[elliotyang]

This is the phase diagram i do not understand. But I tried to list out what the question want. Can anyone tell me whether I am correct or wrong?

[_cheers]

I *think* your T=Tc is right, but where you have 'liquid' is where both phases exist. right above that is the gas phase and above your coexistence curve is vapour. liquid is where you have solid, and solid is to the right of where you have solid. I may have vapour and gas mixed up...

so where T>Tc  to T=Tc should be your vapour and from T=Tc to T<Tc should be your liquid

hope that helps...