Exothermic and Endothermic

When something loses heat we say that the process is exothermic (heat is exiting). A simple example of an exothermic reaction is a fire. We make fires to cook food or warm our homes because the reaction gives off heat (which makes us, or our food, warmer).

When something gains heat, we say that the process is endothermic (heat is “endering” – okay, entering, but that doesn’t fit as well with endothermic). An example of an endothermic reaction is what happens in a chemical cold pack (these are the plastic bags that athletic trainers or school nurses have that, when squeezed, get cold and can be used instead of ice on an injury). Inside the cold-pack is an amount of sodium acetate and a smaller bag of water. When the pack is squeezed, the water bag bursts and the water dissolves the sodium acetate. This process takes in heat (from you).

It is important to recognize that exothermic does NOT mean hot (although exothermic things often feel hot) and that endothermic does NOT mean cold. For example, the freezing of water is endothermic (you have to take heat out of the water to cause the phase change) but it certainly doesn’t feel warm.