Before giving you an injection, a physician swabs your arm with isopropyl alcohol at room temperature. Why does this make your arm feel cold? (?Hint: The reason is not the fear of the injection! The boiling point of isopropyl alcohol is 82.4o C.)
Solution 18DQ Step 1 of 2: The reason for this is latent heat of evaporation. That is, when liquid evaporates it absorbs some certain amount of heat energy from surrounding to go from liquid to gaseous state. That energy is released when gas condenses back to liquid. Similarly as isopropyl alcohol has lower boiling point(temperature at which liquid changes as gas) and is rather volatile; that is, even at room temperature it will evaporate quickly. This evaporation requires energy, and the result is that while alcohol is evaporating, it absorbs heat (and thus feels cold) from both the skin underneath it and also from the air around it. Thus skin feels cool as the alcohol evaporates away.