Which equation represents a synthesis reaction?

• A. H2O -> H2 + O2
• B. NaCl + AgNO3 -> NaNO3 + AgCl
• C. 2 H2 + O2 -> 2 H2O
• D. CH4 + 2 O2 -> CO2 + 2 H2O

Answer: (C)

In a synthesis reaction, two or more substances join to form a single new compound. The reaction where hydrogen gas and oxygen gas come together to make water is a classic example: two reactants form one product, shown by 2 H2 + O2 -> 2 H2O. This illustrates elements combining to build a compound.

The other scenarios are different kinds of reactions: water decomposes into hydrogen and oxygen, which is a breakdown; NaCl and AgNO3 react to form NaNO3 and AgCl, a double-displacement/precipitation process; methane combusts with oxygen to produce carbon dioxide and water, a combustion/oxidation reaction.