# Problem reading thermodynamic tables

Hi!!
I have trouble understanding this:
Given Temperature:
If pressure < saturation pressure => Overheated Vapor
If pressure > saturation pressure =>Compressed Liquid

Why does this happen??

## Answers and Replies

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Chestermiller
Mentor
At a given temperature, you can have both pure vapor and pure liquid present together in equilibrium if the pressure is equal to the equilibrium vapor pressure at that temperature. If you try to increase the pressure by reducing the volume of a fixed amount of liquid and vapor, you won't be able to do it. You will just cause some of the vapor to condense and form more liquid. However, eventually, you will cause all the vapor to condense. After that you will be able to increase the pressure, since only liquid will be remaining.

If you try to decrease the pressure by increasing the volume of a fixed amount of liquid and vapor, you won't be able to do ti. You will just cause some of the liquid to evaporate and form more vapor. However, eventually, youwill cause all the liquid to evaporate. After that, you will be able to reduce the pressure, since only vapor will be remaining. This will be referred to as superheated vapor.