1. The problem statement, all variables and given/known data Hi, I am came across the following question and can't quite make complete sense of it: "Explain, in terms of internal energy, why a liquid cools when it evaporates". 2. Relevant equations 3. The attempt at a solution Could someone tell me whether this questions is implying random surface evaporation or boiling? Internal energy is the sum of the internal kinetic energy and the internal potential energy. I would say that in order to evaporate a liquid energy must be supplied to overcome the bonds (latent heat of vaporisation). Since this must come from the water, the water itself must lose heat energy and cool down. The thing that is confusing me is that it says in my book that vaporisation occurs at constant temperature so wouldn't the liquid water stay at the same temperature? Or by evaporation does it mean it is not boiling therefore there is no heat being supplied to the system hence the water cools down? How would you explain the overall changes to the internal energy of the system? My attempt (assuming we are talking about random evaporation as oppose to boiling where the temperature would therefore not be constant): Some of the energy from the water is transferred to surface molecules giving them enough kinetic energy to escape. Since they have evaporated their potential energy will also increase. The potential energy of the water left behind will decrease as will the kinetic energy (and hence temperature).