Which reaction type involves a compound breaking down into simpler substances when heated?

The reaction type that involves a compound breaking down into simpler substances when heated is known as a decomposition reaction. In this type of reaction, a single compound undergoes a transformation, resulting in two or more products. This process typically requires energy input, which can come from heat, light, or electricity, to break the chemical bonds holding the compound together.

For instance, when calcium carbonate (CaCO3) is heated, it decomposes into calcium oxide (CaO) and carbon dioxide (CO2). This reaction illustrates how a single compound can split into simpler components through the application of heat, highlighting the essence of a decomposition reaction.

In contrast, combination reactions, single substitution reactions, and double substitution reactions involve the formation of new compounds through the combination of elements or the exchange of components between compounds, rather than the breakdown of one compound into simpler substances.