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[Solved. For anyone who is also confused, see the comments under Poutnik's answer. As Poutnik reminds me, remember there is no air involved. I successfully picture that with the piston experiment in Karl's answer.]

When I read the phase diagram, I think it suggests that "at certain temperature and pressure, the phase of water would be (solid, liquid, gas) or two/three of them coexisting." But when I look up NTP on the diagram, it says "liquid", while the saturation pressure of water at 25°C is 0.03atm. So the situation can't exist since the vapor is going to condense until the pressure drops to 0.03atm.

Then what actually is the phase diagram suggesting by "liquid"? What am I missing here?

Edited: Poutnik reminds me that the pressure is the total pressure, not the vapor pressure. Then my question should be "If the vapor and liquid coexist not only at the curve, then what is the meaning of 'liquid' on the diagram?"

enter image description here

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  • $\begingroup$ Then under a total pressure of 1atm, I think liquid water and vapor still coexist when there's 0.03atm vapor in the gas? What is the meaning of "liquid"? I'm confused because in the textbook it says water and vapor coexist at the curve, but I think they also coexist elsewhere like the NTP. $\endgroup$
    – Wang
    Commented Apr 21, 2020 at 8:56
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    $\begingroup$ This diagram is for one compound, just one compound and nothing else. There are no other compounds. There is nothing else. There is no air, in particular. Total pressure is the same as vapor pressure. That's what it says. $\endgroup$
    – Ivan Neretin
    Commented Apr 21, 2020 at 9:09

2 Answers 2

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The phase equilibrium diagram is for a system consisting of just water.

The diagram pressure is the total system pressure.

It may be equal to the saturated vapor pressure, if no external pressure is imposed on the system.

Or, it can be ( much ) higher, if some external pressure higher than the saturated vapour pressure exists. In such a case all vapor is at equilibrium transformed to liquid or solid phase.

Really high pressures can be seen in the water phase diagram for very high pressures, displaying rare ice phases.

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  • $\begingroup$ So, can I interpret the water phase diagram at NTP as "at NTP, liquid water can exist if the vapor pressure is saturated(0.03atm) " ? I'm saying this because at a total pressure of 1 atm, liquid water can't exist if the vapor pressure is under 0.03atm (right?), which I think contradicts the diagram. Also, does the more detailed phase diagram you posted here gives the information of how the high pressure ice would exist? By how I mean like the vapor pressure of the ice. $\endgroup$
    – Wang
    Commented Apr 21, 2020 at 10:01
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    $\begingroup$ No. If pressure is higher than saturated vapour pressure at given temperature, vapour cannot exist in equilibrium. $\endgroup$
    – Poutnik
    Commented Apr 21, 2020 at 10:21
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    $\begingroup$ @Wang If you apply a pressure of more than 0.03atm on water at 25°C, the gas phase condenses completely. $\endgroup$
    – Karl
    Commented Apr 21, 2020 at 10:22
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    $\begingroup$ I guess he is confused, as he may subconsciously still count with air pressure. There is none. $\endgroup$
    – Poutnik
    Commented Apr 21, 2020 at 10:25
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    $\begingroup$ I think I understand now. Yes I was subconsciously still counting air pressure! Big mistake here. Thanks for this clarification! $\endgroup$
    – Wang
    Commented Apr 21, 2020 at 10:30
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Imagine filling the e.g. water into a cylinder with a piston on top. No air inside.

Now the experiment is to control the temperature inside the cylinder, and move the piston to get the wanted pressure inside. If the pressure is above the vapour pressure, your cylinder is completely filled with liquid water, if it is below, all the water will be evaporated.

Of course you have to pull out the piston very far to make room for the complete phase transition. While you still have two phases in the cylinder, the equillibrium T/p value pair is always on one of the lines in your phase diagram.

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  • $\begingroup$ So if I keep the temperature at 25°C, press the piston so hard that the pressure reaches the ice area in this picture Poutnik posted: en.wikipedia.org/wiki/Phase_diagram#/media/… , I will get ice and no gas, no liquid? $\endgroup$
    – Wang
    Commented Apr 21, 2020 at 10:24
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    $\begingroup$ @Wang Of course! $\endgroup$
    – Karl
    Commented Apr 21, 2020 at 10:29
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    $\begingroup$ @Wang Yup. At about 10,000 atmospheres you get Ice VI. At 21,000 atmosphere Ice VII, then on up through Ice X and Ice XI as you run the pressure even higher. You better have a very strong piston! Note that at sufficient pressure you can make a baking dish out of ice, but anything for the stove would be problematic. $\endgroup$
    – Loren Pechtel
    Commented Apr 22, 2020 at 3:14

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