Carbon dioxide at -20°C is pressurized at 100 atm. If the CO2 undergoes isobaric cooling to -65°C followed by isothermal depressurization to 1 atm, what are the original phase and subsequent phase(s) of the CO2?
a) liquid, gas, solid
b) gas, solid
c) liquid, gas
d) liquid, solid, gas
Phase changes are read from a phase diagram, a plot of compound or element phase as a function of temperature and pressure. Note some commonly used terms when describing phase changes: isobaric indicates constant pressure, isothermal indicates constant temperature. Adiabatic indicates no heat transferred, but it applies to thermodynamic cycles more than phase diagrams. A substance’s phase is read by plotting indicated temperatures and pressures on the graph. Here, -20°C and 100 atm indicates a starting phase of liquid. If the CO2 undergoes isobaric cooling to -65°C, it becomes solid (freezes). The isothermal depressurization to 1 atm causes sublimation to a final gas phase.
a) liquid, gas, solid, incorrect, Isobaric cooling leads to a solid phase rather than gas from the starting liquid phase. Also, isothermal depressurization causes a vertical decrease on the graph which cannot result in a gas to solid phase change.
b) gas, solid, incorrect, The starting phase is liquid, and the isobaric change leads to a solid phase followed by isothermal depressurization and a final gas phase.
c) liquid, gas, incorrect, The starting liquid phase undergoes isobaric cooling leading to a solid phase rather than gas.
d) liquid, solid, gas, correct.