endothermic

Endothermic describes a process or chemical reaction that absorbs heat from its surroundings. In thermodynamics, the enthalpy change, ΔH, is positive for an endothermic process, meaning the system requires energy input. This contrasts with exothermic processes, in which heat is released and ΔH is negative.

In chemistry, endothermic reactions often occur when bonds are broken faster than new bonds are formed, requiring energy input. Common examples include the melting of ice at 0°C, sublimation of solid carbon dioxide, and the vaporization of water. Photosynthesis in plants is a well-known endothermic process, driven by absorbed sunlight to convert carbon dioxide and water into glucose and oxygen.

Endothermy also appears in biology as a separate concept: endotherms are animals that generate and regulate body heat metabolically to maintain a relatively constant internal temperature, as seen in mammals and birds. This physiologic endothermy requires substantial energy intake and is influenced by activity, ambient temperature, and physiology. It is distinct from chemical endothermy, though both involve heat flow into a system.

Measurement and applications: Calorimetry determines ΔH for reactions and phase transitions. Endothermic processes underpin cold packs, certain dissolution reactions, and many refrigeration cycles, which rely on absorbing heat during vaporization or dissolution. Understanding whether a process is endothermic helps predict temperature change of surroundings and energy requirements.