When a question lists possible salts, treat each clue as a filter. One clue tests the cation and one tests the anion, so together they can leave a single salt.
This lesson is part of interpreting unfamiliar chemical evidence. It uses the same tests as interpreting qualitative analysis observations.
How does the candidate grid work?
Draw a small table with the candidate salts down the side. Add one column per clue and mark each candidate as kept or removed.
A clue that removes nothing still tells you something: it cannot separate those salts.
Worked example: four possible salts
A crystal X is one of copper(II) sulfate, copper(II) chloride, zinc sulfate or zinc chloride. The data are these.
- Clue 1: the solution is blue, and sodium hydroxide gives a blue precipitate.
- Clue 2: dilute nitric acid then silver nitrate solution gives a white precipitate.
| Candidate | Clue 1 (cation is Cu²⁺) | Clue 2 (anion is Cl⁻) |
|---|---|---|
| Copper(II) sulfate | kept | removed |
| Copper(II) chloride | kept | kept |
| Zinc sulfate | removed | removed |
| Zinc chloride | removed | kept |
Clue 1 alone leaves two salts, and so does clue 2. Only copper(II) chloride survives both, so X is copper(II) chloride.
The mistake that loses marks
A student stops after the first clue that “sounds right”, writing “copper salt, so copper sulfate” because sulfate is familiar. The anion clue was never used.
Finish every column before naming the salt. A gap in the grid is a missing mark in the answer.
Check yourself
Solid Y is one of magnesium carbonate, calcium carbonate, magnesium sulfate or calcium sulfate. Y dissolves in dilute nitric acid with bubbles that turn limewater cloudy. The solution gives a white precipitate with sodium hydroxide that stays in excess, and no precipitate with ammonia solution. Name Y.
Answer
Bubbles turning limewater cloudy show carbon dioxide, so the anion is carbonate. That removes both sulfates.
A precipitate that stays in excess sodium hydroxide fits Ca²⁺ and Mg²⁺. No precipitate with ammonia solution fits Ca²⁺ only, because Mg²⁺ gives a white precipitate.
Y is calcium carbonate.
What to study next
Next, see how to write when the clues run short in writing an evidence-based conclusion when a result is inconclusive. You can also review choosing supported observations.
Log any grid you got wrong in the claim, evidence and explanation revision desk. If you want a teacher to give you new puzzles, see online one-to-one Chemistry tuition.