Combination reactions, also known as synthesis reactions, occur when two or more substances combine to form a single product. During a combination reaction, multiple reactants come together to create a more complex product. The general form of a combination reaction can be represented as:

Reactant A + Reactant B → Product AB

Decomposition reactions, on the other hand, are the opposite of combination reactions. In a decomposition reaction, a single compound breaks down into two or more simpler products. During decomposition, a complex molecule is broken down into smaller constituent molecules or elements. The general form of a decomposition reaction can be represented as:

Compound AB → Reactant A + Reactant B

In summary, the main difference between a combination reaction and a decomposition reaction is that a combination reaction involves the joining of multiple substances to form a single product, while a decomposition reaction involves the breakdown of a single compound into multiple products.