Which statement best describes the reversibility of phase changes?

Which statement best describes the reversibility of phase changes?

• A. None are reversible.
• B. Only some phase changes are reversible.
• C. All phase changes are reversible under appropriate conditions.
• D. Only melting and condensation are reversible.

Answer: (D)

Phase changes can be reversed as long as the system stays near equilibrium and you adjust temperature or pressure slowly enough. When conditions are changed gently, solid can become liquid and liquid can become solid again; liquid can become gas and gas can become liquid again; and so on. In each case the forward and reverse paths occur along the same thermodynamic route, so the process is reversible in principle. This includes melting and freezing, vaporization and condensation, as well as sublimation and deposition. The idea that only melting and condensation are reversible is too limited, because the other transitions between solid, liquid, and gas—freezing, vaporization, sublimation, and deposition—are also reversible under the right conditions.

Phase changes can be reversed as long as the system stays near equilibrium and you adjust temperature or pressure slowly enough. When conditions are changed gently, solid can become liquid and liquid can become solid again; liquid can become gas and gas can become liquid again; and so on. In each case the forward and reverse paths occur along the same thermodynamic route, so the process is reversible in principle. This includes melting and freezing, vaporization and condensation, as well as sublimation and deposition. The idea that only melting and condensation are reversible is too limited, because the other transitions between solid, liquid, and gas—freezing, vaporization, sublimation, and deposition—are also reversible under the right conditions.