Why homologous series of carbon compounds are so called? Write chemical formula of two consecutive members of a homologous series and state the part of these compounds that determines their (i) physical properties, and (ii) chemical properties.

1. Why Homologous Series is So Called

A homologous series is a family of organic compounds that share the same functional group, have similar chemical properties, and can be represented by a general molecular formula. They are called 'homologous' (from Greek homos meaning 'same' and logos meaning 'relation') because successive members of the series differ from each other by a constant structural unit, which is a methylene group ($-\text{CH}_2-$), and a molecular mass difference of $14\text{ u}$.

2. Chemical Formula of Two Consecutive Members

Taking the alkane homologous series as an example:

• Methane: $\text{CH}_4$
• Ethane: $\text{C}_2\text{H}_6$

(Alternatively, for the alcohol series: Methanol $\text{CH}_3\text{OH}$ and Ethanol $\text{C}_2\text{H}_5\text{OH}$)

3. Parts Determining Properties

• (i) Physical Properties: The alkyl group / carbon chain length (molecular mass) determines the physical properties (such as melting point, boiling point, and solubility). As the carbon chain length increases, the molecular mass increases, leading to a regular gradation in physical properties.
• (ii) Chemical Properties: The functional group (e.g., $-\text{OH}$ in alcohols, $-\text{COOH}$ in carboxylic acids, or double/triple bonds in alkenes/alkynes) determines the chemical properties of the compounds in a homologous series.