Not All Hydrocarbons Are Alkanes
Why IGCSE Chemistry students confuse hydrocarbons with alkanes, how alkenes and other homologous series are also hydrocarbons, and the key differences the 0620 mark scheme tests.
Published by IGCSEChemistry.com.my
Chemistry teaching team: K. S. Tan (15+ years teaching IGCSE Chemistry) and Ms Yash (10+ years teaching IGCSE Chemistry) and Ms Kartini (15+ years teaching IGCSE Chemistry).
Mapped to Cambridge IGCSE Chemistry 0620 (2026–2028). Last updated 2026-08-19.
What students typically write
“Hydrocarbons are alkanes.” Or: “All hydrocarbons have the formula CnH2n+2.” Or: “Ethene is not a hydrocarbon because it is an alkene.”
Students learn alkanes first and associate the term “hydrocarbon” exclusively with them. When alkenes are introduced, they treat them as a separate category rather than recognising that both alkanes and alkenes are subsets of the broader hydrocarbon family.
Why it is wrong
“Hydrocarbon” is a composition term: it tells you what elements are present (carbon and hydrogen only). Any compound made exclusively of C and H atoms is a hydrocarbon, regardless of its structure.
Within the hydrocarbon family, different structures define different homologous series:
| Series | Bonding | General formula | Example |
|---|---|---|---|
| Alkanes | Single C-C bonds only (saturated) | CnH2n+2 | CH4, C2H6 |
| Alkenes | At least one C=C double bond (unsaturated) | CnH2n | C2H4, C3H6 |
At IGCSE level, you need to know alkanes and alkenes. Both are hydrocarbons. The general formula CnH2n+2 belongs to alkanes only, not to all hydrocarbons.
This distinction matters in classification questions. If a question asks “name a hydrocarbon”, both methane (alkane) and ethene (alkene) are correct. If it asks “name an alkane”, only methane, ethane, propane, butane etc. are correct.
What the mark scheme actually wants
A typical question: “Ethene (C2H4) is a hydrocarbon. State what is meant by the term hydrocarbon.” [1]
Marking point:
- A compound containing carbon and hydrogen only (1 mark)
Note that the definition says nothing about single bonds or the general formula CnH2n+2. Those would define an alkane, not a hydrocarbon.
Worked example: wrong vs right
Question: Classify the following as alkane, alkene, or neither: (a) C3H8, (b) C3H6, (c) C3H7OH. State which are hydrocarbons. [4]
Wrong answer: “(a) Alkane and hydrocarbon. (b) Neither, because it doesn’t fit CnH2n+2. (c) Not a hydrocarbon.” This scores 1/4. C3H6 is wrongly rejected as a hydrocarbon.
Right answer: “(a) Alkane (fits CnH2n+2) — hydrocarbon (1). (b) Alkene (fits CnH2n, has C=C) — hydrocarbon (1). (c) Not an alkane or alkene; it is an alcohol (1). It is not a hydrocarbon because it contains oxygen (1).” This scores 4/4.
How to avoid this mistake
Use a two-step classification:
- Is it a hydrocarbon? Check: does it contain only C and H? If it contains O, N, Cl or any other element, it is not a hydrocarbon.
- What type of hydrocarbon? Check the bonding. Only single bonds = alkane. Contains C=C = alkene.
Keep “hydrocarbon” as the umbrella term and “alkane”/“alkene” as the specific categories underneath it.
For the full treatment of organic families, see alkanes, alkenes, and homologous series.
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