# Help me in understanding the general principles behind evaporation and condensation of a liquid

I will place my two questions infront of you guys. Let's consider the liquid is water.

1) " How and why exactly does water vapor in air condense at the surface of water? Is vapor in air condensing at some rates all the time at all temperatures without a water surface? What are the factors influencing the rate of condensation? "
( I heard condensation near the surface occurs because vapor molecules at the surface lose energy by getting attracted to the surface water molecules and losing kinetic energy from striking them and form bonds with them and become water. )

2) Say, air is at relative humidity of 100%, which as I understand, means rate of evaporation of water is equal to rate of condensation of vapor and that there is zero net evaporation of water. It implies that after reaching a certain level of concentration of water vapor in air (vapor pressure), even if I increase the rate of evaporation to whatever amount by supplying heat to the water, rate of condensation will get equal to that. I understand this point is also called as dew point.

MY QUESTION explicitly is, why does the amount of increased fraction of molecules escaping the given water surface area will be equal to the increased fraction of water vapor molecules that are gonna get condensed? What's the reason for this saturation point? Also, does this mean if I place water at low temperature i.e for which rate of evaporation is less than the rate of condensation of fully saturated air (RH=100%) above it, will water level rise?

I'm not a physics college student. Just a curious guy trying to make sense of nature in simple mathematical models. So, please explain as simple as you can guys.

• You appear to be trying to use kinetics (rates) to understand thermodynamics (energetics). This is a mistake. Sep 24, 2018 at 16:39
• Sorry but I'm not sure what you mean. Is that a sarcastic remark on the tags I put for my question ? Sep 24, 2018 at 16:46
• No, not at all. Thermodynamics deals with things like the Gibbs free energy of phases, from which one can calculate phase stability and relative phase fractions under specific conditions. Kinetics deals with how fast (if at all) one might get to thermodynamic stability. Visualizing thermodynamic stability in terms of molecule kinetics is going to get you into trouble. Sep 24, 2018 at 16:51
• I can see your point, Jon. I'm waiting for other answers to clear my thinking. Sep 24, 2018 at 17:24
• Related (a very nice answer!): physics.stackexchange.com/a/773781/226902 Jul 27 at 19:31