I was walking through our undergraduate organic chemistry labs the other day and one student noticed that while she was evapourating solvent under vacuum that the flask got quite cold. She asked me why this was. Here is my answer.
Endothermic is the physical term used chemical processes that absorb heat from the environment to proceed. This is opposite to the term exothermic, which are chemical processes that give off heat to the environment as they proceed. In order to for molecules to change state from liquid to gas they require an input of energy. This energy comes from the surrounding environment. If heat leaves the environment and goes into the system you are observing (in this case the from the flask to the solvent), then the environment will become cooler. This is why sweat actually cools your body. As the sweat evapourates from your skin, it takes with it heat leaving you cooler.
Endothermic reactions are also what take place in those cooling packs. The spontaneous reaction inside is endothermic, taking heat from the surrounding environment and the result is the pack is cold. Exothermic reactions are what are in heat packs. The reaction gives off heat, making the environment hot.
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