Condensation, the formation of a liquid from a vapor or gas. It occurs in the formation of clouds and when cooled, water vapor turns into dew or rain. Condensation takes place when the gas or vapor is cooled or compressed (or both), and heat is given off in the process. It is what physicists call a change of state, involving an increase in density of the substance without any chemical change.

Condensation is used in liquefying carbon dioxide and ammonia in refrigerating systems. Natural gases are liquefied for convenient transportation and storage. Artificial rain is produced sometimes by seeding clouds with particles of dry ice around which water vapor condenses. Condensation also is part of the process of distillation.