Which statement best describes an endothermic reaction?

• A. The reaction releases heat to the surroundings.
• B. The energy of the products is lower than the reactants.
• C. The reaction increases the system’s energy; energy of the products is higher than the reactants.
• D. The reaction occurs only at high temperature.

Answer: (D)

Endothermic reactions absorb heat from their surroundings, so the system’s energy increases. That means the products end up with higher energy than the reactants, corresponding to a positive enthalpy change. Temperature alone doesn’t define endothermy—the reaction’s energy flow matters, not the ambient heat level. Some endothermic processes happen with modest heat input, others require more, and heat input is what drives them. For example, photosynthesis stores energy from light in chemical bonds, illustrating how energy enters the system rather than leaves it. The idea that endothermic behavior only occurs at high temperature isn’t correct; the defining feature is heat absorption, not the ambient temperature.