Putrescine is a diamine produced when proteins decompose. It has an unpleasant smell and is responsible for the odour of rotting meat. Putrescine contains two amine groups. When putrescine dissolves in water, the solution formed is alkaline. Explain why the solution is alkaline, and describe what would be observed if a sample of this solution were tested with universal indicator solution.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Putrescine is a diamine produced when proteins decompose. It has an unpleasant smell and is responsible for the odour of rotting meat. Putrescine contains two amine groups.
Model answer (5 marks)
Amine groups have a lone pair of electrons on nitrogen.
When putrescine dissolves, the lone pair accepts a proton from water:
NH₂ + H₂O → NH₃⁺ + OH⁻
Thus hydroxide ions are produced.
The excess OH⁻ raises the pH above 7, so the solution is alkaline.
With universal indicator the colour changes to blue/blue‑green (or violet at very high pH), showing an alkaline solution.
When putrescine dissolves, the lone pair accepts a proton from water:
NH₂ + H₂O → NH₃⁺ + OH⁻
Thus hydroxide ions are produced.
The excess OH⁻ raises the pH above 7, so the solution is alkaline.
With universal indicator the colour changes to blue/blue‑green (or violet at very high pH), showing an alkaline solution.
Examiner tips
- Show the proton‑accepting step and the resulting OH⁻ ions; link to pH rise.
- Mention the colour change of universal indicator for alkaline solutions.
- Use correct chemical symbols and units where appropriate.
Common mistakes
- Writing the reaction as NH₂ + H₂O → NH₂⁺ + OH⁻ (incorrect protonation).
- Forgetting to state that the solution becomes alkaline (pH > 7).
- Using the wrong colour description for universal indicator (e.g., green instead of blue).
Mark scheme (5 marks)
- Amines contain a lone pair of electrons on the nitrogen atom
- The lone pair accepts a proton / H⁺ from water
- This produces hydroxide ions (OH⁻) in solution
- Excess hydroxide ions make the solution alkaline / raise pH above 7
- Universal indicator turns blue / blue-green / violet, indicating an alkaline solution
Key terms in this question
amine · alkaline · universal indicator
Related
- All WJEC A-Level Chemistry (Wales) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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