endothermic

Think of an endothermic process as a "heat sponge." While an exothermic reaction sends heat out into the world, an endothermic one pulls heat in from around it. This is why a chemical ice pack feels cold against your skin: the reaction inside is taking in heat from your body. To keep this kind of process going, you usually need to keep supplying energy. A classic example is melting an ice cube. The ice has to absorb heat from the air or your hand before it can change from a solid into liquid water.

The word "endothermic" comes from Greek roots meaning "within" and "heat." In science, it describes a process, most often a chemical reaction, that absorbs thermal energy from its environment. Because the system is taking in heat, the surroundings often become cooler, which is why these reactions can feel cold to the touch.

In a laboratory setting, chemists identify endothermic reactions by a positive change in enthalpy (ΔH > 0). This indicates that the products of the reaction have more stored energy than the starting materials. A famous example is photosynthesis: plants capture energy from sunlight to transform carbon dioxide and water into glucose. Without that constant intake of light energy, the process simply couldn't happen. Similarly, when you cook an egg, the proteins undergo an endothermic change; they require the heat from the pan to denature and solidify.

Beyond chemistry, the term also has an important meaning in biology. "Endothermic" describes "warm-blooded" animals, such as mammals and birds. These creatures produce most of their body heat through metabolic processes instead of depending mainly on outside warmth. Whether in a test tube or a living organism, "endothermic" points to a system that takes in or produces heat energy in order to function, grow, or change.