high importancemedium difficulty5 min

Homologous Series

A homologous series is a family of organic compounds with the same functional group and the same general formula, where successive members differ by a CH₂ group.

In one line: Same group, same family, one CH₂ more each step.

Practice This Concept

Understand it

Members of a homologous series have similar chemical properties because they share the same functional group. Their physical properties show a gradual change as molecular mass increases. This idea helps students organise carbon compounds into families instead of learning them as isolated names.

Example

Methanol, ethanol, and propanol form a homologous series because each has the -OH group and each next member differs by CH₂.

Rule to remember

Members of the same homologous series have the same general formula, the same functional group, and a CH₂ difference between successive compounds.

See it worked out

Follow the example line by line, then use the same method on new questions.

Worked example

1Methane, ethane, and propane belong to the alkane series.
2Each next member has one extra CH₂ unit, so they form a homologous series with similar chemical behaviour.

Method you can reuse

1Check the functional group.
2Compare the general formula.
3See whether successive compounds differ by CH₂.
4Conclude whether they form a homologous series.

Sketch it in your answer

Write three consecutive members vertically to show the CH₂ difference clearly, such as CH₄, C₂H₆, and C₃H₈.

Check yourself

Answer the question in your head before opening the answer.

Try it yourself

What is the common difference between two successive members of a homologous series?

Think first, then show answer

Successive members of a homologous series differ by one CH₂ unit. They also have the same functional group and similar chemical properties.

Common wrong answer

A common wrong answer is to call any set of related compounds a homologous series. Without the CH₂ difference and same functional group, the answer is incomplete.

Don't mix this up

Students often think any group of carbon compounds is a homologous series. The same functional group and a fixed difference of CH₂ are both required.

Exam tip

When comparing two compounds, check the functional group first and then see whether the formulas differ by CH₂.

Write it in the exam

Each point is worth about one mark. Write point 1 for a 1-mark question and keep adding points as the marks go up.

How CBSE asks it

Questions may ask you to identify a series, state the common difference, or explain why members behave similarly.

1
Enough for 1 mark
A homologous series is a family of organic compounds with the same functional group and the same general formula, where successive members differ by a CH₂ group.
2
Add for 2 marks
Example: Methanol, ethanol, and propanol form a homologous series because each has the -OH group and each next member differs by CH₂.
3
Add for 3 marks
Members of a homologous series have similar chemical properties because they share the same functional group.

MCQ Quiz

Practice this concept with focused MCQs

Open the concept quiz intro first, review the test details, and then start a focused MCQ set from this concept only. Instant score and answer review are live now.

10 MCQs5 MinutesInstant Results