Explain the difference between saturated and unsaturated hydrocarbons. Give the general formulas of alkanes, alkenes and alkynes with the structure of the two-carbon member of each, and explain what a homologous series is.

1. 1 · Define. Hydrocarbons are compounds of carbon and hydrogen only.
2. 2 · Saturated and unsaturated. Saturated hydrocarbons have only single C-C bonds; unsaturated ones have a double or triple bond.
3. 3 · Three classes.
   • Alkanes, CₙH₂ₙ₊₂: ethane, C₂H₆
   • Alkenes, CₙH₂ₙ: ethene, C₂H₄
   • Alkynes, CₙH₂ₙ₋₂: ethyne, C₂H₂
4. 4 · Draw the structures. Draw ethane (one line between the C atoms), ethene (two lines) and ethyne (three lines), adding H atoms so each C has four bonds.
5. 5 · Homologous series. A family with one general formula whose members differ by CH₂ (14 u), such as methane, ethane and propane.
6. 6 · Real life. LPG (butane and propane) and CNG (methane) are alkanes used as fuels.
7. 7 · Close in one line. So the type of bond decides the class and its general formula.

Scoring tip: Check that every carbon in your structures has four bonds. A carbon with three or five lines loses the diagram mark.