To narrow an unknown, list the candidates, then let each stated result rule some of them out. You do not need to recognise the answer at once. You need to be able to say what each result excludes.
This lesson builds on interpreting a chromatogram, where you also reached a conclusion from evidence. It is part of separation and chemical analysis.
How does elimination work?
Every candidate substance has known properties. A test result either fits a candidate’s properties or it does not. If it does not fit, the candidate goes.
Some results point towards a substance, and some only point away from one. Both are useful. A table with one column per result keeps your reasoning visible.
How do you narrow an unknown step by step?
- Write the candidates as a short list first.
- Take the results one at a time, in the order given.
- For each result, cross out every candidate that would not produce it, and write the reason.
- Check the survivor against every result, not just the last one.
- Write the conclusion as “the unknown is consistent with …”.
Worked example
Invented scenario. A white solid has four possible identities: calcium carbonate, sodium chloride, sodium carbonate, sodium sulfate. Three results are given:
- Result 1: the solid dissolves in water.
- Result 2: adding dilute acid to the solution produces no bubbles.
- Result 3: adding acidified barium chloride solution to a fresh sample gives a white precipitate.
Step 1, result 1: calcium carbonate is insoluble in water, so it is ruled out. Remaining: sodium chloride, sodium carbonate, sodium sulfate.
Step 2, result 2: carbonates give bubbles of carbon dioxide with acid. No bubbles rules out sodium carbonate. Remaining: sodium chloride, sodium sulfate.
Step 3, result 3: acidified barium chloride gives a white precipitate with sulfate ions. Sodium chloride contains no sulfate, so it is ruled out.
Step 4, conclusion: the unknown is consistent with sodium sulfate, and it explains all three results.
For reference, the reaction that result 2 relies on is Na₂CO₃ + 2HCl → 2NaCl + H₂O + CO₂. You can confirm the balance with the equation balance reasoning trainer.
The mistake to watch for
A common slip is to name the substance from one result and ignore the rest.
Mistaken answer: “It dissolves in water, so it is sodium chloride.”
The student used only result 1. Sodium carbonate and sodium sulfate also dissolve.
The correction is that a result which several candidates share cannot separate them. Keep going until every remaining result has been used, and check that the final candidate explains each one.
Check yourself
Try these, then open each answer.
1. Candidates: magnesium carbonate, sodium chloride, calcium carbonate. A white solid does not dissolve in water and fizzes with dilute acid. Which candidates remain?
Show answer
Sodium chloride dissolves, so it is ruled out. The two carbonates are both insoluble and both fizz, so magnesium carbonate and calcium carbonate remain. A further test is needed to separate them.
2. Why does “no bubbles with dilute acid” help, even though it does not name a substance?
Show answer
It rules out carbonates, which would produce carbon dioxide. A negative result narrows the list just as a positive one does.
3. A solution gives a white precipitate with acidified barium chloride. A student concludes the solid must be sodium sulfate. What is missing?
Show answer
The result shows sulfate ions are present. It does not say which cation accompanies them, so sodium sulfate is one possibility but not the only one.
Where this leads next
Next, distinguish observation from inferred identity, which sharpens the wording you use in each step of the table. Test the route with the mixed practice set.
If elimination questions feel like guesswork, our teachers can build the routine with you in online one-to-one Chemistry tuition.