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I was reading about the vaporisation of liquid when I came across the following statement.

The average energy of the particles in a liquid is governed by the temperature. The higher the temperature, the higher the average energy. But within that average, some particles have energies higher than the average, and others have energies lower than the average.Some of the more energetic particles on the surface of the liquid can be moving fast enough to escape from the attractive forces holding the liquid together. They evaporate.

Now if some particles have energies lower than the average, then why don't they convert into solids in the same way the particles with higher energies convert to gases.

If we cover a vessel filled with water with a plate, after some time, we observe droplets of water accumulated at the plate because of vaporisation of water. Why don't we observe any changes that show liquid particles converting to solid?

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If water molecules go into the gas phase from the liquid then they are in a different phase. But water molecules go into the gas phase one at a time. If there are the same number of water molecules leaving the liquid as coming into it then the atmosphere above the liquid would have 100% humidity and be in equilibrium with the liquid.

Now if some small number molecules of water happen to line up to form an ice crystal within the water itself what happens? Such an ice crystal is thermodynamically unstable and then "melts" within the liquid phase. Since the whole process takes place in the liquid phase it is virtually undetectable.

Now you also have to realize that form ice crystals big enough to detect visually requires more than a handful of molecules. You'd need billions of billions of molecules. That is just a statistical impossibility.

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  • $\begingroup$ why is the crystal thermodynamically unstable? $\endgroup$ – Arishta Jan 31 '17 at 18:01
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    $\begingroup$ Thermodynamics says that you shouldn't have ice in water that is above 0 C. $\endgroup$ – MaxW Jan 31 '17 at 18:07
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    $\begingroup$ Thermodynamics also says that you shouldn't have liquid water that is below 0 degrees C. Kinetic barriers allow it to happen anyway. Just a comment on your comment, not relevant to the question ;) $\endgroup$ – airhuff Jan 31 '17 at 21:35
  • $\begingroup$ @airhuff - Understand, but where do you try to cut such an answer off? $\endgroup$ – MaxW Jan 31 '17 at 21:47
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    $\begingroup$ Not sure I understand the statement. I +1'd the excellent answer and made it clear that my commentary was only directed at your previous comment. Just highlighting the point that thermodynamics alone doesn't rule all physical / chemical processes. For further example, you can also have, with great difficulty, superheated ice. In one experiment ice was superheated to 17 degrees C (albeit for a few hundred picoseconds). I think these things would be of interest to the OP even if his question has been appropriately answered. I hope I haven't misunderstood your question. $\endgroup$ – airhuff Jan 31 '17 at 22:13
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The situation you describe, where both solid and liquid exist, would be possible around the freezing point. Coexistence of vapour, solid and liquid is possible if there is a the triple point (for water: 273.16 K; 611.657 Pa).

But in standard conditions, MaxW is right the crystal melts, because the entropy gained by spatial randomization of the particles compensates the energy lost by breaking their packing.

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