Originally Posted by
Abu Mahendra
What effects, if any, does air humidity play in this? Colder air will be drier. Anything here?
Water decreases the density of air.
Originally Posted by
andr0id
Let's suppose we have a bucket of a given size into which you can place molecules.
Suppose for our experiment H20 weighs 18. O2 weighs 32, and N2 weighs 28.
It starts out full of only O2 and N2. Now take some of those out and put in some H2Os. The bucket is lighter and therefore less dense. I guess spread out may be imprecise, but a water molecule is less dense for its 3 atoms than 2 atoms of either of the other gases.
You are on the right track but have gone off the rails just a little and made it more complicated than it needs to be. It all has to do with the molar mass of the water and the air. On average, dry air has a molar mass of about 29 g/mol. Water has a molar mass of 18 g/mol. At a given temperature and pressure, the number of molecules in a volume of gas is constant. If you add water to a mass of dry air, the number of dry air molecules decreases to keep the pressure from increasing and the density of the mass of air decreases.
In layman's terms, some of the oxygen and nitrogen has to skooch over to make room for the water. The distance between the molecules doesn't change.