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Unsaturated Hydrocarbon

An unsaturated hydrocarbon is a hydrocarbon that contains at least one double bond or triple bond between carbon atoms.

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Student-friendly explanation

Unsaturated hydrocarbons are more reactive than saturated hydrocarbons because double and triple bonds can open up for addition reactions. Ethene and ethyne are common examples. In school tests, unsaturation is often checked using bromine water or alkaline potassium permanganate, which get decolourised by such compounds.

How to write this in exams

  1. 1

    Start with the exact idea

    An unsaturated hydrocarbon is a hydrocarbon that contains at least one double bond or triple bond between carbon atoms.

  2. 2

    Then show how to use it

    1. Check the bond between carbon atoms. 2. Identify double or triple bonds. 3. Match the compound with the unsaturated family. 4. Use the bond type to explain higher reactivity.

  3. 3

    Add one concrete example

    Ethene, C2H4, is an unsaturated hydrocarbon because it contains a carbon-carbon double bond.

  4. 4

    Avoid this incomplete answer

    A common wrong answer is to call propane unsaturated because it burns well. Burning is not the test for unsaturation; the bond pattern is.

Definition

An unsaturated hydrocarbon is a hydrocarbon that contains at least one double bond or triple bond between carbon atoms.

Example

Ethene, C2H4, is an unsaturated hydrocarbon because it contains a carbon-carbon double bond.

Rule to remember

Open-chain alkenes generally follow CnH2n, and alkynes generally follow CnH2n-2. The important condition is the presence of double or triple carbon-carbon bonds.

Memory hook

Double or triple bond means unsaturated.

Examples and method

Worked example

Ethene has the formula C2H4. Because it contains a double bond between the two carbon atoms, it is unsaturated and can undergo addition reactions easily.

Method to apply

1. Check the bond between carbon atoms. 2. Identify double or triple bonds. 3. Match the compound with the unsaturated family. 4. Use the bond type to explain higher reactivity.

Diagram support

Draw ethene with a double bond or ethyne with a triple bond to highlight the unsaturated part clearly.

How CBSE asks it

Questions often ask you to identify compounds that decolourise bromine water, undergo addition, or contain a double or triple bond.

Avoid common mistakes

Common confusion

Students often think any hydrocarbon with many hydrogens is saturated. The actual bond pattern must be checked first.

Common wrong answer

A common wrong answer is to call propane unsaturated because it burns well. Burning is not the test for unsaturation; the bond pattern is.

Exam tip

Look for C=C or C≡C. If either is present, the hydrocarbon is unsaturated.

Quick check

What bond pattern tells you that a hydrocarbon is unsaturated?

A hydrocarbon is unsaturated if it contains at least one double bond or triple bond between carbon atoms. That bond pattern is the main sign of unsaturation.

Study the unsaturated hydrocarbon diagram carefully

Use the labelled diagram to keep unsaturated hydrocarbon clear in short answers and revision.

What this diagram makes clear

This diagram keeps the labels and direction of unsaturated hydrocarbon in the right order.

Where this helps in exams

Use this for labelled diagram work and short exam answers on unsaturated hydrocarbon.

Revision cue

Revise unsaturated hydrocarbon through the labels before writing the answer.

Answer writing and exam use

1-mark answer

An unsaturated hydrocarbon is a hydrocarbon that contains at least one double bond or triple bond between carbon atoms.

2-mark answer

An unsaturated hydrocarbon is a hydrocarbon that contains at least one double bond or triple bond between carbon atoms. Open-chain alkenes generally follow CnH2n, and alkynes generally follow CnH2n-2. The important condition is the presence of double or triple carbon-carbon bonds. Ethene, C2H4, is an unsaturated hydrocarbon because it contains a carbon-carbon double bond.

3-mark answer

Unsaturated hydrocarbons are more reactive than saturated hydrocarbons because double and triple bonds can open up for addition reactions. Ethene and ethyne are common examples. In school tests, unsaturation is often checked using bromine water or alkaline potassium permanganate, which get decolourised by such compounds. Open-chain alkenes generally follow CnH2n, and alkynes generally follow CnH2n-2. The important condition is the presence of double or triple carbon-carbon bonds. Ethene has the formula C2H4. Because it contains a double bond between the two carbon atoms, it is unsaturated and can undergo addition reactions easily. Questions often ask you to identify compounds that decolourise bromine water, undergo addition, or contain a double or triple bond. A common wrong answer is to call propane unsaturated because it burns well. Burning is not the test for unsaturation; the bond pattern is.
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