# Vapor-Liquid Equilibrium of a Pure Substance

Gas Phase
Liquid Phase
Measures Total Pressure,
Ptotal
Measures Vapor Phase Mole Fraction, yi = 1
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### Ch 2, Lesson D, Page 1 - Vapor-Liquid Equilibrium of a Pure Substance

• In this lesson we will study vapor-liquid equilibrium in more detail.
• We will define some new variables and formulate some equations that will help us describe the way in which temperature and pressure interact when we have vapor and liquid in equilibrium.
• Let’s begin by considering the vessel shown here.  It contains a pure substance in both the liquid and the vapor phases.  For example, let’s assume the substance is water.
• It is very important for you to realize that there is no AIR in the vessel…just water !
• OK, so what is going on here ?  We assume we are at equilibrium.  That’s the magic word.  Everything we will discuss in this lesson applies ONLY when the vapor and liquid phases are in equilibrium.
• What does equilibrium mean ?  It is not a static motionless state, it is dynamic.  Water molecules are moving back and forth between the vapor and liquid phases all the time.  The key is that water molecules are vaporizing and condensing at exactly the same rate.  The result is that there is no NET change in the number of molecules in either phase.
• Equilibrium also implies that both phases are at the same temperature and pressure as well.
• In order to describe the behavior of MIXTURES at vapor-liquid equilibrium, we need to define an important new variable: the partial pressure.
• Flip to the next page to see how partial pressure works and how it can be interpreted for our system that contains pure water.