ACS Organic Chemistry Practice Test 2026 – Complete Study Resource

Combination (synthesis) reactions are when two substances react to form a single new substance. You’ll see two reactants on the left and one product on the right, with the product typically built from the two starting pieces. The equation A + B -> AB shows exactly that: two species come together to form one compound. The other scenarios illustrate different processes: AB -> A + B is decomposition, where a single compound splits; AB + CD -> AD + CB is double displacement, where partners swap to form two new compounds; AB + C -> AC + B is single displacement, where one element replaces another in a compound to yield a different pair. Therefore, the synthesis example is the one where two substances combine to give a single product.