These eight questions use original data and only the tests in your syllabus. Write a full answer first, then open the explanation.
The set belongs to interpreting unfamiliar chemical evidence. The reference tables are in interpreting qualitative analysis observations.
Questions
Q1. A student heats a blue crystalline solid and sees it turn white while droplets form on the cool tube wall. State one observation and one explanation.
Answer
Observation: the blue solid turns white, and a colourless liquid collects on the tube wall.
Explanation: the crystals lose water of crystallisation, so the hydrated salt becomes anhydrous. Naming “water” or “anhydrous” belongs in the explanation.
Q2. Sort into observation or explanation: (a) a gas relights a glowing splint; (b) oxygen is produced.
Answer
(a) is an observation, a result of a test that can be seen. (b) is an explanation, because the gas is named from that result.
Q3. Solution A gives a white precipitate with sodium hydroxide that stays in excess, and a white precipitate with ammonia solution that stays in excess. A student writes “A contains Zn²⁺”. Is this supported?
Answer
No. Zn²⁺ would dissolve in excess of both reagents.
Both results fit Mg²⁺, and the stays-in-excess sodium hydroxide result also rules out Al³⁺ and Pb²⁺. The supported ion is Mg²⁺.
Q4. Solution B is colourless. Dilute nitric acid then silver nitrate gives a white precipitate.
A student adds “a brown ring formed, so nitrate is present”. Comment.
Answer
The chloride result is supported. The brown ring test was not supplied, so it cannot be used as evidence. Write “nitrate ions were not tested”.
Q5. Salt C is one of iron(II) sulfate, iron(III) sulfate, iron(II) chloride or iron(III) chloride. Sodium hydroxide gives a brown precipitate.
Dilute hydrochloric acid then barium chloride gives a white precipitate. Name C.
Answer
Brown precipitate means Fe³⁺, which removes both iron(II) salts. The white precipitate with barium chloride after acid means sulfate, which removes the chloride. C is iron(III) sulfate.
Q6. Solution D gives a white precipitate with sodium hydroxide that dissolves in excess. With ammonia solution it gives a white precipitate that stays in excess. Give the conclusion, including any ion you cannot separate.
Answer
Dissolving in excess sodium hydroxide fits Zn²⁺, Al³⁺ or Pb²⁺. The precipitate staying in excess ammonia rules out Zn²⁺, which leaves Al³⁺ or Pb²⁺.
The results do not separate these two. A further test on a fresh sample decides: potassium iodide solution gives a yellow precipitate only with Pb²⁺.
Q7. Copper turnings are placed in silver nitrate solution. After some time the solution turns pale blue and a grey solid forms on the copper. Write the observation and an explanation.
Answer
Observation: the colourless solution turns pale blue, and a grey solid appears on the copper.
Explanation: copper is more reactive than silver, so copper displaces silver from the solution. Cu²⁺ ions give the blue colour, and the grey solid is silver.
Q8. Barium chloride solution gives a white precipitate in solution E. No acid was used. Write a four-part conclusion.
Answer
Evidence: a white precipitate forms with barium chloride.
It supports barium carbonate or barium sulfate. It does not exclude either carbonate or sulfate ions. Adding dilute hydrochloric acid decides: the precipitate dissolves with bubbles for carbonate, and stays for sulfate.
If you got these wrong
Match the slip to the lesson.
- Q1, Q2 and Q7: observation versus explanation.
- Q3 and Q4: using only the supplied results.
- Q5 and Q6: combining two clues.
- Q8: incomplete evidence.
Record each slip in the mistake log and paper-error review. For a teacher to go through new questions with you, see online one-to-one Chemistry tuition.