COMPRESSIBILITY FACTOR GRAPH

Hello,

I have a question about our faculty assignment. It is about the compressibility factor z. The professor was introducing us to the COMPRESSIBILITY FACTOR GRAPH and there was something interesting, the curves of the graph that were showing behaviour by different temperatures were crossed at almost the same point, like here: http://www.chem.quee...ressibility.gif, or for air (from wikipedia) here: http://upload.wikime...50_-_1000_K.png

And the question was, if that was a coincidence or there is something behind it...

Now I have searched for a long time and I did not find an explanation for this, my opinion is that it is a coincidence, because at more exsact graphs you can see that the lines are not so close, not crossed at the same point, but that could be from the deviance...

I would be glad if anyone could help me,

Thanks in advance!

L