Phase Diagram (Binary Mixture)

In our analysis of phase diagram, we shall consider only 2-component mixture, e.g. A (more volatile) and B (less volatile).

There are 2 types of phase diagram: constant pressure or constant temperature.

Click here for examples of phase diagrams.

 

Constant Pressure Phase Diagram

The Figure below shows a constant pressure phase diagram for an ideal solution (one that obey Raoult's Law)

Fig001. Constant Pressure Phase Diagram

Note: usually the mole fraction of the more volatile component is plotted on the horizontal axis, whereby x is the mole fraction in the liquid phase and y is the mole fraction in the vapour phase.

 

From the above constant pressure phase diagram; we can see that:

bulletSince the boiling point of pure A (tA at xA = 1.0) is lower than boiling point of pure B (tB at xA = 0.0, i.e. xB = 1.0), therefore component A is more volatile than component B

bulletBoiling point and condensation point changes with concentration: Saturated liquid curve : x vs. T, bubble point curve Saturated vapour curve : y vs. T, dew point curve

bulletAt each temperature, the vapour and the liquid are in equilibrium

bulletThree different regions exist where only vapour, only liquid and a vapour-liquid mixture can exist.

Click here for an example analysis of how the concentrations of the more volatile component in the liquid and vapour changes with temperature.

 

Constant Temperature (Isothermal) Phase Diagram

The Figure below shows the constant temperature phase diagram. The constant pressure phase diagram is more commonly used in the analysis of VLE, but the constant temperature phase diagram is also useful in the analysis of solution behaviour that deviates from Raoult's Law.

Fig002. Constant Temperature Phase Diagram

[ From this Figure (constant temperature phase diagram) we see that the more volatile liquid will have a higher vapour pressure (i.e. pA at xA = 1.0). Note also the regions of vapour-only, liquid-only and vapour-liquid mixture. ]

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