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I understand that at freezing and condensation points, potential energy of molecules are given off to form intermolecular bonds while the kinetic energy of the molecule stays constant.

My question is, why is this so? Why can't kinetic & potential energy be lost concurrently? It seems reasonable for, say, half of the molecules to lose their kinetic energy while the other half lose their potential energy. What is so special about these intermolecular bonds?

Is the "intermolecular bonds formed" just stronger intermolecular electrostatic forces of attraction? If so, then wouldn't it make more sense to for these bond strengths to be continuously increasing instead of only increase at 2 distinct points (freezing and condensation points)?

P.S. I have chosen to ask this question from a cooling perspective because, even though it is symmetrical to heating perspective, the argument here is generally more hand-wavy.

Tham
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1 Answers1

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wouldn't it make more sense to for these bond strengths to be continuously increasing instead of only increase at 2 distinct points

There's a misconception here: the bond strengths don't increase at those points, that's when they form at all.

It's probably easier to think of it like that: the molecules, as they lose energy (i.e., as the system cools down), at some point are not able any more to escape the energy well of the intermolecular attraction and so form bonds (condense or freeze).

And then there's a key point, which is often left implicit, which can be confusing: traditionally, temperature is only defined for systems in equilibrium. This means you have "to wait" until any energy change spreads through the entirety of the system — and then it's clear that any infinitesimally cooler portion of the system, say, a cool chunk of ice, will absorb heat from the surrounding water, freezing it: and the system can only start to cool down further after it's wholly frozen.

stafusa
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