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I'm confused about why the boiling of liquids happen. The boiling point of a liquid is defined as the temperature in which it's vapor pressure is equal to the pressure of the gas above the liquid( for example the atmospheric pressure ), and vapor pressure is defined as the (temperature dependent) partial pressure of the gas state of the liquid in which the rate of condensation is equal to the rate of evaporation. So for example, let say we have a beaker of water at sea level( 1 atm of atmospheric pressure) and we heat it constantly( for example with a bunsen burner ) to 100ºC, since at that temperature the vapor pressure of water is equal to the atmospheric pressure of water, we have heated it to it's boiling point. What I don't understand is why after some time all beaker's water will became a gas and no liquid water will be left in the beaker, because the bubbles of water gas that form in the beaker will rise( due to lower density ), but once the reach the level of water they should constantly condense and evaporate at the same rate( due to the vapor pressure ) so the level of water should remain constant, because the amount of water that leaves as a gas is the same that condenses to the liquid, but we know that after some time all water will evaporate and diffuse into the atmosphere, so no liquid water will be left in the beaker.

I'm confused about this, why does the liquid completely evaporates when according to the vapor pressure, gas and liquid should be in equilibrium and some water should remain liquid in the beaker? What am I missing?. Thank you for your help, I haven't really found any explanation to this in the internet or in a textbook.

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The water vapour dissipates into the room, so the vapour pressure near the water surface is a bit lower than equilibrium. If we stop the water vapour escaping by placing the water in sealed, but variable volume (to keep the pressure at atmospheric) container, the water may not all turn to vapour at the boiling point. Whether it does so or not depends on the volume of the container and the amount of water

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  • $\begingroup$ I thought you said that the volume is not constant. You can easily evaporate all the water if the pressure is held constant and the volume is allowed to increase. $\endgroup$ Nov 20 '20 at 16:43
  • $\begingroup$ @Chet Miller: You are right. I was not thinking clearly because I was worried wwhat would happen the to any air. If there is only water and water vapour, and one holds at fixed $T$, then it is the volume that determines the fractions of water and vapour. $\endgroup$
    – mike stone
    Nov 20 '20 at 18:16
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We are dealing with a non-equilibrium situation here. Plus, the release of water vapor at the surface is not controlled by evaporation at the surface.

The bubbles that form within the water below the surface are all basically 100% water vapor (that has already evaporated). When they rise to the surface, the water vapor is not released by evaporation. Once the bubble breaks the surface, it is virtually entirely released into the room air, and rapidly disperses (convects and diffuses) into the room air, so that its partial pressure essentially drops close to the bulk room air value (way less than 1 atm), and cannot condense back at any significant rate.

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