1. Which feature identifies an alkene?
- An OH group
- A carbon–carbon double bond
- Only single carbon bonds
Check the answer and reasoning
A carbon–carbon double bond
The C=C bond is the alkene functional group.
CHEM / IGCSE
Organic families, fuels and polymers
Organic compounds are classified by functional groups and homologous series. Alkanes undergo combustion and substitution; alkenes contain a carbon–carbon double bond and undergo addition.
Learning objectives
- Explain organic families, fuels and polymers using a concrete example.
- Apply the worked method and justify each step.
- Recognise the limitation described in the final check.
The big idea
Organic compounds are classified by functional groups and homologous series. Alkanes undergo combustion and substitution; alkenes contain a carbon–carbon double bond and undergo addition.
Alcohols, carboxylic acids and esters connect fuels with chemical synthesis. Addition polymers repeat alkene-derived units, while condensation polymers form links with loss of small molecules. Cracking changes molecular size rather than separating a mixture.
Equations and relationships
General formula for acyclic saturated alkanes.
Worked example
What is the molecular formula of propane?
Follow the solution
Use n=3 in CₙH₂ₙ₊₂: hydrogen count is 2×3+2=8.
Investigate and explain
Predict the worked result before calculating. Change one input, record the new result, and explain which assumptions still hold.
Related interactive model. Compare its assumptions with the explanation; it is not a complete syllabus lesson.
Check your understanding
2. What is the molecular formula of propane?
- C₃H₆
- C₃H₄
- C₃H₈
Check the answer and reasoning
C₃H₈
Use n=3 in CₙH₂ₙ₊₂: hydrogen count is 2×3+2=8.
3. Does fractional distillation itself break hydrocarbon bonds?
- No
- Yes
- Only for large molecules
Check the answer and reasoning
No
Distillation separates by boiling behaviour; cracking breaks covalent bonds.