0

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)

Al Brown
  • 3,385

2 Answers2

3

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.

Chemomechanics
  • 30,163
2

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.

fraxinus
  • 9,585