A reaction that involves breaking down a compound into simpler substances is known as what?

A reaction that involves breaking down a compound into simpler substances is termed a decomposition reaction. This type of reaction typically occurs when a single compound undergoes transformation into two or more different products, often requiring energy input in the form of heat, light, or electricity to break the chemical bonds. Common examples of decomposition reactions include the breakdown of water into hydrogen and oxygen gases or the decomposition of calcium carbonate into calcium oxide and carbon dioxide when heated.

The other types of reactions listed do not fit this definition. A combination reaction involves two or more substances combining to form a single new product, while a single-replacement reaction entails one element replacing another within a compound. Electrolysis specifically refers to the process of using electricity to drive a non-spontaneous chemical reaction, often involving decomposition but under the influence of an electric current rather than natural breakdown. Thus, the term that accurately describes the breaking down of a compound into simpler substances is indeed a decomposition reaction.