I am facing difficulties with understanding the relationship with saturation pressure and the phase change (from liquid to gas) of a pure substance.
Let's suppose I have some amount of water at p = 5 bar and T = 200 ºC.
To know if it is liquid, vapor or in the phase change, I could go and check the saturation temperature for given p = 5 bar.
That would be 151,71 ºC.
I perfectly visualize that, being the saturation temperature below the actual temperature, the phase change "already happened". And I can say it already happened because temperature does not change while being in the phase-change, so, being the temperature greater than the saturation temperature, that change already happened and the water is now vapor.
But if I choose to apply the same reasoning the other way, given that the saturation pressure for T = 200 ºC is 15,54 bar, I do not understand how this would tell me that the water is vapor.
What I expect is that, since the saturation pressure is greater than the actual pressure, the water is still liquid. And I say still liquid because a lower pressure somehow translates as less energy, and less energy means to me that it still remains liquid. It must increase it's energy (just like with temperature) before going for the phase-change. But it happens to be exactly the opposite. And I am not able to visualize this.