Qualitative analysis asks you to name an ion from what you see. The observation goes in one column and the inference in another, and each needs its own wording.
This lesson sits in SPM Chemistry acids, bases and salts. Read planning salt preparation, too, because you need to know which salt you hold before you test it.
How do I get from an observation to an ion?
Use each reagent to cross names off a list. Start with the cations on your syllabus: Zn²⁺, Al³⁺, Pb²⁺, Ca²⁺, Mg²⁺, Cu²⁺, Fe²⁺, Fe³⁺ and NH₄⁺.
The table shows the pattern you are working from.
| Cation | NaOH(aq), a little | NaOH(aq), excess | NH₃(aq), excess |
|---|---|---|---|
| Zn²⁺ | white precipitate | dissolves | dissolves |
| Al³⁺ | white precipitate | dissolves | stays |
| Pb²⁺ | white precipitate | dissolves | stays |
| Ca²⁺ | white precipitate | stays | no precipitate |
| Mg²⁺ | white precipitate | stays | stays |
| Cu²⁺ | blue precipitate | stays | dissolves to deep blue solution |
Read each row as a fingerprint. Two tests cut nine ions down to one or two.
Worked example: a white precipitate with two candidates
Solution P is colourless. A student adds sodium hydroxide solution drop by drop, then in excess, and repeats the test with ammonia solution.
| Test | Observation | What it rules out |
|---|---|---|
| NaOH, a little | white precipitate | Cu²⁺, Fe²⁺, Fe³⁺ (coloured precipitates) |
| NaOH, excess | precipitate dissolves, colourless solution | Ca²⁺, Mg²⁺ (precipitate stays) |
| NH₃, excess | white precipitate stays | Zn²⁺ (it would dissolve) |
Only Al³⁺ and Pb²⁺ remain. The honest inference is “Al³⁺ or Pb²⁺, one more test needed”. Adding potassium iodide solution to a fresh portion gives a yellow precipitate if lead is present.
The mistake that loses marks
A student sees a white precipitate and writes “Zn²⁺ present”. The observation matches zinc, but it matches four other cations too.
The fix is to finish the table. Record every reagent result, then name the ion only when all rows agree with one fingerprint. If two ions survive, say so.
Check yourself
Solution Q is pale green. Sodium hydroxide gives a green precipitate that does not dissolve in excess. Ammonia solution also gives a green precipitate that stays. Name the cation and write the observation and inference separately.
Answer
Observation: a green precipitate forms with both reagents and does not dissolve in excess of either.
Inference: the cation is Fe²⁺. The green colour rules out Zn²⁺, Al³⁺, Pb²⁺, Ca²⁺ and Mg²⁺, which give white precipitates. Fe³⁺ gives a brown precipitate and Cu²⁺ a blue one.
What to study next
Identification is half of the job, because the same ions appear in writing ionic equations with correct charges. When a result does not fit any fingerprint, see interpreting unfamiliar chemical evidence.
To log which steps you keep missing, try the mistake log and paper-error review. For a teacher to go through your written answers with you, see online one-to-one Chemistry tuition.