Here's why:

* Chemical reactions involve the breaking and forming of chemical bonds. Decomposition reactions specifically involve the breaking down of a single reactant into two or more products.

* The chemical composition changes. The original compound is broken down into simpler substances with different chemical formulas.

Example:

* The decomposition of hydrogen peroxide: 2H₂O₂ → 2H₂O + O₂

In this reaction, hydrogen peroxide (H₂O₂) breaks down into water (H₂O) and oxygen gas (O₂). The chemical bonds within the hydrogen peroxide molecule are broken, resulting in the formation of new substances.