When heat is removed from a system and no state change necessarily occurs, which of the following is possible?

• A. The temperature may decrease, and the substance may change state
• B. The temperature may increase
• C. The temperature increases, and state changes occur
• D. Heat is added to the system

Answer: (A)

When heat is removed, the system’s temperature tends to fall. If the system reaches a phase-change temperature, removing heat will cause a change of state while the temperature stays constant during that transition. After the phase change completes, further cooling will lower the temperature again. So it is possible to see both a temperature decrease and a change of state during the process. The other scenarios require adding heat or a temperature rise, which isn’t what happens when heat is removed.