1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Cooling when evaporating

  1. Jan 9, 2012 #1
    Can someone please explain to me why a liquid cools down when it evaporates:-D
     
  2. jcsd
  3. Jan 9, 2012 #2

    russ_watters

    User Avatar

    Staff: Mentor

    Re: coolling when evaporating

    Evaporation removes heat. "Heat of vaporization".
     
  4. Jan 9, 2012 #3

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    Re: coolling when evaporating

    A more 'nuts and bolts' explanation is as follows. Molecules in the water have a range of kinetic energies. The faster molecules on the surface have enough energy to take them up into the surrounding air and to become part of the surrounding water vapour.
    It is the Mean KE of molecules in the water that determines the Temperature and, having lost some of the faster molecules, the mean KE is reduced - hence the temperature drops.

    Once the air near the surface is 'saturated', there will be as many molecules returning (adding their KE to the rest) as leaving so net evaporation will be zero and the temperature will cease dropping. This loss and return of molecules is often described in terms of 'vapour pressure' (changes of Momentum on the surface).

    When there is a current of unsaturated air over the surface, the escaping molecules get swept away from the surface and so more evaporation will take place, reducing the temperature further (e.g. on the washing line on a windy day or standing in a wind after a swim).

    This explanation does away with the notion that air is, somehow, like a sponge - absorbing the water. The molecules of water vapour are in no way different from the O2 and N2 molecules also in the air; all three gases exert their own pressure and add it to the final pressure in the air. The same vapour pressure would exist over the surface of water if it were in a container which had been initially evacuated.
     
  5. Jan 9, 2012 #4
    Re: coolling when evaporating

    That's a really nice explanation.
     
  6. Jan 9, 2012 #5
    I don't believe that's always true, Russ. Cold water will evaporate into warmer air quite readily. If we define heat as energy that moves from a warmer body to a cooler one solely because of the difference in temperature, then this KE of vaporization is not heat.

    Moreover, these vaporizing water molecules do not "carry" any latent heat. We know a lot more about molecular structures now than we did when Black first proposed his hypothesis of "latent" (hidden) heat back in the 1850's. Quantum physics has given us a pretty good insight into both the internal and external mechanisms by which energy is stored and expressed by molecules. There has been no convincing evidence of latent heat or enthalpy of transformation stored anywhere--hidden or overt--in liquid or gas molecules.

    If the water is hotter than the air, then some net amount of heat will be transferred from the water to the air. If the water is the same temperature or cooler than the air, then no net amount of heat will be transferred to the air. Enthalpy transfer, yes. Heat transfer, no.
     
  7. Jan 9, 2012 #6

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    Re: coolling when evaporating

    Energy will be lost due to evaporation but gained due to the temperature difference. If you covered the surface of the water with a thin layer of oil, it would warm up quicker than with the surface uncovered. This is how one can normally stay alive in a sauna!
    The actual net flow direction will depend on the relative temperature difference and the relative humidity. You will die in a sauna full of vapour because your sweating will not produce sufficient evaporation.

    You are right to say that molecules don't "carry latent heat" because one is a microscopic concept whilst the other is macroscopic. The escaping molecules still carry energy away, though.
     
  8. Jan 10, 2012 #7
    Re: coolling when evaporating

    Also note that the hot air surrounding the liquid heats up the liquid and increases the KE of the molecules in the liquid. There are cases (combination of certain liquids and initial temperatures) where the cooling due to evaporation is smaller than the heating due to convection of heat from the surrounding into the liquid, and the net effect is a heating of the liquid. But the cooling 'term' in the heat balance equation is due to evaporation.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook