Endothermic and Exothermic Reactions
The concept of exothermic and endothermic reactions was first explained in the Introduction page of the "Chemical Reactions" unit, but we will briefly review it now.
Exothermic reactions are processes that release heat, such as combustion. Although it may not seem like it, the condensing and freezing of a substance are also exothermic reactions, since heat must be released to change to the lower state.
Endothermic reactions are processes that absorb heat from their surroundings. Melting water may be the most familiar reaction, since melting a block of ice in your hand clearly takes heat away from your hand.
Note that if the reaction is reversed, the heat change is also reversed. If this explanation is not clear, please review the more detailed text at the beginning of the "Chemical Reactions" chapter.