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removing nitrogen, oxygen etc. from a volume of air so that only water vapour remains, what will be the temperature? If it isn't above 100C why doesn't the water vapour condense?
but I thought it was the most energetic particles that left a liquid?so if you took a volume of air at room temperature and pressure, somehow disappeared all other particles apart from water particles would you be left with a volume of pure water vapour at 23C (no equilibrium)?imagine you could somehow do it instantaneously so the particles did not experience a pressure drop
To explain take a triple point cell, whic is a simple device to provide a temperature reference point that is dependent only on the physical properties of water. It is a cell where water can exist simultaneously as a solid, a liquid and a gas, but only at around -0.1C Outside this precise point the water is either all liquid and gas or solid and gas, with it becoming all gas at well above 100C.At the triple point all 3 phases exist, at the same temperature.The probability that a molecule is vapour or liquid is dependent on the energy of the molecule, whilst the temperature is a measure of the average energy of the particles. Thus as the energy is spread over a large range, some have enough energy to be a vapour.
mcjhn Your thought experiment verges on magic but could just about be performed and we need to think it through properly. Let us assume we have some air saturated with water vapour at room temperature in a closed vessel and then removed almost all the oxygen and nitrogen by exposing some non volatile solid or liquid chemical absorbing agent to the air (this would leave a few residual gasses but that is not relevant to the experiment. the pressure in the vessel would drop considerably causing the remaining gas and water vapour to expand to fill the volume
ok, sorry, add to the room the a glass of water, the water vapour in the air and the liquid water are in equilibriumthe glass, water and air are at room temperature/pressure.is it not only the more energetic water molecules that evaporate? (this is why wet skin gets chilly quickly in a breeze)Graph of liquid water moleculestherefore if you were to magically remove everything from the room apart from the water vapour, if you measured the instantaneous temperature it would be above room temperature as only the water molecules of higher than average energy evaporate??Red box around the energies of water vapour moleculesif the water vapour molecules did not have these higher than average energies why don't they condense back into a liquid?yeah your right i don't think its possible but as a thought experiment..
The killer point is, I think, that in order to leave the liquid, the molecules have to lose some energy to it. A molecule at the surface is attracted back by dipole-dipole interactions with the water below it.They have to overcome that force and, in doing so they lose (on average) their "excess" energy.