An endothermic reaction is defined as one that

An endothermic reaction is defined as one that

• A. Releases heat
• B. Absorbs heat
• C. Produces light
• D. Occur spontaneously

Answer: (B)

Endothermic reactions are defined by heat flowing into the system. The energy required to drive the reaction comes from the surroundings, so the system gains energy and the surroundings often feel cooler. That’s why the best choice is the one that says the reaction absorbs heat.

In practical terms, when heat is absorbed, enthalpy increases (ΔH is positive). If a reaction released heat instead, it would be exothermic, not endothermic. Producing light isn’t a defining feature of endothermic behavior—it can happen in reactions that release energy as light, but that describes energy output, not the direction of heat flow. Occurring spontaneously relates to thermodynamic favorability but isn’t the defining trait of endothermic versus exothermic.