First order phase transition (van der waals gas)

[jasony]

I am looking at some of the notes but don't quite understand this.

What are 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"

Why V_{g} decreases with temperature but V_{l} increases with temperature?

 

[jasony]

Anyone who can help? please...

 

[DrDu]

Look at the Maxwell construction: When the temperature rises, the two loops in the theoretical p-V curve become less and less pronounced until they disappear at the critical point. At the critical point Vg and Vl coalesce, hence the distance of Vg and Vl has to decrease with temperature.