I was thinking about the difference between air and water, and how water evaporates as steam and becomes air. This led me to believe that air is just evaporated water. However when I read about air it said it consists of nitrogen and other gases, which water doesn't contain. So my question is what is the difference between air and evaporated water (steam) and what happens to steam when it mixes with air?

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    $\begingroup$ Look at pretty much everything around you. Wood is very different from concrete, which is different from metal, which is different from glassware. Why? $\endgroup$ Mar 29, 2018 at 8:11
  • $\begingroup$ @IvanNeretin: I haven't studied any chemistry yet, but is it because of the different arrangement of atoms? $\endgroup$
    – Raghib
    Mar 29, 2018 at 8:25
  • $\begingroup$ Well, yes, and sometimes different types of atoms, too. Same thing with air and water steam: they are about as different as it gets. $\endgroup$ Mar 29, 2018 at 8:54
  • $\begingroup$ Evaporated water doesn't become air it becomes a gas, water vapour. Your confusion is between the terms gas and air: air is a specific mixture of gases; water vapour is just water in gas rather than liquid form. $\endgroup$
    – matt_black
    Mar 30, 2018 at 10:53

1 Answer 1


Air is a mixture of many gases. It contains elemental gases such as $\ce {N2}$ (nitrogen; $\ce {78.1\%}$), $\ce {O2}$ (oxygen; $\ce {20.9\%}$), and $\ce {Ar}$ (argon; $\ce {0.9\%}$), as well as gaseous compounds such as $\ce {CO2}$ (carbon dioxide; $\ce {0.03\%}$). The percentages are given by volume for dry air. That means, the first two gases, $\ce {N2}$ and $\ce {O2}$, make up $\ce {99.0\%}$ of the atmosphere by volume. It also has some low level Ozone ($\ce {O3}$) and under ambient conditions, has varying quantity of water vapor as well (as clouds in the sky).

That water vapor in air comes from different sources such as steam water, water evaporated from sea bed, etc. You always hear how much vapors in air during everyday weather forecast. That has specific name--humidity. And there is an another measure to give you an idea how much vapor in air in a given day. That called the dew point. The dew point is the temperature the air needs to be cooled to (at constant pressure) in order to achieve a relative humidity (RH) of $\ce{100\%}$. At this point the air cannot hold anymore water in the gas form. If the air were to be cooled even more, water vapor would have to come out of the atmosphere in the liquid form, usually as fog or precipitation (you'd see this more often in spring day mornings). Hope, this'd be enough to give you an idea what happens to vapor when they get into air.

  • $\begingroup$ Note that the percentages mentioned in the first paragraph are on a dry basis, i.e. without considering water vapour. The percentage of water in air saturated with water vapour, i.e. at 100%RH, is around 4%, for instance. $\endgroup$ Mar 29, 2018 at 11:17
  • $\begingroup$ @Eashaan Godbole: Thank you for the clarification. I have edited the phrases, accordingly. $\endgroup$ Mar 29, 2018 at 18:57
  • $\begingroup$ santimirandarp: If actual temperature falls behind the dew point (say, dew point is $\ce {65}$ degrees but during the night atmospheric temperature down to $\ce {55}$ degrees), then precipitation occurs. That's why we see water droplets on plant leaves (called dew!). $\endgroup$ Mar 29, 2018 at 20:51

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