Which type of reaction is characterized by a substance breaking down into simpler substances?

The type of reaction characterized by a substance breaking down into simpler substances is a decomposition reaction. In this type of reaction, a single compound undergoes a process where it splits into two or more simpler products, which can be elements or smaller compounds. This process often requires an input of energy, such as heat, light, or electricity, to occur.

For example, when water (H2O) is decomposed through electrolysis, it breaks down into oxygen (O2) and hydrogen (H2) gases. This illustrates the fundamental principle of decomposition reactions, where a complex molecule is transformed into simpler components.

In contrast, synthesis reactions involve combining simpler substances to form a more complex compound, replacement reactions involve the exchange of ions between compounds, and combustion reactions typically involve a substance reacting with oxygen to produce energy, often releasing heat and light. These other types do not fit the definition of breaking down into simpler substances like decomposition reactions do.