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Suppose the pressure is 1 atm and we put water (at 25 °C) in an open container. After some time the water will disappear as the water evaporates. We know that water at 25 °C and 1 atm has a vapour pressure. And this vapour pressure is obviously less than the atmospheric pressure or external pressure as no boiling is occurring at this pressure, temperature. If they were equal, boiling would occur which is clearly not happening.

So, if the vapour pressure is less than the atmospheric pressure, then how are are liquid molecules converting into gaseous molecules? I mean the atmospheric pressure above the liquid is exerting a greater force on the liquid and it's vapour pressure is less than the atmospheric pressure.

Then how are the liquid molecules overcoming the atmospheric pressure and changing into gaseous molecules if their vapour pressure is less than the atmospheric pressure? They can only change into gaseous molecules when they can overcome the atmospheric pressure which can occur when their vapour pressure equals atmospheric pressure.

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I remember my days when I started confusing myself in the exact same way. The issue I believe is that you haven't been explained the details of how all this happens.

Evaporation: All molecules in a liquid have various amount of energy. But the ones at the top, the ones that form the surface - they have the maximum energy in them because they are present there by overcoming any forces of attraction. (The other way to imagine this would be, the layer of molecules at the surface will have mostly downward forces of attraction on them compared to the distributed force of attraction from all directions on a bulk molecule.) Now these surface molecules have the tendency to escape. These molecules will individually leave the liquid by getting rid of the forces of attraction because it is easier for them compared to bulk molecules. And when individual molecules leave the liquid, it is called evaporation. Although this is a form of vapourisation, it doesn't need a constant temperature where the change of state occurs. They are just individual molecules that leave the liquid and escape and cause a vapour over the liquid.

Boiling: But in boiling, a lot of constant heat supply, constant temperature, constant pressure. All this is required. And when this is obtained, the liquid changes its state and becomes a gas. This gas forms bubbles and escapes from the bulk of the liquid as well and not only from the surface as in case of evaporation.

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  • $\begingroup$ Surely having mostly downward forces of attraction is a good reason for them not to escape? $\endgroup$
    – user20574
    Commented Apr 20, 2021 at 17:13
  • $\begingroup$ Imagine you are trying to climb up a tree in order to escape zombies. Consider two situations: 1) held from all directions by zombies 2) held only from below by zombies. In which case are you more likely to escape and climb up the tree? Also, the surface area of a molecule exposed to attractive forces is much lower at the surface of the liquid relative to bulk. Just like one zombie pulling you from below versus 5 holding on from all directions. You are just overthinking. This happens alot even to me as well. As Poutnik has already mentioned, once the energy is enough to escape, it escapes. $\endgroup$
    – Desai
    Commented Apr 20, 2021 at 19:52
  • $\begingroup$ I'm more likely to move upwards if there are zombies pulling me upwards, than if there aren't. $\endgroup$
    – user20574
    Commented Apr 21, 2021 at 8:59
  • $\begingroup$ Yeah ofcourse but there are like five pulling you in all directions why are you biased? $\endgroup$
    – Desai
    Commented Apr 21, 2021 at 9:09
  • $\begingroup$ if there are five pulling me downwards, I'm pulled downwards on net, and if there are five pulling me downwards and five pulling me upwards, I'm not. $\endgroup$
    – user20574
    Commented Apr 21, 2021 at 9:13

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