The lighter water vapor molecules replace an equal number of heavier oxygen and nitrogen molecules, which makes the volume of air less what?

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Each water molecule is made up of 2 atoms of hydrogen and 1 atom of oxygen. The molecular weight of water is 18 g. Nitrogen molecule is made up of 2 nitrogen atoms and hence has a molecular weight of 28 g. Similarly, an oxygen molecule is made up of 2 oxygen atoms and has a molecular weight of 32 g. Thus, water is lighter as compared to nitrogen and oxygen molecules. This means that when lighter water molecules replace equal number of nitrogen and oxygen molecules, the air becomes lighter. 

Since density is the ratio of mass and volume, we can also state that the density of air decreases as lighter water vapor molecules replace heavier nitrogen and oxygen molecules. In other words, the air volume becomes less dense.

 

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