Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Boiling/ Melting .

  1. Aug 22, 2004 #1
    Boiling/ Melting.....

    I take it everyone knows the graph of temperature when you heat ice / water - with the flat bits when it melts and boils. My question is why is this the case? Obviously the heat energy you are putting in is going to kinetic energy of the particles when the graph is not flat, but when it is flat and the temperature is not changing what is the heat energy being converted into?
    Thanks in advance. :wink:
  2. jcsd
  3. Aug 22, 2004 #2

    Claude Bile

    User Avatar
    Science Advisor

    The extra energy goes into overcoming the various forces that hold a solid (or liquid) together, that is, it contributes to the molecules' potential energy rather than its kinetic energy.

  4. Aug 22, 2004 #3
    there is boiling point which the liquid will not go any higher then that temperture, extra heat energy put in only keeps at that level. other heat energy is just wasted.

    if that heat energy keeps applying i guess the liquid will vaporize.

    (note the process do not include any extra pressure to the liquid, so that the boiling point will not vary)
  5. Aug 22, 2004 #4

    Doc Al

    User Avatar

    Staff: Mentor

    Just to add to what Claude already explained: Realize that even when water is being heated, most of the energy goes into weakening the intermolecular bonds, not into increasing KE.
  6. Aug 24, 2004 #5

    You refer to the phase diagram or the T-S (temp - entropy) chart?

    Either way, what is really happening is you put energy into the system, in your case liquid water. The energy is not absorbed unevenly, so you must raise the entropy of the entire system a certain degree before you can begin to change phases, i.e. from liquid to gas. This is commonly called the heat of transformation,

    [tex]\triangle S=\triangle Q /T_{abs} [/tex]

    Where S is the change in entropy (note you cannot calculate S, only a change in S), dQ is the heat added to the system, and Tabs is the absolute temp (in deg. R).

    Entropy was really invented to describe the second law of thermodynamics, and it really tells you which direction a phase chance will proceed at a given set of parameters (will it go back to water, or continue to change to gas?).

    Hope this helps.
    Last edited: Aug 24, 2004
  7. Aug 24, 2004 #6
    I was refering to the phase diagram.

    The one thing that bugs me is when you are heating the substance (in my example ice/ water), when you reach the changes in state is it that with heat energy you are adding no longer is used to increase kinetic but potential energy OR that the particles begain to gain potential energy as they drift apart (ie - ice to water), and the kinetic energy being lost is replenished by the heat energy you are adding? (thus flat points on graph)

    Also, can anyone please explain why the potential energy of the particles increases at these points rather than kinetic? Ive tried, but i find its rather hard to explain :wink:
  8. Aug 24, 2004 #7
    Electric attraction between hydrogen atoms in one water molecule and oxygen atoms in other water molecules have to be overcome if individual water molecules are to leave the surface of the liquid and become vapour.As more heat is added to the liquid water, the average distance between hydrogen atoms of one molecule and oxygen atoms of another molecule increases (and this amounts to an increase in potential energy) but there is resistance to this happening
    (think of stretching a spring - it gets more difficult to do the more you stretch it)
    and a lot of heat energy is needed.There comes a point at which the spring snaps -
    the hydrogen-oxygen attraction between different molecules is overcome - and large numbers of water molecules can escape from the liquid.This is when the liquid boils.
    Last edited: Aug 24, 2004
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?