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I Liquid boiling and Evaporation

  1. Nov 27, 2017 #1
    In the normal conditions (sea level) water evaporates at 100 C.

    In thermodynamics, we say: the amount of energy Q, can raise temperature of the liquid by the formula Q1=cm(t2-t1); when the liquid reaches the boiling point (100 C), we write Q2=Lm.
    Q2 is entirely spent on changing liquid state into gaseous state, but as I know it's not necessary for liquid to evaporate (liquid goes into the gaseous state more and more rapidly as we approach the higher temperatures).

    The question is: isnt the energy we give, is partially spent on changing state on the <100 C temperatures; and if so - why we dont consider it
  2. jcsd
  3. Nov 27, 2017 #2


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    Staff: Mentor

    I think I understand your question and it is really a matter of specifying what part of a chain of events you are referring to. So the way I view it is this: when the temperature is <100C, all of the energy put into the water is to change its temperature. But simultaneous to that, some energy is being removed from the water (lowering its temperature) by convection and evaporation (if we're talking about a pot on a stove). You can add the three effects (superposition) to find the net effect (temperature is only rising).
  4. Nov 27, 2017 #3


    Staff: Mentor

    Your question is a bit muddled.

    It sounds like you are saying that Q1 is the heat required to warm the water, and Q2 is the heat of vaporization. But if you put in Q2 additional heat, all the water will be boiled off into steam while the temperature remains constant at 100C. But you don't need to boil any of the water.

    You may be confusing evaporation of water from the surface with boiling. It's explained better in this WIKI ariticle.
  5. Nov 27, 2017 #4


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    But we DO consider every possible mechanism of Energy transfer when we try for the highest accuracy. Evaporation is very dependent on the experimental details and it is very hard to estimate so heat experiments are often done in sealed containers and, of course, they are well insulated and their thermal capacity is also included in calculations.
    Calorimetry is a highly sophisticated branch of measurement but it is not one of the most accurate - not compared with Frequency and Time measurement, for instance.
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