A single positive test shows that one property is present. It does not prove identity unless no other substance in the question shares that property. To evaluate a test, ask: what else would give this result, and what second test separates them?
This lesson combines the reasoning from narrowing an unknown and separating observation from inference. It is the last lesson in separation and chemical analysis.
Why can one test fall short?
Tests are designed around a property. Several substances can share that property, so the result fits more than one candidate. The test is reliable only for the question it was designed to answer.
Two patterns appear in questions. One is an ion that shares a result with other ions, such as a white precipitate with sodium hydroxide. The other is a second substance in the sample that interferes, such as carbonate in a halide test.
How do you evaluate a test step by step?
- State what the result shows in one sentence.
- List the other candidates that would give the same result.
- Choose a second test that gives a different result for those candidates.
- Predict the outcome for each candidate.
- Write a conclusion that matches the strength of the evidence.
Worked example
Invented scenario. A colourless solution forms a white precipitate with aqueous sodium hydroxide. The candidates are aluminium, calcium and zinc ions. Is the result enough?
Step 1, what it shows: all three ions give a white precipitate, so the result is consistent with any of them. It is not enough.
Step 2, second test: add excess sodium hydroxide. The precipitate dissolves for aluminium and zinc, and stays for calcium. So a precipitate that dissolves rules out calcium.
Step 3, third test: use aqueous ammonia on a fresh sample and add it in excess. The white precipitate of zinc hydroxide dissolves, while aluminium hydroxide does not.
Step 4, conclusion: if the precipitate dissolves in excess sodium hydroxide but not in excess ammonia, the ion is aluminium. If it dissolves in both, the ion is zinc. If it stays in excess sodium hydroxide, it is calcium.
The original test opened the question. The other two closed it.
The mistake to watch for
A common slip is to accept a positive result without asking what could interfere.
Mistaken answer: “Bubbles form when acid is added to the grey solid, so it is a carbonate.”
The student assumed one result pointed to one substance.
The correction is that reactive metals also give bubbles with acid, and that gas is hydrogen rather than carbon dioxide. Testing the gas separates them: hydrogen burns with a squeaky pop, and carbon dioxide turns limewater milky.
Check yourself
Try these, then open each answer.
1. A solution gives a white precipitate with silver nitrate. Why might adding dilute nitric acid first improve the test?
Show answer
Carbonate ions also give a white precipitate with silver nitrate. Acid first removes carbonate as carbon dioxide, so a precipitate that remains is more reliably a halide.
2. A white precipitate with sodium hydroxide does not dissolve in excess. Which of aluminium, calcium and zinc ions remain?
Show answer
Aluminium and zinc hydroxides dissolve in excess, so they are ruled out. Calcium ions remain, consistent with the result.
3. A solution contains chloride ions, according to the silver nitrate test. A student says the solid is sodium chloride. What is missing?
Show answer
The test identifies only the anion. A test for the cation is needed before the compound can be named.
Where this leads next
Bring the five lessons together in the mixed practice set. If you want a teacher to question your reasoning as an examiner might, our online one-to-one Chemistry tuition starts with a paid one-hour trial class.