AQA A-Level Chemistry Paper 3, June 2022: Question 33

1 mark · Easy difficulty · Multiple Choice

Identify the correct statement about the amino acid serine, HOCH2CH(NH2)COOH.

Practise this question

Question

Multiple choice question 33 asking: 'Which statement about HOCH2CH(NH2)COOH is correct?' followed by four options: A 'It decolourises bromine water.', B 'It is a component of DNA.', C 'It is insoluble in water.', and D 'It reacts with hydrochloric acid.', each with an answer selection box.
Question text

33 Which statement about HOCH2CH(NH2)COOH is correct?

[1 mark]

A It decolourises bromine water.

B It is a component of DNA.

C It is insoluble in water.

D It reacts with hydrochloric acid.

Mark scheme

Show the mark scheme Mark scheme table row for question 33 showing the correct answer as 'D (AO1)', worth 1 mark, with the additional notes confirming 'It reacts with hydrochloric acid.'

33 D (AO1) 1 It reacts with hydrochloric acid.

How to answer it

Properties and Reactions of 2-Amino Acids (Serine)

📋 What this question tests

This question assesses your ability to identify functional groups within condensed structural formulae, deduce their characteristic acid-base properties, and distinguish biological molecules (amino acids/proteins vs DNA constituents).

  • Functional group identification: Recognising primary alcohol (-OH), primary amine (-NH₂), and carboxylic acid (-COOH) groups.
  • Acid-base chemistry of amino acids: The basic nature of the amine group reacting with strong acids (e.g. HCl).
  • Solubility and bonding: Zwitterion formation and extensive hydrogen bonding with water.
  • Biochemical components: Understanding the molecular units that make up proteins versus nucleic acids (DNA).

Question 33

Multiple Choice: Which statement about HOCH₂CH(NH₂)COOH is correct?

Mark scheme: D (AO1, 1 mark)

✅ Correct Answer: D

It reacts with hydrochloric acid.

The molecule contains a basic amino group ( -NH₂ ). The lone pair of electrons on the nitrogen atom acts as a proton acceptor (Brønsted-Lowry base):

HOCH₂CH(NH₂)COOH + HCl → HOCH₂CH(NH₃⁺Cl⁻)COOH

This forms a soluble substituted ammonium salt.

💡 Key Knowledge Breakdown

  • Structure: The compound shown is serine (2-amino-3-hydroxypropanoic acid), an α-amino acid.
  • Amphoteric nature: Amino acids can act as both acids (via -COOH ) and bases (via -NH₂ ).
  • Why A is incorrect: Decolourising bromine water is a test for carbon-carbon double bonds ( C=C ) or phenols. Serine is completely saturated and aliphatic.
  • Why B is incorrect: Amino acids are monomers of proteins. DNA consists of 2-deoxyribose, phosphate ions, and nitrogenous bases (A, T, C, G).
  • Why C is incorrect: Amino acids exist as zwitterions in solid and solution forms and possess multiple polar groups ( -OH, -NH₂, -COOH ), allowing strong hydrogen bonding and electrostatic attraction with water molecules, making them readily soluble.

🧠 Exam Technique: Systematic Elimination

  1. Unpack the formula: Write out each functional group:
    • HOCH₂- (alcohol)
    • -CH(NH₂)- (amine)
    • -COOH (carboxylic acid)
  2. Evaluate options against functional groups:
    • Any C=C ? No → Rule out A immediately.
    • Protein monomer or DNA? Protein → Rule out B.
    • Highly polar / zwitterionic? Yes → High solubility, rule out C.
  3. Confirm the remaining option: Amine + Acid → Salt formation. This is a definitive chemical reaction.

❌ Common Misconceptions & Traps

  • Confusing DNA and Proteins: Rushing students often see a biological organic molecule and confuse amino acid monomers with nucleotide monomers.
  • Overlooking the Amine Base: Forgetting that an amino acid can react with acids as well as bases. Because "acid" is in the name, some candidates only search for reactions with alkalis.
  • Assuming large organic molecules are insoluble: Neglecting the ionic character of amino acids (zwitterionic state at neutral pH) which grants high water solubility despite the carbon chain length.

Topics

Organic Chemistry · 3.3.11 Amines · 3.3.13 Amino Acids, Proteins and DNA

Question and mark scheme from the AQA A-Level Chemistry examination, Paper 3, June 2022. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.