What is a homologous series?

A homologous series is a group of organic compounds that:

1. Have the same general formula (e.g., alkanes: CₙH₂ₙ₊₂, alkenes: CₙH₂ₙ).
2. Differ by a constant unit — usually a –CH₂– group between successive members.
3. Show similar chemical properties because they contain the same functional group.
4. Show a gradual change in physical properties (like boiling point, melting point, solubility, density) as the molecular mass increases.

For example:

• The alkane series: methane (CH₄), ethane (C₂H₆), propane (C₃H₈), butane (C₄H₁₀) …
  Each member differs from the next by –CH₂–.

In short: A homologous series is a “family” of organic compounds with the same functional group, similar chemistry, and a regular difference in structure.