Which of the following chemical reactions is an example of a single substance breaking down into simpler substances?

• A. C + O2 → CO2
• B. H2 + Cl2 → 2HCl
• C. 2H2O2 → 2H2O + O2
• D. NaCl + AgNO3 → AgCl + NaNO3

Answer: (C)

Verified step by step guidance

1. Understand the definition of a decomposition reaction: it is a type of chemical reaction where a single compound breaks down into two or more simpler substances.
2. Examine each given reaction to identify if it involves one reactant breaking down into multiple products.
3. For the reaction \(\mathrm{C + O_2 \rightarrow CO_2}\), note that there are two reactants combining to form one product, so this is a synthesis reaction, not decomposition.
4. For the reaction \(\mathrm{H_2 + Cl_2 \rightarrow 2HCl}\), two reactants combine to form one product, which is also a synthesis reaction.
5. For the reaction \(\mathrm{2H_2O_2 \rightarrow 2H_2O + O_2}\), observe that one compound, hydrogen peroxide, breaks down into two simpler substances, water and oxygen, which fits the definition of a decomposition reaction.