Alcohol – IGCSE Chemistry Definition
IGCSE Chemistry definition of alcohol: an organic compound with the −OH functional group. Covers ethanol, production methods, reactions, and exam tips for 0620.
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.
Alcohols, especially ethanol, are heavily tested in IGCSE Chemistry. Examiners focus on the two production methods (and their comparison), combustion, and the reaction with carboxylic acids to form esters. Ethanol questions alone can carry 5–8 marks across a paper.
The 0620 definition
Alcohols are a homologous series of organic compounds containing the hydroxyl functional group, −OH. Their general formula is CₙH₂ₙ₊₁OH.
The first three alcohols
| Name | Formula | Boiling point |
|---|---|---|
| Methanol | CH₃OH | 65 °C |
| Ethanol | C₂H₅OH | 78 °C |
| Propanol | C₃H₇OH | 97 °C |
Boiling points increase with chain length (stronger intermolecular forces).
Two methods of making ethanol
Method 1: Fermentation
C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂
Conditions: yeast (enzyme zymase), 25–35 °C, anaerobic (no air). See fermentation.
Method 2: Hydration of ethene
C₂H₄ + H₂O → C₂H₅OH
Conditions: phosphoric acid catalyst, 300 °C, 60 atm.
| Feature | Fermentation | Hydration |
|---|---|---|
| Raw material | Sugar / glucose (renewable) | Ethene from crude oil (non-renewable) |
| Rate | Slow (days) | Fast (continuous) |
| Purity | Low (needs distillation) | High |
| Temperature | Low (25–35 °C) | High (300 °C) |
| Type of process | Batch | Continuous |
Reactions of ethanol
Combustion (as a fuel)
C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O
Ethanol burns with a clean, blue flame. It is used as a biofuel and in spirit burners.
Reaction with carboxylic acids (ester formation)
Ethanol + ethanoic acid → ethyl ethanoate + water
See ester for full details.
Oxidation
Ethanol can be oxidised to ethanoic acid (carboxylic acid) by heating with acidified potassium dichromate(VI). The solution changes from orange to green.
Worked exam question
Compare the production of ethanol by fermentation and by hydration of ethene. Give one advantage of each method. [2]
Mark scheme
Fermentation: uses renewable raw material / sugar / low energy cost / low temperature needed [1]
Hydration: faster / produces purer ethanol / continuous process [1]
Common exam mistakes
- Writing the fermentation equation unbalanced — remember the 2 in front of both C₂H₅OH and CO₂.
- Confusing the −OH group in alcohols with NaOH — alcohols are covalent, not ionic.
- Saying ethanol is only a fuel — it is also a solvent and a raw material for making esters.
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