Which is an example of an exothermic reaction?

• A. Photosynthesis.
• B. Melting of ice.
• C. Dissolution of salt in water.
• D. Combustion.

Answer: (D)

Exothermic reactions release energy as heat (and often light) to the surroundings. Combustion fits this idea clearly: when a fuel burns in oxygen, chemical bonds are rearranged to form carbon dioxide and water, and the energy released from forming these bonds exceeds the energy required to break the initial bonds. That surplus energy shows up as heat (and a visible flame).

In contrast, processes like photosynthesis absorb energy from the surroundings (light) to build glucose, so they are endothermic. Melting ice requires heat input from the surroundings to overcome the solid’s lattice, another endothermic process. Salt dissolving in water is typically endothermic or only weakly exothermic, depending on the substance, but it’s not the classic example of releasing energy like combustion.