What is a decomposition reaction?

A decomposition reaction is characterized by the breakdown of a single compound into two or more simpler substances. This type of reaction occurs when the bonds holding the compound together are broken, resulting in the formation of smaller molecules or elemental substances.

In the context of the other choices, a reaction where two or more substances combine to form one product describes a synthesis reaction, not decomposition. An exchange of ions is indicative of a double displacement reaction, where ions in two compounds swap places. Lastly, while some decomposition reactions may release energy, it is not a defining feature, as others may require energy input to occur, typically in the form of heat, light, or electricity. Therefore, the defining characteristic is the breakdown of a single reactant into simpler products.