Bibloski
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Hi,

Struggling to answer this question from the OCR A Level Physics textbook(A2):

"Explain, in terms of internal energy, why a liquid cools when it evaporates".


Any help is much appreciated.
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Stonebridge
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It cools because it is losing its more energetic (i.e. faster) molecules by evaporation.
And what, in terms of the energy of the molecules, does the temperature depend on?
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Bibloski
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(Original post by Stonebridge)
It cools because it is losing its more energetic (i.e. faster) molecules by evaporation.
And what, in terms of the energy of the molecules, does the temperature depend on?
Thanks for the reply. I would guess that since molecules with enough kinetic to evaporate are leaving the liquid then the average kinetic energy of the water does down and therefore cools. However, I thought phase change occurred at constant temperature? In this example what would happen to the internal potential energy? Would that not increase since evaporating molecules are further apart?
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Stonebridge
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(Original post by Bibloski)
Thanks for the reply. I would guess that since molecules with enough kinetic to evaporate are leaving the liquid then the average kinetic energy of the water does down and therefore cools. However, I thought phase change occurred at constant temperature? In this example what would happen to the internal potential energy? Would that not increase since evaporating molecules are further apart?
Yes, read my answer to this question.

http://www.thestudentroom.co.uk/show...php?p=37051473
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Bibloski
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(Original post by Stonebridge)
Yes, read my answer to this question.

http://www.thestudentroom.co.uk/show...php?p=37051473
Thanks for the link. I'm still confused though. If it is happening at constant temperature then how can the water cool down? I'm still not clear about the situation the question is implying. Is it implying evaporation of boiling water or evaporation of surface molecules where the temperature of the main volume of the water is below the boiling point? Also, if the potential energy is increasing doesn't that increase the internal energy so how does it then cool?
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Stonebridge
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If you have a glass of water at room temperature some molecules evaporate for the reasons already mentioned. The water is not boiling. The evaporation has the effect of cooling the remaining water. In practice, heat enters the glass from the room, keeping the water at a constant temperature. This means the system attains thermal equilibrium and there is no overall change in the internal energy. The energy lost to evaporation is replaced by heat from outside.

When water boils the energy supplied increases the potential energy of the water and this is seen in the breaking down of the bonds between the molecules. This is change of state.
Boiling and evaporation are not quite the same thing.

The thermodynamics is simple.
If you increase the internal energy you increase either the pe, the ke or both, of the molecules.
Temperature depends only on the ke.
The intermolecular bonds in the material are broken if enough energy is supplied. This requires increasing the pe of the molecules.
Internal energy is the sum of ke + pe.
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Bibloski
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(Original post by Stonebridge)
If you have a glass of water at room temperature some molecules evaporate for the reasons already mentioned. The water is not boiling. The evaporation has the effect of cooling the remaining water. In practice, heat enters the glass from the room, keeping the water at a constant temperature. This means the system attains thermal equilibrium and there is no overall change in the internal energy. The energy lost to evaporation is replaced by heat from outside.

When water boils the energy supplied increases the potential energy of the water and this is seen in the breaking down of the bonds between the molecules. This is change of state.
Boiling and evaporation are not quite the same thing.

The thermodynamics is simple.
If you increase the internal energy you increase either the pe, the ke or both, of the molecules.
Temperature depends only on the ke.
The intermolecular bonds in the material are broken if enough energy is supplied. This requires increasing the pe of the molecules.
Internal energy is the sum of ke + pe.
Thank you, that info is very helpful. So, if a liquid is evaporating when the water is at room temperature the water cools (until it gains energy back from the surrounding) whereas during boiling it stays at that temperature constantly. Is that right? So during boiling the liquid isn't cooling when the water evaporates? In you glass water example what would happen to the overall internal energy as you said the kinetic energy would go down but the potential energy would increase?
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tutorpace
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Liquids cool down during evaporation when the particles with higher energy escape from the liquid. Because of this, the liquid left behind has less energy than it did before evaporation occurred. Therefore, the temperature of the liquid is lower. Evaporation does not stop suddenly when the highest energy particles break away from a liquid. In a liquid, particles are moving at different speeds, and they interact. This interaction causes some to slow down and others to go faster. After evaporation begins, the total amount of energy in the liquid is lower, but some of the particles that speed up are able to free themselves. Therefore, evaporation continues.
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Stonebridge
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(Original post by Bibloski)
Thank you, that info is very helpful. So, if a liquid is evaporating when the water is at room temperature the water cools (until it gains energy back from the surrounding) whereas during boiling it stays at that temperature constantly.
That's it.

So during boiling the liquid isn't cooling when the water evaporates?
Correct because there is a constant supply of heat. Switch it off and the boiling stops.

In you glass water example what would happen to the overall internal energy as you said the kinetic energy would go down but the potential energy would increase?
If the glass/water is in thermal contact with its surroundings and remains at the same temperature there is no change in the internal energy (per mole) as the ke lost by the evaporating molecules is replaced by heat from the surroundings. All that happens is you lose some of the molecules, so the total internal energy is only less in so far as there is less water.

If the water is not allowed to absorb heat from outside then it cools. It actually eventually reaches an equilibrium where the number of molecules escaping from the surface is equal to the number coming back in from the atmosphere.
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