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I know that when the temperature of the air rises, the maximum amount of Water it can hold before the water condenses to water droplets increases. But why is this - has it got something to do with entropy or thermodynamics?

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  • $\begingroup$ A given vol. of air, when is warm, can carry more water because it has more space. $\endgroup$ – user36790 Dec 7 '14 at 2:21
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This has everything to do with entropy: when the temperature is higher, the benefit of having more water molecules in the air (giving rise to greater entropy) become energetically more favored. This is why water "dissolves better in air" at higher temperatures.

Another way of looking at this (pure statistical thermodynamics): when water is cold, few molecules have the energy to "escape" the surface tension forces that hold molecules in the liquid - and thus few molecules escape. By contrast any water molecule in the air that hits the water has a good chance of becoming part of the liquid. When the rate of escape is low, the equilibrium concentration is also low.

Contrast this with high temperature water: lots of molecules escape so you can have far more molecules in the vapor phase at equilibrium.

Both approaches give you the same answer... saturated vapor pressure increases with temperature.

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This phenomenon has nothing to do with the properties of the air, but the properties of the water in it. Hot air means hot water in the air. Cold air means cold water in the air. Cooling water causes it to condense. This is considering a constant volume.

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protected by ACuriousMind Feb 1 '17 at 16:02

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