When heat is removed from a system, which may occur?

• A. The temperature may decrease, and the substance may change state
• B. The temperature increases
• C. The temperature may decrease, and the substance may change state
• D. Only external pressure changes

Answer: (C)

When heat is removed, the system loses energy, so its temperature tends to fall. If the temperature reaches a phase boundary, the substance can change state—for example, liquid water can freeze at 0°C, or water vapor can condense to liquid at 100°C (at a given pressure). During a phase change, the temperature can hover at the transition temperature until the change completes, and only then continue to drop as heat removal continues. External pressure changes aren’t a guaranteed result of removing heat; they depend on how the system is confined or controlled. So removing heat can lead to a lower temperature, and it can also cause a change of state under the right conditions.