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I apologize in advance if this is too basic for this site.

Considering that hydrogen is lighter then oxygen, and water is 2/3 hydrogen, and only 1/3 oxygen, and our "air" is comprised mainly of oxygen, why isn't water lighter then air?

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Water vapor is lighter than air. Liquid water is likewise lighter than liquid "air" (either liquid nitrogen or liquid oxygen is a suitable replacement). –  tpg2114 Mar 14 at 4:11
    
Also, air is mostly nitrogen, not oxygen. If it were mostly oxygen, the atmosphere would be combustible. Which would be cool, but not conducive for life. –  tpg2114 Mar 14 at 4:12
    
@tpg2114 so simply put, liquid is denser then vapor? –  tryingToGetProgrammingStraight Mar 14 at 4:23
    
@tpg2114 oxygen gas isn't directly combustible but in high concentrations it makes just about everything else highly combustible. You can't burn O2 by itself. –  Brandon Enright Mar 14 at 5:15
    
@tryingToGetProgrammingStraight: yes, liquid is denser than vapour. –  John Rennie Mar 14 at 9:35

3 Answers 3

up vote 4 down vote accepted

Air is lighter because there are fewer molecules per unit volume compared with a unit volume of liquid water. A mole of water is 18 grams, so a liter of water contains about 55 moles (1000 grams).

A mole of air at standard temperature and pressure, however, occupies a volume of 22.4 liters, much more. Dividing a mole of 02 (32 grams) by 22.4, you have just a bit more than a gram for that liter of air.

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Air is mostly dinitrogen and dioxygen.

At room temperature water is liquid and the molecules are close together due to strong attraction of water molecules to each other. This intermolecular interaction is called "hydrogen bonding". The oxygen atom of water has a partial negative charge and the hydrogens partial positive charges. A hydrogen atom of one molecule is attracted to the oxygen atom of another molecule. The hydrogen atoms are often shared with two oxygen atoms, and clusters of molecules are rapidly formed and destroyed.

On the other hand, dinitrogen and dioxygen have no permanent dipole moment and are only weakly attracted to each other. They remain in the gas phase with the molecules far apart due to this weak interaction.

The relative masses are:

water (HOH) 18

nitrogen (NN) 28

oxygen (OO) 32

So if the molecules were equally spaced apart, water would be least dense. This is what actually occurs in the gas phase, water vapor is less dense than air. Water is mostly in the liquid phase, with molecules much closer to each other, at room temperature and atmospheric pressure due to intermolecular forces.

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You forgot to mention what amount of water and air you are taking as a comparison. Weight is proportional to mass. So if you take more amount of air and less amount of water in two equal containers, then i am sure air is going to be heavier. don't go for taking the same 'volume' of both the things, since it is not necessary that same volume of water and air has same amount of matter... You see, it is the amount of matter that gives something weight.

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