The temperature of a sample during the process of a phase change will remain constant. Even though heat may be added or removed during this process, it is utilized to directly impact the formation and breaking of intermolecular interactions within the sample. The result is a change in the internal properties of the sample (the phase), with no actual change in temperature.

When a substance freezes it transitions from a liquid to a solid. The result is a much more ordered structure, as opposed to the fluidity associated with the liquid phase. Increasing order corresponds to a decrease in entropy because entropy is a measure of disorder.

When a substance freezes, the average kinetic energy of the particles decreases. This means that the particles will move more slowly. Most substances also contract when they cool to increase the organization of the molecules (crystallization). Thus, for most substances, particles will get closer together as they freeze. One notable exception is water, which expands as it freezes (the same mass of water that freezes into ice will float in liquid water due to an increase in volume).

The section point A is in is the solid portion of the phase diagram. At low temperatures and high pressures, substances form solids. Increasing the temperature while keeping pressure high, ice melts into water. Point G is in the liquid portion of the phase diagram.

Distillation is the process by which liquids are purified of impurities. Distillation first requires vaporization of liquids to become pure gases. The gases are then cooled and turned back into pure liquids via condensation into a separate container. A good distillation will remove all impurities from the liquid. This is why distilled water is used for chemical solutions; it does not contain ions or other impurities that could interfere with reaction.

Vaporization refers to the phase change from liquid to gas, also known as evaporation. When becoming a gas, atoms spread out and expand to fill whatever container they are in. Conservation of mass hold that atoms are never created nor destroyed. Atoms becoming more organized and forming a more rigid shape describes a phase change toward becoming solid.

Some substances will transition from a solid to a gas and skip the liquid phase entirely at standard conditions. This change from a solid to a gas is called sublimation. The reverse process of a gas going to a solid is known as deposition. As an example, solid carbon dioxide (dry ice) will sublimate to produce gaseous carbon dioxide at room temperature.

Evaporation is the process by which a liquid transitions to a gas.

Sublimation refers to the phase change whereby a substance goes directly from solid to gas. At high temperature and pressure water will be more likely to melt and than evaporate. At low temperature and low pressure, the water will likely stay solid. Likewise at low temperature and high pressure. At high temperature and low pressure, the ice will be most likely to sublimate. This is clear if one looks at the phase diagram for water. High pressure makes it energetically favorable for water to melt before evaporating. Keeping the pressure low, however, makes it more favorable to pass straight into the gaseous phase.

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.