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MCAT Physics Question — Phase Changes

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

Click for Explanation

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.

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