Identify the synthesis reaction among the following equations.

• A. A + B → AB
• B. AB → A + B
• C. AB + CD → AD + CB
• D. CH4 + O2 → CO2 + H2O

Answer: (A)

The main concept being tested is synthesis (combination) reactions, where two or more substances combine to form a single product. In the equation A + B → AB, two reactants come together to make one compound, which is the hallmark of a synthesis reaction.

The other equations illustrate different reaction types: AB → A + B is decomposition, where a single compound breaks into simpler substances; AB + CD → AD + CB is a double displacement (metathesis) reaction, where partners swap to form two new compounds; CH4 + O2 → CO2 + H2O is combustion, a hydrocarbon reacting with oxygen to yield carbon dioxide and water (often with energy release).

So the equation where two substances combine to form one product best represents a synthesis reaction.