Trimethylamine, (CH₃)₃N, is the compound responsible for the characteristic smell of rotting fish. When trimethylamine dissolves in water, the solution turns litmus paper blue. Explain why trimethylamine produces an alkaline solution when dissolved in water, and describe two other chemical properties you would expect trimethylamine to show.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Trimethylamine, (CH₃)₃N, is the compound responsible for the characteristic smell of rotting fish. When trimethylamine dissolves in water, the solution turns litmus paper blue.
Model answer (5 marks)
Trimethylamine has a lone pair of electrons on the nitrogen atom, which can accept a proton from water:
(CH₃)₃N + H₂O → (CH₃)₃NH⁺ + OH⁻
The formation of hydroxide ions makes the solution alkaline.
Two further properties:
1. It reacts with acids to give the corresponding ammonium salt, e.g. (CH₃)₃NH⁺Cl⁻.
2. It is a volatile, pungent compound with a characteristic fishy odour.
(CH₃)₃N + H₂O → (CH₃)₃NH⁺ + OH⁻
The formation of hydroxide ions makes the solution alkaline.
Two further properties:
1. It reacts with acids to give the corresponding ammonium salt, e.g. (CH₃)₃NH⁺Cl⁻.
2. It is a volatile, pungent compound with a characteristic fishy odour.
Examiner tips
- Show the proton‑accepting mechanism to gain the 2 marks for alkalinity; use the formula to demonstrate OH⁻ formation. Mention the salt formation and the smell to cover the remaining 3 marks. Use the exact wording "lone pair" and "hydroxide ions" as the scheme rewards.
- common_mistakes
- :
- Writing the reaction as (CH₃)₃N + H₂O → (CH₃)₃NH⁺ + H₂O (missing OH⁻). Forgetting to state that trimethylamine is a base. Describing the smell as "sweet" instead of "fishy".
Mark scheme (5 marks)
- Trimethylamine contains a lone pair of electrons on the nitrogen atom
- The lone pair accepts / accepts a proton (H⁺) from water, producing hydroxide ions (OH⁻), making the solution alkaline
- Trimethylamine reacts with acids to form a salt
- Trimethylamine acts as a base / is a base
- Trimethylamine has a characteristic / pungent / unpleasant smell (fishy odour) and is a volatile compound
Key terms in this question
Related
- All WJEC A-Level Chemistry (Wales) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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