What is the density of water vapor and why?

Question

i'm just wondering.......

Answer 1

It all depends on the temperature. The hotter it is, the less dense it is. The cooler the water vapor is, the denser it gets. Think about this: if it gets cool enough, the water vapor condenses into the density of liquid water. Keep going, and if you let it freeze into ice, then the density goes up, which is why ice floats. Water is the only substance I've heard of where the solid is less dense than the liquid.

Answer 2

The density of any vapour or gas is defined by the mass of the vapour in a particular volume. i.e. if you increase the volume wihtout adding more vapour the density decreases so there is no absolute answer.

Relative to air water vapour is less dense because it has a lower molecular weight than the average for air mostly becasue 2/3 of the atoms in a water molecule are hydrogen which is the lightest atom.

Air is basically 80% molecular nitrogen (N2) which has a molecular weight of 28 and 20% molecular oxygen, molecular weight 32, so the average molecular weight of air is about 28.8. OTOH the molecular weight of water (H2O) is 2 x atomic weight of hydrogen + 1 x atomic weight of oxygen i.e. 1 + 1 + 16 = 18 i.e less than air.

Answer 3

It increases with temperature as more molecules have the energy to escape from the liquid surface and become vapour. Incidentally when water freezes its density gets less.