Which type of reaction involves the breaking down of substances?

A decomposition reaction is characterized by the process of breaking down a compound into its constituent elements or simpler compounds. In this type of reaction, one substance undergoes a transformation that results in the formation of two or more products, which are typically simpler molecules or atoms. This process can be triggered by various factors such as heat, light, or catalysts.

For example, when water (H2O) is decomposed through electrolysis, it breaks down into hydrogen (H2) and oxygen (O2) gases. This exemplifies how a single compound can divide into simpler substances, which is the very essence of a decomposition reaction.

In contrast, synthesis reactions involve the combination of two or more substances to form a more complex compound, which is the opposite of decomposition. Combustion reactions usually involve a substance reacting with oxygen to produce heat and light, commonly resulting in the formation of carbon dioxide and water from hydrocarbons. Redox reactions encompass the transfer of electrons between substances, which may involve oxidation and reduction but do not specifically define the breaking down of compounds in the same way that decomposition does. Thus, the distinctive feature of the decomposition reaction makes it the correct choice for identifying the breakdown of substances.