Which term refers to a physical or chemical change accompanied by the evolution of heat?

• A. Exothermic
• B. Thermal reactivity
• C. Runaway reaction
• D. Endothermic

Answer: (A)

Exothermic describes a process in which heat is released to the surroundings. This applies to both chemical changes, like combustion, and certain physical changes, such as freezing or condensation, where the system gives off heat as its particles rearrange. The defining idea is that the enthalpy change is negative (the system loses heat). If heat is absorbed instead, the process is endothermic. The other terms don’t fit: thermal reactivity isn’t a standard descriptor for heat evolution, and a runaway reaction describes a hazardous scenario where heat generation outpaces removal, not the general concept of heat release.