Which statement best defines a decomposition reaction?

• A. A reaction where two elements form a compound.
• B. A decomposition reaction in which a single compound breaks down into simpler substances.
• C. A reaction where oxygen reacts with fuel.
• D. A reaction in which a compound trades partners with another.

Answer: (B)

A decomposition reaction is when a single compound is broken down into simpler substances. This is the reverse of making a compound from elements, so you start with one substance and end with two or more products. It often needs energy input, like heat or electricity, to break the bonds inside the compound. For example, heating calcium carbonate gives calcium oxide and carbon dioxide, and the electrolysis of water yields hydrogen and oxygen. The other descriptions describe synthesis (elements forming a compound), combustion (fuel reacting with oxygen), or an exchange/displacement process, which aren’t about a compound splitting into simpler parts.