What do you call a reaction where one substance breaks down into two or more products?

A reaction in which one substance breaks down into two or more products is known as a decomposition reaction. This type of reaction involves the breakdown of a compound into simpler substances, which can be elements or simpler compounds.

For example, when water is decomposed through electrolysis, it breaks down into hydrogen and oxygen gases. This clearly illustrates the breakdown of a single compound into multiple products, which is the defining characteristic of a decomposition reaction.

The other options represent different types of reactions. A combination reaction involves two or more substances combining to form a single product, whereas electrolysis is a specific process that can be considered a method for facilitating certain types of decomposition reactions, particularly in electrochemical contexts. An endothermic reaction refers to a type of reaction that absorbs heat, which does not specifically define the breakdown process of a single substance into multiple products.

Thus, the nature of decomposition reactions is focused on the splitting or breakdown of a single reactant, making it the accurate term for the described process.