Let's assume we have a fixed container filled with some amount of a liquid and its vapor. If we heat it up, obviously the fraction of mass that's in vapor form will increase (being in gas is higher energy state, fraction of stuff that's in a higher energy state increases as energy increases in a thermodynamic equilibrium; I might be wrong if there's something really wonky going on with surface tension). What about volume fraction? At first glance it seems that it can change in either direction.
@robryk My gut feeling is that if the pressure inside the container is low enough, the increased pressure from the vapor would not counteract the change in the liquid’s density due to temperature. So for water it could change in either direction depending on the starting temperature!
@ruuda It's not the increased pressure from vapor that counteracts ~anything as long as the liquid is incompressible: it's just that more of the liquid evaporates. I agree about that case, and at some point the trend has to reverse, because eventually all the water will boil (or, if the density in the container is too high, we will reach the critical conditions first).
