On the CO2 phase diagram, the locations of the triple point and the normal sublimation points are shown. At 3 atm, we note that CO2 sublimes at about -65oC (at this point, the red line crosses the blue solid-vapor equilibrium curve). At 6 atm, CO2(s) melts at about -58oC, and boils near -40oC. At these temperatures, the red 6 atm line crosses the blue solid-liquid equilibrium curve, and the blue liquid-vapor curve.
Something else to think about: at a constant temperature of -30oC, under what pressure does CO2(g) liquefy?
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