminal point on the segment that divides the liquid and gas regions of the phase diagram. Beyond this point, liquids and gases become indistinguishable. The critical point occurs at a very high temperature and pressure. Increasing the termperature beyond this point cannot result in a phase change, regardless of pressure change.