Justify an ion identification with the results the question supplies, and nothing else. A true test result that was never reported is still an invented result in that answer.
This lesson belongs to interpreting unfamiliar chemical evidence. The standard tests are in interpreting qualitative analysis observations.
How do I pick the supporting results?
Copy out the supplied results first, each on its own line. Then ask of each one: which ion does this narrow down, and which does it rule out?
Give an ion a “supported” label only when a listed result points to it. Write “not tested” for ions the data never touches.
Worked example: salt T
Salt T is a white solid. The question supplies these results.
- T dissolves in water to give a colourless solution.
- Dilute nitric acid then silver nitrate solution: a white precipitate.
- Sodium hydroxide solution: a white precipitate that dissolves in excess.
- Ammonia solution: a white precipitate that dissolves in excess.
Result 2 supports chloride ions. Results 3 and 4 together support Zn²⁺, because Al³⁺ and Pb²⁺ stay undissolved in excess ammonia. Result 1 rules out a coloured cation such as Cu²⁺.
A strong answer reads: “T is zinc chloride. Chloride: a white precipitate with acidified silver nitrate. Zinc: the precipitate dissolves in excess of both sodium hydroxide and ammonia.”
The tempting invented result
A student adds “a pungent gas turned damp red litmus blue when warmed, so ammonium is absent”. Nobody supplied that test.
The accurate wording is “ammonium ions were not tested”. You may add that warming with sodium hydroxide would test for them, but mark it clearly as a suggestion.
Check yourself
Salt U gives a blue solution. Sodium hydroxide gives a blue precipitate that stays in excess. Barium chloride after dilute hydrochloric acid gives a white precipitate. Which ions are supported, and which result might a student wrongly claim?
Answer
Supported: Cu²⁺ (blue solution, blue precipitate insoluble in excess) and SO₄²⁻ (white precipitate with barium chloride that survives acid). So U is consistent with copper(II) sulfate.
A student might wrongly claim “ammonia gives a deep blue solution in excess”. That test was not supplied, so it cannot be used as evidence, though it would be a valid further test for Cu²⁺.
What to study next
Go on to combining two supplied clues, or return to observation versus explanation if your results column still contains names.
The claim, evidence and explanation revision desk gives you a place to rebuild justifications. For a teacher to check them with you, see online one-to-one Chemistry tuition.