# What does the state of a substance at a specific T and P mean?

From the phase diagram of water, we see that water is a liquid at 20°C, 1 atm. The state of the water at 20°C, 1 atm is liquid. But there is actually also a water vapor in equilibrium with liquid water. What is the definition of “the state of a substance at a specific temperature and pressure?”

• The state of a substance is probably understood as the most important phase describing this substance at a specific temperature and pressure. At $20°$C, and $1$ atm., the concentration of water in the liquid phase is greater than the concentration of water in the vapor phase. So water is considered as liquid under these conditions of $T$ and $p$. Jun 16 at 8:44
• The water phase diagram means the single component ( but its phases ) system. So at 20 deg C, the pressure 1 atm will finally convert all water vapour to liquid. Jun 16 at 11:43

I will start by addressing the posted question: the "state of a substance at a specific temperature and pressure" refers to "the most stable phase of the homogeneous substance at the specific p and T". The phase is indicated in the phase diagram by looking up the appropriate T,p point. At $$\pu{20 ^\circ C}$$ and 1 atm pressure that would be pure liquid water.

Now, addressing the confusion, the system you are referring to (with vapour) is not at 1 atm, at least not 1 atm of water vapour. You are probably thinking of a closed container partly occupied by liquid water and a mixed gas phase with contents under a pressure of 1 atm. Yes, the liquid water in the container will be in equilibrium with vapour (if the liquid has a volume smaller than the container's, and if the temperature is not too high, causing all of the water to form vapour). But the pressure of the water vapour (that is, its partial pressure) will not be 1 atm. It will be much less, closer to the ideal equilibrium vapour pressure of water at that temperature.

Note the question is similar that posed here, from which I borrow the following phase diagram (not of water): That post states

However, this had me slightly confused because in the situation initially described (at a point somewhere within the liquid region - NOT on the phase boundary) some vapour existed above the liquid in the container ...

I quote the above because it is a related confusion. When water is in equilibrium with vapour the state is necessarily one described by a point on the liquid-gas coexistence line, and most decidedly not an interior point in the region labelled "liquid". An interior point is not one describing equilibrium with vapour (or, there is no coexisting vapour at such an interior point).