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MCAT Chemistry Question — Pressure Reduction

The pressure above a solid compound is slowly reduced. Which of the following is true?


A. If the substance is H2O, it will transition from solid directly into gas.
B. For all substances except H2O, the substance will transition into liquid phase and then the gas phase as the pressure is reduced.
C. For all substances except H2O, the substance will transition from solid directly into gas.
D. If the substance transitions from solid directly into gas, it must be H2O.

Click for Explanation

For most substances, the solid phase is more dense than the liquid phase, so lowering the pressure will cause the solid phase to transition either to the liquid phase or the gas phase, depending on the temperature. Because this transition (melting vs. sublimation) is temperature-dependent, neither (B) nor (C) is always true. Similarly, both water and other substances are capable of subliming at certain temperature/pressure combinations, so (D) is not always true. You should also remember that although H2O is rare in having a less-dense solid phase, it is not unique.

Thus (A) remains as the only option, and it is true. Because reductions in pressure favor the less-dense phase, ice will transition directly to gas as the pressure is reduced.

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