Define endothermic and exothermic reactions with examples.

• A. Endothermic: absorbs heat; Example decomposition of ammonium chloride. Exothermic: releases heat; Example combustion of methane.
• B. Endothermic: releases heat; Example combustion of methane. Exothermic: absorbs heat; Example dissolving salt.
• C. Endothermic: requires no heat; Example dissolving salt. Exothermic: produces heat; Example freezing water.
• D. Endothermic: releases heat; Example combustion of methane. Exothermic: absorbs heat; Example melting ice.

Answer: (B)

Endothermic and exothermic describe how heat moves during a process. In an endothermic process, heat is absorbed from the surroundings into the system, so the surroundings often feel cooler because energy is being taken in. A classic example is the decomposition of ammonium chloride, which requires heat input to proceed. In an exothermic process, heat is released from the system to the surroundings, warming the surroundings; combustion of methane is a standard example that releases a lot of heat.

Other everyday examples help anchor the idea: dissolving certain salts can be endothermic (it takes in heat), while freezing water is exothermic (it releases heat). The key is the direction of heat flow relative to the surroundings, not the magnitude of energy change.