Which pair consists of heating processes?

• A. Melting and Evaporation
• B. Condensation and Freezing
• C. Melting and Condensation
• D. Evaporation and Freezing

Answer: (A)

Endothermic phase changes absorb heat, so a pair that includes two processes requiring energy input is the one that’s being tested. Melting, where a solid becomes a liquid, needs energy to weaken the crystal structure; evaporation, where a liquid becomes a gas, also needs energy to allow molecules to escape into the vapor phase. That means both steps in the pair are heating processes.

In the other options, at least one step releases heat rather than absorbing it. Condensation (gas to liquid) and freezing (liquid to solid) are exothermic, releasing heat as the substance changes state. So they aren’t both heating processes. Similarly, mixing a heating process with a cooling one in the remaining pairs shows they aren’t both heating either.