Combination: In chemistry, combination refers to the process of two or more substances combining to form a new substance. The starting substances, known as reactants, undergo a chemical reaction during which their atoms and molecules rearrange to form the new substance, known as the product. The process can be represented as:

Reactant A + Reactant B -> Product AB

For example, when hydrogen gas (H2) and oxygen gas (O2) combine, they react to form water (H2O).

Decomposition: Decomposition is the opposite process of combination. It involves the breakdown or splitting of a compound into simpler substances. During decomposition, the starting substance, known as the reactant, breaks down into two or more new substances, known as the products. The process can be represented as:

Reactant AB -> Product A + Product B

For example, when water (H2O) is decomposed, it breaks down into hydrogen gas (H2) and oxygen gas (O2).

In summary, combination is the process of combining substances to form a new substance, while decomposition is the process of breaking down a substance into simpler substances. These two processes play important roles in various chemical reactions and natural phenomena.