Temperature, pressure and density are intrinsically linked. If you know 2 of them, you can calculate the third. If you know 2 of them, and you have an appropriate chart or table, you can look up the phase(s) of matter.

The easiest way to talk about it here is in terms of pressure and temperature. I'm ignoring density in this post.
It's like describing a circuit in terms of Volts and Ohms, but not the Amps. The circuit is still completely described.

I'll be using atmospheres for pressure. 1 atm is the mean air pressure at sea level.
1 atm = 14.7 psi = 760 mmHg = 101.325 kPa

Quote Originally Posted by Renton View Post
Are freezing/boiling points of substances completely hard numbers? By that I mean, can water in liquid form exist below 0 degrees celsius, or likewise can it exist at above 100?
For a given pressure, these temperatures are hard numbers. Like NG said, impurities in the water will subtly change the numbers. Still, for a given substance, the thermodynamic behavior is deterministic.

Water can exist at temperatures below 0 C or above 100 C if the pressure is greater than 1 atm.

Here's a useful graph, called a P-T diagram, for water. This shows pretty much everything you're asking and more for this question.

Notice that a horizontal line across the upper portion of the graph shows exactly what you're asking.

There's also a cool phenomenon at 0.006 atm and 0.01 C where water can exist equally as steam, water or ice. This is called the triple point of water.

The top-right portion of the graph, beyond the 'C' (critical point) water becomes a supercritical fluid, where there are not distinct liquid and gas phases.

Here's a link to an article that says the coldest water can exist without becoming ice is -48 C. I scanned it quickly and didn't see any mention of what pressure they had it.


Quote Originally Posted by Renton View Post
I'm aware that running water cannot freeze as easily as standing water, but is that because the movement of the water raises the temperature or what?
This one is really tough, actually. It's a very complicated system with a ton of variables.
While geothermal heating can play a part, water flowing in mountain streams and rivers can definitely get below -2 C. So it's not necessary that the water is above 0 C.

I could only find partial explanations over and over again in my search.

Part of the reason it doesn't freeze is that the Hydrogen bonds that form between water molecules when it freezes are quite weak. The turbulence in the flowing water can break apart newly formed Hydrogen bonds, and effectively lower the freezing point of the flowing water.

Another part is that the water requires a nucleation site to begin crystallization. This partly explains why rivers freeze at the banks first.

Thermal energy is lost to evaporation on the surface of the flow, which partly explains why rivers freeze on top first. That frozen layer acts as an insulation layer against further evaporation, which partly explains why a river that is frozen over is still flowing beneath the ice.

Ice forms in hail storms under very turbulent conditions, though. Waterfalls do freeze.