Saturated and unsaturated: inspect carbon–carbon bonds
Apply the definition to carbon–carbon bonding rather than every double bond or every oxygen-containing group.
Section C12.3 · Homologous series
Cross-referenced from the related Cambridge IGCSE Chemistry 0620 or Physics 0625 teaching to the 0652 syllabus. Exact 0652 Core/Supplement objective coverage and this final-series course’s examiner fit still need specialist review. Written practice is self-checked and does not predict a Cambridge grade or demonstrate supervised laboratory competence.
Before you begin
- Distinguish C–C, C=C and C=O in a displayed drawing.
- Count a double bond as two shared pairs in the ordinary model.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which notation directly shows a carbon–carbon double bond?
What you will learn
- Define saturated molecules by all carbon–carbon bonds being single.
- Identify unsaturation when at least one carbon–carbon bond is not single.
The definition names a particular bond type
A saturated compound has molecules in which all carbon–carbon bonds are single. An unsaturated compound has molecules with one or more carbon–carbon bonds that are not single, such as the C=C in ethene. In the required alkane/alkene comparison, ethane is saturated and ethene unsaturated.
Inspect the bond endpoints, not just the number of lines somewhere on the page. A C=O carbonyl double bond is not a carbon–carbon double bond. Ethanoic acid's displayed example has C–C and C=O; the carbonyl does not make its carbon skeleton unsaturated under this stated definition. This classification is different from deciding whether the compound is a hydrocarbon or an acid.
Keep structure and formula evidence distinct
The simple alkane and alkene formulas support the studied acyclic-series comparison, but a molecular formula generally gives totals rather than full connectivity. A supplied displayed structure directly identifies which carbon atoms are joined and the bond order. Use that evidence when the question concerns saturation.
Adding oxygen or an OH group alone is not a definition of carbon–carbon unsaturation. Likewise a saturated substance is not guaranteed chemically inert, harmless or unable to burn. The later lessons describe the particular reactions of alkanes, alkenes and oxygen-containing families rather than inferring every property from this one bond classification.
Sources for this lesson
Pause and explain
Does a C=O group alone establish the required carbon–carbon unsaturation?
Worked example
Compare ethane, ethene and the displayed ethanoic-acid example for the stated carbon–carbon saturation definition.
Show the worked solution
- Ethane has only a single C–C bond, so is saturated.
- Ethene has a C=C bond, so is unsaturated.
- Ethanoic acid's C=O is not C=C; its carbon–carbon bond is single under this definition, while its acid group gives a different family classification.
Answer Ethane saturated; ethene unsaturated; ethanoic acid's C=O is not carbon–carbon unsaturation.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
State both definitions and classify CH₃–CH₃ versus CH₂=CH₂.
Give me a hint
Focus on the carbon–carbon connection.
Compare my reasoning
- Saturated means all C–C bonds are single.
- Unsaturated means one or more C–C bonds are not single.
- CH₃–CH₃ is saturated; CH₂=CH₂ is unsaturated because of C=C.
All C–C single: saturated. A non-single C–C: unsaturated. Ethane saturated; ethene unsaturated.
Look for these in your work
- Both definitions name carbon–carbon bonds.
- The examples are classified from the actual connection.
A displayed CH₃COOH example contains C=O but no C=C. Explain why a report calling that carbon–carbon unsaturation has misread the bond.
Give me a hint
Read the atom at each end of the double line.
Compare my reasoning
- The double line connects carbon to oxygen.
- Its carbon–carbon connection is single.
- Thus that C=O alone does not meet the stated carbon–carbon unsaturation definition; the COOH group separately classifies the acid.
C=O is carbon–oxygen, so it does not establish carbon–carbon unsaturation in CH₃COOH.
Look for these in your work
- The bond endpoints are named.
- Family and saturation classifications remain distinct.
A student calls ethanol unsaturated solely because it contains O, and calls ethane unable to burn because it is saturated. Evaluate both claims.
Give me a hint
Neither oxygen presence nor saturation supplies those conclusions.
Compare my reasoning
- Ethanol's stated carbon–carbon bond is single; its OH group does not create a C=C bond.
- Ethane's saturated classification concerns its carbon–carbon bond order.
- Alkanes can combust, so saturation does not establish inability to burn.
Neither claim follows: ethanol's oxygen is not C–C unsaturation, and saturated ethane can combust.
Look for these in your work
- Both claims are assessed using the relevant definition.
- Bond classification is not made a universal reaction rule.
Common mistakes
- Counting any double bond as carbon–carbon unsaturation without reading its atoms.
- Equating oxygen content with saturation status.
- Calling every saturated substance inert or unable to burn.
- Using a molecular atom total as a complete displayed structure.
What can you explain now?
What bond feature establishes carbon–carbon unsaturation in the stated definition?
Compare with the explanation
At least one carbon–carbon bond that is not single.
The endpoints must both be carbon; a C=O double bond is a different feature.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Read three fresh structures, identify the carbon–carbon bond orders and distinguish saturation from functional-group family or combustion behaviour.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.