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A homologous series is a group of organic compounds that have a similar structure, general formula, and chemical properties due to the presence of the same functional group. The consecutive members of the series differ by a CH2 group or an increase of 14 in their molecular mass.

The main characteristics of a homologous series include: 1. Similar Structure: All members of the homologous series have a similar structure with the same arrangement of atoms and the same functional group. 2. General Formula: The members of a homologous series follow the same general molecular formula. For example, the general formula for alkanes is CnH2n+2, where n is the number of carbon atoms in the compound. 3. Physical Properties: Physical properties, such as boiling points, melting points, and density, show variations among members of the same homologous series. The difference in physical properties can be attributed to the increase in molecular mass or the increase in the number of carbon atoms. 4. Chemical Properties: The members of a homologous series exhibit similar chemical properties due to the presence of the same functional group. They generally undergo the same chemical reactions. 5. Gradation in Properties: In a homologous series, there is a gradation (or regular change) in physical and chemical properties as the molecular size and mass increase. For example, boiling and melting points increase with the increase in molecular mass.

Some common examples of homologous series include: 1. Alkanes: These are saturated hydrocarbons following the general formula CnH2n+2 (e.g., methane, ethane, propane, etc.). 2. Alkenes: These are unsaturated hydrocarbons with a carbon-carbon double bond, following the general formula CnH2n (e.g., ethene, propene, butene, etc.). 3. Alkynes: These are unsaturated hydrocarbons with a carbon-carbon triple bond, following the general formula CnH2n-2 (e.g., ethyne, propyne, butyne, etc.). 4. Alcohols: These organic compounds contain a hydroxyl (-OH) functional group and follow the general formula CnH2n+1OH (e.g., methanol, ethanol, propanol, etc.).