IGCSE Chemistry 0620 · Common mistakes
Common mistakes in IGCSE organic chemistry
Updated · by the Fahmazing team
Organic chemistry is full of names that look alike and reactions that sound alike. These are the slips that most often cost marks, each shown next to the answer that earns them.
Saying bromine water turns "clear"
Clear means see-through, and orange bromine water is already clear. The mark scheme wants colourless.
What is seen when ethene is bubbled through bromine water?
✗ Wrong: It turns from orange to clear.
✓ Right: It turns from orange to colourless.
Free lesson: Alkenes
Giving alkanes the reactions of alkenes
Only alkenes (with a C=C bond) decolourise bromine water or undergo addition. Alkanes are saturated. With chlorine in UV light they undergo substitution: one H swaps for one Cl.
Methane reacts with chlorine in UV light.
✗ Wrong: CH₄ + Cl₂ → CH₄Cl₂ (addition)
✓ Right: CH₄ + Cl₂ → CH₃Cl + HCl (substitution)
Free lesson: Alkanes
Writing carbon monoxide for complete combustion
With plenty of oxygen, an alkane burns to carbon dioxide and water. Carbon monoxide and soot come from incomplete combustion.
Complete combustion of methane
✗ Wrong: CH₄ + O₂ → CO + 2H₂O
✓ Right: CH₄ + 2O₂ → CO₂ + 2H₂O
Free lesson: Alkanes
Balancing only the carbon in cracking
Both sides must have the same number of carbon and hydrogen atoms. Subtract both.
C₁₀H₂₂ → C₈H₁₈ + ?
✗ Wrong: C₂H₂
✓ Right: C₂H₄ (ethene): 10 − 8 = 2 carbons and 22 − 18 = 4 hydrogens
Tip: Cracking needs both a high temperature and a catalyst. Giving only one loses the mark.
Free lesson: Alkenes
Mixing up the two ways to make ethanol
Fermentation uses yeast at 25–35 °C with no oxygen: slow but renewable. Hydration of ethene uses steam at about 300 °C and 6000 kPa with an acid catalyst: fast and continuous, but uses ethene from petroleum.
State the conditions for fermentation.
✗ Wrong: 300 °C with an acid catalyst
✓ Right: Yeast, 25–35 °C, in the absence of oxygen
Free lesson: Alcohols
Confusing ethanol, ethanoic acid and ethanoate
Look at the ending: -ol is an alcohol, -oic acid is a carboxylic acid, and salts of ethanoic acid are ethanoates.
Name the salt formed from ethanoic acid and sodium hydroxide.
✗ Wrong: Sodium ethanol
✓ Right: Sodium ethanoate
Free lessons: Carboxylic acids · Naming organic compounds
Naming an ester the wrong way round
The alcohol gives the first word (ending -yl); the acid gives the second (ending -oate).
Ester from ethanol and propanoic acid
✗ Wrong: Propyl ethanoate
✓ Right: Ethyl propanoate
Free lesson: Naming organic compounds
Leaving the C=C in a polymer
In addition polymerisation each C=C opens and links to the next monomer. The repeat unit has only single C–C bonds in the chain, and no other product forms.
Repeat unit of poly(ethene)
✗ Wrong: –CH₂=CH₂–
✓ Right: –CH₂–CH₂–
Free lesson: Polymers
Questions students ask
How do you test for an alkene?
Add bromine water. An alkene turns it from orange to colourless. An alkane leaves it orange.
What is the difference between addition and substitution?
In addition, atoms join across a C=C double bond and one product forms. In substitution, one atom swaps for another, so there are two products, such as CH₃Cl and HCl.
Why is fermentation done without oxygen?
With oxygen, yeast respires aerobically and ethanol can be oxidised to ethanoic acid, so little ethanol is made.