# What happens to water cooling at constant volume but above freezing?

Related to this popular question: How strong is the force of ice expanding when freezing?

We are cooling H2O to absolute zero while keeping the volume (density) constant. The answers focus on the solid portion as H2O normally expands when freezing.

But what about the liquid portion of the cooling?

We are holding overall density, but imo best to think of it as the equivalent “a chamber of set size and heat being removed”.

If we start at room pressure and temperature, during the cooling to 0C water, shouldn’t the liquid part decrease (very slightly) in density? I can’t decide whether:

1. Liquid would pool under a tiny mostly-vacuum with some water vapor, still almost exactly at room pressure.

2. Pressure would decrease until the 0C liquid density matched the 30C liquid density and water still fills chamber.

What would happen? Does presence of gravity change it? (Please be sure to consider both questions)

• At what temperature and pressure (or density) you start cooling? Jul 29, 2021 at 13:24
• Room temp and pressure Jul 29, 2021 at 16:03

According to the phase diagrams in the link, if you start with liquid water at, say, 20°C and 1 atm and cool it, maintaining phase equilibrium, the liquid will shrink and evaporate (at the top, in a gravity field) to maintain a pressure of 3.2 kPa, dropping to 0.6 kPa at 0°C. (The liquid will start to expand at 4°C but not enough to fill the container again.) Then, the vapor will disappear, and the space will be filled completely with a liquid-ice Ih slush until you reach -22°C, at which point the liquid will freeze to produce a mixture of ice-Ih and ice-III.

• Ok cool. Thanks. I saw compression coefficients and density by temp. looks like the water can only cool like a hundredth of a degree IF pressure went down to zero (which it wouldnt) for the pressure to expand the water enough to maintain density. I dont get the 3.2 atm you wrote? Then vapor comes like you said. So when first gets to 0C, it is water and vapor, then ice forms in the water part and that has lower density; so when enough ice forms, the vapor goes away, then pressure starts to increase as more ice?? I dont see how on diagrams whether is evaporation or boiling. Is vapor same as gas? Jul 29, 2021 at 17:17
• I meant 3.2 kpa not atm. I also see it must be p for phase transition at 20C, so nevermind that. Also, if we have ice and vapor and water that has to be the triple point. aaand i just checked the 0.6 is triple point pressure 😀. That leaves the last two sentences in the above, two questions, if willing. Thanks again Jul 29, 2021 at 17:34
• Vapor often means gas in equilibrium with a condensed phase underneath it, which is the case here. The liquid wouldn't boil (i.e., nucleate bubbles), as all of these processes are presumed to occur slowly enough that surface evaporation is adequate to maintain equilibrium. (But if you pull a vacuum on liquid water quickly, it can boil.) Yes, when the vapor disappears at 0°C, the pressure increases tremendously, as discussed in the other answer. Jul 31, 2021 at 22:20
• Thank you. I always thought of vapor and evaporation as fundamentally different, with vapor as not really a phase just a bit of vapor, or even just particles from the liquid, kindve migrating or diffusing around. I suspect most people do, even many technical folk. Thats not right though obviously. I guess theres no huge difference just the process of “quick evaporation” of a sort is called boiling. Anyway have a good one. Jul 31, 2021 at 23:47

You can use the phase diagrams from the previous Q/A you linked.

Generally, if the pressure drops outside of what a phase can sustain at a certain temperature, another phase appears in order to "fix" the pressure to a value acceptable for both phases.

If you start with like e.g. 100C water at 1 bar pressure, the "helping" phase will be vapor, at least down to 0C. All the constant-volume states below this temperature will have lower pressure AND some volume occupied by water vapor.

At the triple point, part of the water will freeze and expand, using up the vapor phase volume.

Then, you will get a mixture between liquid and the usual ice (Ih) all the way down to the ice III formation.

• Thanks. Great. So when first gets to about 0C, it is water and vapor below 1 atm, then ice forms in the water part and that has lower density; so when enough ice forms, the vapor goes away, then pressure starts to increase as more ice?? I dont see how on diagrams whether is evaporation or boiling. Is vapor same as gas? Jul 29, 2021 at 17:20
• errr... I am wrong. Update. Jul 29, 2021 at 17:28
• Never wished so much could give two green checks lol. Your update answers my first question above, just the last sentence of my comment if willing Jul 29, 2021 at 17:36