Question
Thu June 21, 2012 By:

rubbing isopropyl alcohol gives a cooling sensation on the skin. why?

Expert Reply
Thu June 21, 2012
Any object will absorb heat from its surroundings if the surroundings are warmer than it. Since room temperature is usually lower than body temperature, alcohol is usually colder than your skin. When you put alcohol on your skin, first, your body will warm the alcohol until it is the same temperature as your skin, and that feels a little cold to you (just like when you touch any object at a lower temperature than body temperature, like a piece of metal).

Also, evaporation plays an important role. This is because to turn a liquid into a gas, you have to break all of the intermolecular forces that hold that liquid together (these can include Van der Waals foces, dipole-dipole forces, and in the case of an alcohol, hydrogen bonding). Breaking all those attractive interactions requires a lot of energy. When alcohol evaporates on your skin, your body is supplying most of the energy to break all of those intermolecular interactions. Any endothermic reaction will feel cold. When you use one of those instant cold-packs, a special endothermic chemical reaction inside the pack is what makes it so cold suddenly.

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