I know that density of moist air is less than density of dry air becuase water molecules replace air molecules, and hence as the average molecular mass of water is less than that of air, the density decreases.

Now my doubt is why do the water molecules replace air molecules, why don't they just get mixed up with air molecules without replacing the already existing ones?

I assume the answer might be due to atmospheric pressure( to maintain it almost constant). But I am not able to find a logic to it.

Please explain. Also suggest if I have to add any extra tags relating to the topic.

  • $\begingroup$ The water vapor and air separate because of their difference in densities. $\endgroup$
    – Sam
    Commented Jan 20, 2020 at 6:02
  • $\begingroup$ Kindly elaborate or refer to a source $\endgroup$
    – sheshin
    Commented Jan 20, 2020 at 6:43
  • $\begingroup$ Basic Archimede's Principle. It's just like how oil floats on top of water. $\endgroup$
    – Sam
    Commented Jan 20, 2020 at 9:15
  • $\begingroup$ The water molecules DO just get mixed up with air molecules; they don't replace air molecules. $\endgroup$ Commented Nov 22, 2021 at 16:48

2 Answers 2


The volume of the Earth’s atmosphere isn’t fixed. It can vary slightly for a number of different reasons, including a slight increase due to water evaporation and So locally, dry air molecules can move out of the way to make room for gaseous water molecules during vaporization and evaporation.

Hope this helps


I'm not a professional. It is only my thought 👎may you find it useful

If water molecules can dissolve in air then, their would be no change in volume. But as we know, water dosen't dissolve instead it only makes a mixutre with air. So water vapors fight for their volume too therefore increasing volume and decreasing density. Now think about water vapors given off by evaporation. They contain relatively less energy as compared to vapors of boiling water. Therefore the vapors of evaporation must take some energy from air to stay in atmosphere. That's why these vapors cause only a little increase in volume of air than vapors of boiling do.

  • $\begingroup$ I don't understand why there would be a difference in water vapor coming from evaporation versus boiling. The original question stands regardless of the source of the water vapor $\endgroup$ Commented Nov 22, 2021 at 14:43
  • $\begingroup$ Yes the question is not about this. I myself added this. I don't know if it actually happpens. But the reason i think is that in evaporation the energy from atmosphere and sunlight is used to make vapor so their is very little increase in temperature of system hence lesser volume of air. While in case of boiling, the energy is given by heating which goes into atmosphere and increases volume of air. You know that evaporation occurs at temperature lower than 100°C at 1atm. Also Volume increases at increasing temperature $\endgroup$
    – user316791
    Commented Nov 26, 2021 at 8:09

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.