In which type of reaction do two molecules combine to form a single product?

• A. Decomposition reaction
• B. Synthesis reaction
• C. Single displacement reaction
• D. Double displacement reaction

Answer: (B)

The correct answer is a synthesis reaction. In a synthesis reaction, two or more reactants combine to form a single, more complex product. This type of reaction can be represented by the general form:

[ A + B \rightarrow AB ]

where A and B are reactants that come together to form the compound AB. This process is fundamental in various biological and chemical systems, as it underpins the formation of larger molecules from simpler precursors, such as in the synthesis of proteins from amino acids.

In contrast, a decomposition reaction involves a single reactant breaking down into two or more products, which is the opposite process. A single displacement reaction, also known as a single replacement reaction, involves one element displacing another in a compound, leading to changes in the products but not the combination of two molecules into one. A double displacement reaction features the exchange of components between two compounds but does not result in the formation of a single product. Thus, the definition of a synthesis reaction aligns perfectly with the requirement of two reacting molecules combining to form a singular product.