Which of the following factors characterize members of the same homologous series?

Analyze the question: You are asked which statement correctly describes members of the same homologous series (e.g., alkanes: CH₄, C₂H₆, C₃H₈...).

Concept: A homologous series is a family of organic compounds that:

• share the same general formula (e.g., CₙH₂ₙ₊₂ for alkanes)
• have the same functional group
• have similar chemical properties
• show a gradual change in physical properties (boiling point, density) as molar mass increases
• differ from each other by a CH₂ unit between successive members

Check each option:

• A. Chemical properties change gradually: Wrong. Members have similar chemical properties because they share the same functional group. It is the physical properties that change gradually.
• B. Physical properties are similar: Wrong. Physical properties vary gradually, for example boiling points rise as the chain gets longer (methane is a gas, while hexane is a liquid).
• C. Same molecular formula: Wrong. Members have the same general formula but different molecular formulae. Same molecular formula describes isomers, not homologues.
• D. Successive members differ by CH₂: Correct. For example, CH₄ → C₂H₆ → C₃H₈, with each step adding one CH₂ (a difference of 14 mass units).

Answer: D

Exam tip: Remember the homologous series rules as "Same general formula, same functional group, similar chemical properties, gradual physical change, CH₂ difference." If an option says chemical properties change or physical properties are the same, it is a trap, because the two are swapped.