Decomposition reactions are considered the opposite of combination reactions because they involve breaking down a single reactant into two or more simpler products, while combination reactions involve combining two or more reactants into a single product.
Combination Reactions:
Two or more reactants combine to form a single product.
General form: A + B → AB.
Example: 2H₂ + O₂ → 2H₂O (Hydrogen burning in oxygen to form water).
Decomposition Reactions:
A single reactant breaks down into two or more products.
General form: AB → A + B.
Example: 2HgO → 2Hg + O₂ (Decomposition of mercury(II) oxide).