# 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.