Imagine a room at a “normal” temperature. It is filled with a mixture of dry air and water vapor. Each fills the room, so their volumes are equal. However, their densities are far different, because water vapor is a very light gas. At ordinary room temperatures, it constitutes only about 1/2 to 1 1/2% of the total weight of the mixture that we call “air.”
Excerpt from Technical Institute Manual #2.