Question

Co2(g) ---> CO2(l) + heat

This equation was given to me and asked why high pressure was needed to keep carbon dioxide in the liquid state. With the information given; I don't exactly know how to answer the question. How do you increase the pressure of a liquid. For gases you can simply decrease the volume.

Answer 1

One reason is that in the gaseous state the molecules are so far apart that there is little attraction between them. At high pressures, it forces the molecules closer together (smaller volume), there is more attractions, and London forces play a bigger role in the liquifaction process. Here is an article about critical pressure and critical temperature. Perhaps it will help.
http://www.chem.purdue.edu/gchelp/liquids/critical.html

Answer 2

Thanks :D