Re: How can the phase diagram of water be physically represented without air?

If no air is present in a closed system how can the pressure be altered? Pressure would be altered in the same way as for any other system. We must remember the distinction between pressure (force per unit area) and compressibility (change in volume with change in pressure). Pressure can be exerted on any substance -- solid, liquid or gas -- but only gases have noticeable compressibility at familiar pressures (up to two or three bars).

At higher pressures, liquids and solids are also compressible. There's a nice demonstration of the compressibility of dry versus hydrated rock in the Nova video "Venus Unveiled."

What would the triple point of water truly look like? You do not need to reduce the overall pressure to 6 millibars to have the vapor, liquid and solid phase coexisting; when you mix ice and water and allow the temperature to reach 0°C, there will be 6 millibars of water vapor above the slush. This is called the vapor pressure.

Solids and liquids are called "condensed phases," to distinguish them from gases. Every solid or liquid has a vapor pressure, that is, a pressure of gaseous material which will be maintained above the condensed phase at a given temperature. Thus, if you release the pressure above a solid or liquid, some of the solid or liquid will pass into the gas phase in order to balance the pressure release.

Vapor pressure is independent of the presence of any other gas, and will depend on two things: how strongly the material holds together (metallic bonds? ionic bonds? network covalent bonds? intermolecular forces?) and the temperature. For example naphthalene, a molecular solid, has a vapor pressure at room temperature of 0.1 mm of mercury (compare liquid water, which has a vapor pressure of 24 mm of mercury at the same temperature). On the other hand, more strongly-bonded solids (like quartz) have vapor pressures which approximate zero.

The liquid phase, by the way, can exist only under a narrow range of conditions. For example, one can easily conceive of a situation in which the pressure is so low that liquid water cannot exist: at very low pressure, only at temperatures low enough to freeze will water have a vapor pressure low enough to remain condensed. This is, in fact, the situation in outer space; liquid water vented into space will boil and freeze simultaneously!

On the other hand, so-called supercritical phases are under high enough pressure that there is no boundary between "gas" and the "liquid" phases; pressure is high enough that the molecules are essentially in contact, while the temperature is high enough to prevent liquifaction. The critical point is the highest temperature (at any pressure) at which gas and liquid phases can coexist. For water, the critical point is reached at a pressure of 221 bar and a temperature of 647 K.