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.

The definition of an endothermic reaction is that the products have higher energy than the reactants, resulting in a positive enthalpy of reaction. For the reaction to run, there must be an input of energy. The opposite is true for exothermic reactions: the products have lower energy than the reactants, enthalpy of reaction is negative, and heat is released.

Nothing is can be definitively stated about the rates of either type of reaction without additional information, as that will depend on the specific reactions and their respective activation energies.

In an exothermic reaction, heat has been released to the surroundings from the system. As a result, the molecules are at a lower final energy state after releasing the energy to the surroundings. 

Going from a solid to a gas (as well as liquid in between) is an endothermic reaction. Energy must be absorbed in order to raise the energy of the molecules so that the phase change can take place. Boiling water, melting ice, or sublimating carbon dioxide all require an input of energy.

The opposite is observed when magma cools. The liquid magma releases energy to the surroundings, allowing it to cool and form igneous rock. The magma is essentially "freezing," turning from a liquid to a solid.

When graphically tracking the energy of a reaction, you can see that energy is always needed to start a reaction, regardless of its enthalpy. This necessary energy is called the activation energy. Exothermic reactions, however, have a lower activation energy compared to the reverse endothermic reaction. This is because there is a net energy release from an exothermic reaction because the products have less energy than the reactants. To reverse this reaction would be to go from the low energy products back to the high energy reactants, resulting in a net increase in energy (an endothermic process).

Condensation of steam is an exothermic process. Heat must be released since the high energy steam is becoming lower energy water.

Combustion reactions occur when a compound is oxidized in a highly exothermic reaction. Most commonly, the reactant is a hydrocarbon (such as propane) and the oxidizing agent is oxygen gas. The result is a large release of heat energy, frequently visualized as a flame.

Note that exothermic reactions by definition release heat, while endothermic reactions absorb heat.

When a reaction is exothermic ("exo-" meaning out and "-thermic" having to do with heat), it means that the reaction is giving off heat into the environment. Therefore, the reactants have a net heat loss throughout the process of the reaction.

The change in enthalpy, , is a measure of the change in heat energy during a reaction.  is always negative for an exothermic process because the products always have less heat energy than the reactants.