Question

Using the phase diagram for `CO_2`, what phase is carbon dioxide in at -20 C and 1 atm pressure?

Answer 1

The gas phase

Explanation:

A phase diagram for `"CO"_2`:

chem.libretexts.org

What we do is take the vertical line of `-20` `""^"o""C"` and the horizontal line of `1` `"atm"` (not explicitly shown here, but it is essentially the bottom of the graph) and find in what phase region they intersect.

Carbon dioxide appears to be in the gaseous state at `-20` `""^"o""C"` at all pressures below `20` `"atm"`, so it will be in the gas phase.