About Group 17 in A Level Chemistry
Group 17 is one of the 18 main topics in our A Level Chemistry (Cambridge (CAIE) 9701) question bank, and about 5% of its real questions come from it. Tayyari Karo has 30 real questions on this topic from 2021 to 2026, from Paper 1, Paper 2; 25 are multiple choice and marked instantly, and the rest are structured questions with a model answer. By difficulty: 18 medium, 9 hard, 3 easy.
Work through the questions below, then open the explanation for each one. Redo the ones you missed a few days later.
Try 6 real questions
These are real past-paper questions. Think of your answer first, then open the explanation.
May/June 2026 · Paper 1 · Group 17
Which statement is correct?
- Bromide ions react to form a white precipitate when added to silver nitrate solution.
- Bromine does not oxidise chloride ions when added to sodium chloride solution.
- Fluorine atoms form cations by accepting electrons when they react.
- Chloride ions are stronger reducing agents than iodide ions.
Show answer and explanation
Answer: B. Bromine does not oxidise chloride ions when added to sodium chloride solution.
Br2 is a weaker oxidising agent than Cl2, so it cannot oxidise Cl-. AgBr is cream, F forms anions, and I- is a stronger reducing agent than Cl-.
May/June 2025 · Paper 1 · Group 17
Which reagent(s) and conditions will oxidise chlorine, Cl2, into a compound containing chlorine in the +5 oxidation state?
- AgNO3(aq) followed by NH3(aq) at room temperature
- concentrated H2SO4 at room temperature
- cold dilute NaOH(aq)
- hot concentrated NaOH(aq)
Show answer and explanation
Answer: D. hot concentrated NaOH(aq)
Hot concentrated NaOH disproportionates Cl2 to give ClO3⁻ (chlorine at +5) and Cl⁻, whereas cold dilute NaOH gives ClO⁻ (+1).
May/June 2024 · Paper 1 · Group 17
A solid sodium halide, NaX, is reacted with concentrated sulfuric acid; the lowest oxidation state of sulfur in the products is +4. Halogen Y2 is less volatile than halogen X2. What are the identities of sodium halide NaX and halogen Y2?
- sodium bromide and chlorine
- sodium bromide and iodine
- sodium iodide and bromine
- sodium iodide and astatine
Show answer and explanation
Answer: B. sodium bromide and iodine
Sulfur being reduced only as far as +4 (SO2) means X⁻ is a moderate reducing agent — bromide (not iodide, which reduces S further to H2S/S). Y2, being less volatile than Br2, must be iodine.
October/November 2023 · Paper 1 · Group 17
HOCl(aq) is the molecule that kills bacteria when chlorine is added to water. The following reaction produces this molecule: Cl2(g) + H2O(l) ⇌ HOCl(aq) + H⁺(aq) + Cl⁻(aq). Which statement about this reaction is correct?
- Chlorine is both oxidised and reduced.
- Chlorine is oxidised but not reduced.
- Hydrogen is both oxidised and reduced.
- Hydrogen is oxidised but not reduced.
Show answer and explanation
Answer: A. Chlorine is both oxidised and reduced.
This is a disproportionation: Cl2 (0) becomes Cl⁻ (−1, reduced) and Cl in HOCl (+1, oxidised) — chlorine is both oxidised and reduced.
May/June 2022 · Paper 1 · Group 17
In the treatment of domestic water supplies, chlorine is added to water to kill bacteria. Some ClO⁻ ions are formed. What is the change in oxidation number of chlorine when forming the ClO⁻ ion from aqueous chlorine?
- −1
- 0
- +1
- +2
Show answer and explanation
Answer: C. +1
Chlorine in Cl2 has oxidation number 0; in ClO⁻ it is +1, so the change is +1.
May/June 2021 · Paper 1 · Group 17
A 5 cm³ sample of 0.05 mol dm⁻³ sodium chloride is mixed with a 5 cm³ sample of 0.05 mol dm⁻³ potassium iodide. 10 cm³ of acidified 0.05 mol dm⁻³ silver nitrate is then added, followed by concentrated ammonia solution. What is seen after the addition of an excess of concentrated ammonia solution?
- a cream precipitate
- a white precipitate
- a yellow precipitate
- no precipitate
Show answer and explanation
Answer: C. a yellow precipitate
AgCl (white) dissolves in excess concentrated NH3, but AgI (yellow) is insoluble even in concentrated ammonia, so a yellow precipitate of AgI remains.
