When an ice molecule hits a water molecule, the water molecule adds kinetic energy to the ice molecule. Why do the water molecules lose kinetic energy? In other words, why does heat like to go where there is less heat? I know thermodynamics tells us energy can't be created or destroyed. It doesn't really make intuitive sense, though. In the macroscopic world, if a ball hits a moving object in a non-opposing direction to the direction of force initially applied, the object's velocity actually increases, while the ball suffers from Newton's equal-and-opposite-reaction rule.


1 Answer 1


Basically a "molecule" of water cannot heat up ice.

I think what you are trying to say is, how does heat transfer take place on a molecular level? If that's the case, then its something like this. In the interface between water and ice, water molecules are moving, while ice molecules are static. on contact, some molecules of ice acquire velocity (due to no binding forces in one direction, and cohesive forces towards liquid water molecules). as a result, surface molecules of ice start acquiring velocity, hence changes state from solid to liquid (simply, ice melts). and due to conservation of energy, an equivalent amount of kinetic energy (macroscopically, heat) is lost by water. hence water cools a bit.

Of course there may be better ways to explain this, but i tried my best. hope you understand :)

  • $\begingroup$ "I thing what you are trying to say is, how does heat transfer take place on a molecular level?" Yes, that is precisely what I'm asking. $\endgroup$
    – moonman239
    Nov 26, 2015 at 1:21
  • $\begingroup$ The law of conservation energy allows for energy to be converted. Do the water molecules have any other sources of energy? If so, why must they lose kinetic energy? $\endgroup$
    – moonman239
    Nov 26, 2015 at 17:33
  • $\begingroup$ You are right! Any particle, including water, can have 2 kinds of energy: potential and kinetic energy. Water has both. potential energy results from the H-bonding between molecules, which is electrical in origin. Actually, both energies decrease in water molecule. But the potential energy decrease happens in very small amount (because if you study the mechanism in minute detail then you'll see that the potential energy change happens indirectly, as a result of kinetic energy change) compared to the kinetic energy change. Thus that change can be neglected in most cases, including this. $\endgroup$ Nov 27, 2015 at 20:25
  • $\begingroup$ When a water molecule loses kinetic energy, is it converted back into potential energy? $\endgroup$
    – moonman239
    Dec 18, 2015 at 19:50

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.