A synthesis reaction combines reactants to form a more complex product, represented by A+B→ AB. For example, magnesium combines with oxygen to form magnesium oxide. A decomposition reaction breaks a compound into simpler substances, represented by AB →A+B. Heating calcium carbonate forms calcium oxide and carbon dioxide. These patterns describe how substances are related, not whether energy is released or absorbed.
A valid reaction equation must still use correct formulas and conserve atoms. In 2H2O→2H2+O2, water decomposes, and both sides contain four hydrogen atoms and two oxygen atoms. Classify the stated overall pattern rather than assuming every reaction with heating is decomposition or every reaction with bubbles is the same type.
Example 1
Classify 2Mg+O2→2MgO.
Two reactants combine to form one product substance, so this is synthesis. Both Mg and O atom counts are conserved.
Example 2
Classify CaCO3→CaO+CO2 and check oxygen.
One compound forms two simpler substances, so it is decomposition. The left has three O atoms; the right has one plus two, also three.
Watch out: Heating is a condition. Classification depends on the reactant–product pattern.