A Level Chemistry OCR Practice Exam

What characterizes a substitution reaction?

1. A reaction where two molecules combine

2. A reaction in which an atom is replaced with a different atom

3. A reaction that involves no change in chemical species

4. A reaction that produces multiple products

The correct answer is: A reaction in which an atom is replaced with a different atom

A substitution reaction is characterized by the replacement of one atom or a group of atoms in a molecule with a different atom or group of atoms. In this type of reaction, the original compound remains largely intact, but part of it is exchanged for another atom or functional group. This is fundamental in organic chemistry, where such reactions often involve hydrocarbons and can lead to the formation of diverse compounds by altering their functional groups. The other characteristics do not align with the definition of a substitution reaction. For instance, the first option describes a combination reaction, where two reactants form a single product. The choice stating that there is no change in chemical species applies more to reactions that do not involve any transformation, such as physical changes or certain equilibrium processes. Lastly, while some substitution reactions can lead to multiple products, this is not a defining trait of all substitution reactions, as many can yield a single product. Thus, the correct characterization of a substitution reaction is indeed that it entails the replacement of one atom or group with another.