Answer
a. Solid or vapor.
b. At $5.11atm\leq P\leq73atm$ and $-56.6^{\circ}C\leq T\leq31^{\circ}C$
Work Step by Step
a. Figure 13–23, the phase diagram for carbon dioxide, shows that at atmospheric pressure, carbon dioxide can exist as solid or vapor.
b. The phase diagram shows that for carbon dioxide to be liquid, $5.11atm\leq P\leq73atm$, and $-56.6^{\circ}C\leq T\leq31^{\circ}C$.
Those conditions are necessary but not sufficient. That is, not all points in those ranges correspond to liquid carbon dioxide, but the pressure and temperature must each fall within their respective boundaries.
