So I was wondering why does condensation occur at the molecular level. I can understand evaporation as the external atmospheric pressure being less than the vapor pressure ( energy of the water molecules). This would help the the molecules to escape the hydrogen bonding holding them. But I can’t understand why would molecules want to attach nearby a cold surface as in condensation.

  • $\begingroup$ Actually, condensation occurs due to change in state of the water. Perhaps I have read your query wrongly? $\endgroup$ – QuIcKmAtHs Mar 17 '18 at 8:49
  • $\begingroup$ @QuIcKmAtHs well look, in the monsoon season when I keep something cold out in the open, after sometime I would find water droplets on the outside of the glass. This happens because of condensation. In evaporation due to higher energy state of the molecules they escape the liquid and ‘evaporate’. Evaporation can also occur due to low atmospheric pressure. What actually causes the deposition of water molecules on a cold surface In condensation. I know it has something to do with the concentration of water molecules in the air(you wouldn’t see this in summer because the air is very dry). $\endgroup$ – physics2000 Mar 17 '18 at 8:56
  • $\begingroup$ Condensation happens when water moves from gaseous to liquid state. It is merely a state change. $\endgroup$ – QuIcKmAtHs Mar 17 '18 at 9:02
  • $\begingroup$ @QuIcKmAtHs exactly, condensation is a state change but in the larger sense. I want to know what happens on the atomic scale. $\endgroup$ – physics2000 Mar 17 '18 at 10:44
  • $\begingroup$ @physics2000, note that the vapor pressure of water is less than atmospheric pressure until you reach or exceed the boiling point of water. $\endgroup$ – David White May 12 '20 at 18:33

When water molecules in water vapour come in contact with cold surface, they transfer a part of their kinetic energy to the cold surface, which causes them to condense as water droplets. When humidity is high, the air is saturated with water vapour, so condensation occurs pretty easily. When the vapour concentration in air is low, there are less chances of transfer of kinetic energy to take place, thus condensation is difficult.


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