Which phase changes require energy input, and which release energy?

Which phase changes require energy input, and which release energy?

• A. Melting, vaporization, and sublimation release energy; freezing, condensation, and deposition require energy.
• B. Melting, vaporization, and sublimation require energy input; freezing, condensation, and deposition release energy.
• C. All phase changes release energy.
• D. None involve energy changes.

Answer: (B)

Phase changes involve energy flow in or out as molecules rearrange. To melt a solid into a liquid, energy must be absorbed to weaken and break the attractions holding the particles in a rigid structure. The same idea applies to vaporization, where a liquid becomes a gas, and to sublimation, where a solid goes straight to gas; all of these require energy input, making them endothermic. On the other hand, when a liquid freezes into a solid, a gas condenses into a liquid, or a gas deposits into a solid, energy is released as the new bonds form, so these are exothermic processes. At a given pressure, the temperature remains constant during these phase changes while the energy goes into changing the state rather than increasing temperature.

Phase changes involve energy flow in or out as molecules rearrange. To melt a solid into a liquid, energy must be absorbed to weaken and break the attractions holding the particles in a rigid structure. The same idea applies to vaporization, where a liquid becomes a gas, and to sublimation, where a solid goes straight to gas; all of these require energy input, making them endothermic. On the other hand, when a liquid freezes into a solid, a gas condenses into a liquid, or a gas deposits into a solid, energy is released as the new bonds form, so these are exothermic processes. At a given pressure, the temperature remains constant during these phase changes while the energy goes into changing the state rather than increasing temperature.