Key Terms Associated with Phase Diagrams

Ch 2, Lesson B, Page 2 - Key Terms Associated with Phase Diagrams

• We’re going to start by focusing on vapor-liquid equilibrium, VLE for short. The concept of saturation is the key to understanding VLE. We all know that at 1 atm water boils at 100oC, right ?  This tells us two very important things.
• 1 – The saturation temperature, Tsat of water at 1 atm is 100oC.
• 2 – The vapor pressure of water at 100oC, P*(100oC) is 1 atm.
• The idea of a saturation temperature is pretty intuitive, but vapor pressure can be confusing.
• Vapor pressure is the pressure that a liquid COULD overcome in order to boil.  Vapor pressure depends on temperature only.
• For example, the vapor pressure of water at 80oC is LESS than 1 atm.  Water at 1 atm and 80oC doesn’t boil.  We would have to reduce the pressure to less than a half an atm in order to get the water to boil at 80oC.
• The vapor pressure of water at 110oC is GREATER than 1 atm.  This means that water at 110oC would boil even if the pressure were greater than 1 atm. OK, so now what does saturation mean ?
• A sat’d liquid is a liquid that is AT the saturation temperature or boiling point that corresponds to the existing pressure.
• So, liquid water at 1 atm and 100oC is a saturated liquid.  In this case, Tsat = 100oC and P = P*(100oC) = 1 atm.
• Now, if you put a tiny bit more energy into a sat’d liquid and keep the pressure constant, some of the liquid would BOIL or evaporate and create a vapor phase.
• The cool part is that the temperature would not change.  The liquid and the vapor would both exist, in equilibrium, at 100oC.
• The vapor bubble that forms would be a sat’d vapor at Tsat = 100oC and P = P*(100oC) = 1 atm
• A sat’d vapor is a vapor at the saturation temperature that corresponds to the existing pressure.
• If you take a tiny bit of energy out of a sat’d vapor, keeping the pressure constant, some of the vapor will condense and create a liquid phase.  That liquid phase will be a sat’d liquid.
• Do you see ?  Sat’d vapor and liquid are phases that ARE or at least COULD be in equilibrium with each other.
• So what if we have liquid water at 1 atm and only 25oC ?  Is this a sat’d liquid ?  NO, because T (25oC) is less than Tsat at 1 atm (100oC).  Understand ?  This is called a subcooled liquid because it exists at a temperature BELOW the saturation temperature for that pressure.
• Similarly, what if we have water vapor at 1atm and 125oC ?  This NOT a sat’d vapor because T > Tsat.  This is called a superheated vapor.