Which term refers to the process by which hydrogen and oxygen combine to form water?

• A. Chemical Reaction
• B. Physical Change
• C. Fire
• D. Exothermic Reaction

Answer: (A)

Transformations where atoms rearrange to form new substances are chemical reactions. When hydrogen and oxygen combine to form water, their molecules break apart and reassemble into H2O, a substance with different properties, so this is a chemical reaction. It is also highly exothermic, releasing energy as heat, but that energy detail describes how the reaction behaves, not the process label. The other terms don’t fit as well: a physical change would involve no new chemical substance, fire is a rapid burning process that isn’t the precise name for forming water, and exothermic reaction focuses on energy change rather than the process itself.