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What are the conditions for the evaporation of water?

I know that for the boiling of water, the vapor pressure of water must equal the partial pressure of water vapor.

But what about for evaporation? Is it the same condition above in addition to the requirement that the relative humidity cannot be 100%?

Can we have boiling when the relative humidity is 100%?

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Evaporation is a surface phenomenon, boiling is a bulk phenomenon. Evaporation will always be occurring for any liquid with an exposed surface. Water molecules in a liquid sample have a distribution of kinetic energies. It takes less work for a surface water molecule to break free from its neighbors, then for a water molecule in the bulk to escape, since the former has fewer neighbors (no molecules on top). Those molecules with enough kinetic energy to break away from their neighbors will escape.

For water to boil its vapor pressure must equal the atmospheric pressure.

The only requirement for evaporation to occur is an exposed liquid surface. I don't think relative humidity affects the boiling point, but I suspect it would affect the rate at which a liquid will evaporate from an open container. Evaporation is always occurring, but to notice a change in the amount of material in an open system, more material must leave the surface then returns to the surface and the rate of return will be affected by the composition (water content) of the air above the open sample.

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  • $\begingroup$ So is delta G for evaporation always negative? $\endgroup$ – Dissenter Jul 26 '14 at 23:20
  • $\begingroup$ @Dissenter No, $\Delta G$ is not negative for metals, or if it is, then it must be very close to zero. $\endgroup$ – LDC3 Jul 27 '14 at 1:02
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    $\begingroup$ $\Delta G$ is negative if the process occur spontaneously, which is to say if the partial pressure of the substance's vapor happens to be less than the vapor pressure of the substance at the given temperature. $\Delta H$ for evaporation is always positive, of course; but because the entropy of a gas is greater than the entropy of the same substance in the liquid phase, the value of $\Delta G = \Delta H - T\Delta S$ for evaporation can be positive, negative or zero. If negative, net evaporation occurs; if positive, net condensation; if zero, equilibrium. $\endgroup$ – Silvio Levy Jul 29 '14 at 0:13
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Water evaporates (in the sense that more water leaves a body of liquid or solid water than comes back to it) whenever the partial pressure of water vapor in contact with the liquid or solid water is less than the vapor pressure of water at the given tempearture. This is the same as saying that the relative humidity is less than 100%, by the definition of relative humidity.

Can we have boiling when the relative humidity is 100%?

You can. Suppose you have a flask of water with a narrow neck and surrounded by thermal insulation. If you heat the water to the boiling point you will soon displace all the air, so both the liquid and gas phases are 100% water, and the water will continue to boil as long as you keep supplying heat.

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  • $\begingroup$ Would there be any net evaporation at a relative humidity of 100%? $\endgroup$ – Dissenter Jul 28 '14 at 23:55
  • $\begingroup$ No net evaporation when the relative humidity is 100%; the rate at which molecules leave the liquid matches the rate at which (other) vapor molecules enter the liquid. $\endgroup$ – Silvio Levy Jul 29 '14 at 0:04

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