First order phase transition (van der waals gas)

[jasony]

Homework Statement

Give the physical explanation of the graphs (Fig 4(a) and 4(b)) on Page 4
http://www.pma.caltech.edu/~mcc/Ph127/b/Lecture3.pdf"

Homework Equations

1) Why $$V_{g}$$ decreases with temperature but $$V_{l}$$ increases with temperature
2) Why must phase transition happen at constant pressure (Maxwell construction)

The Attempt at a Solution

4(a) As temperature increases, greater proportion of molecules acquire sufficient energy to escape from liquid and consequently a higher pressure is needed to establish equilibrium between liquid and vapour

4(b) don't really know how to explain

 

[jasony]

Anyone who can help? please...