International General Certificate of Secondary Education (IGCSE) Chemistry Practice Exam

What defines a homologous series in chemistry?

1. A sequence with increasing atomic number

2. A family of compounds with the same general formula

3. A series of organic reactions

4. A sequence of elements in the periodic table

The correct answer is: A family of compounds with the same general formula

A homologous series in chemistry is defined as a family of compounds that share the same general formula, which means they have similar chemical properties. This series consists of molecules that differ from each other by a specific unit, often a –CH₂– group, which leads to a gradual change in physical properties such as boiling point and melting point. For example, in the case of alkanes, the general formula is CₙH₂ₙ₊₂, where "n" represents the number of carbon atoms. This uniformity allows members of the series to undergo similar chemical reactions, making it easier to predict their behavior. In contrast, the other choices do not accurately capture what constitutes a homologous series. A sequence with increasing atomic number relates more to the arrangement of elements in the periodic table, while a series of organic reactions refers to the chemical transformations that compounds undergo rather than a family of compounds. Similarly, a sequence of elements in the periodic table describes the organization of different elements based on properties, rather than the characteristics of a chemical series. Thus, the definition provided by the correct choice accurately represents the core concept of a homologous series in chemistry.