Heat is involved in chemical change in two ways; chemical changes either result in heat being added to or removed from an object, and chemical changes require a certain amount of heat before they can take place. Both of these two mechanisms of chemical change involve the heat characteristics of the involved substance or substances and the environment in which the chemical change is taking place.
Some changes in chemistry alter the entire structure of a substance itself. In chemical changes that involve a change of state, there is usually an exchange of heat between the substance changing and the environment it is in. When heat is added to a substance, the process is classified as endothermic. This occurs in changes such as melting and vaporization. Conversely, when heat is taken away from a substance, the change is called exothermic. Examples of exothermic reactions include freezing and condensation.
Heat is important to chemical changes in another way. Chemical reactions require a certain amount of energy. The specific amount of energy required varies from one type of change to the next and is called the activation energy for that reaction. Heat, as a form of energy, is necessary to facilitate these chemical changes.