TeXes Science Practice Test

Which of the following represents a decomposition reaction?

1. H2 + O2 → H2O

2. H2SO4 → H2O(l) + SO3(g)

3. 2H2 + O2 → 2H2O

4. C + O2 → CO2

The correct answer is: H2SO4 → H2O(l) + SO3(g)

A decomposition reaction is defined as a chemical reaction in which a single compound breaks down into two or more simpler products. The selected answer reflects this definition accurately. In the case of H2SO4 → H2O(l) + SO3(g), sulfuric acid (H2SO4) decomposes into water (H2O) and sulfur trioxide (SO3). This transformation shows a clear breakdown of one compound into multiple products, fulfilling the criteria for a decomposition reaction. The other reactions provided do not represent decomposition. For instance, H2 + O2 → H2O and 2H2 + O2 → 2H2O are examples of combination reactions where hydrogen and oxygen combine to form water. Similarly, C + O2 → CO2 is a combustion reaction where carbon reacts with oxygen to produce carbon dioxide. In summary, the key aspect of the correct answer is that it illustrates the breakdown of a single substance into simpler components, which is fundamental to understanding decomposition reactions.