IGCSE CHEMISTRY · QUALITATIVE ANALYSIS
IGCSE Chemistry qualitative analysis: ion tests, gas tests and inference
Use the official Notes accurately: sample, reagent and condition, observation, inference and confirmatory evidence.
Read in VietnameseThe evidence chain in qualitative analysis
The qualitative-analysis evidence chain
State what portion or solution is being tested.
Name the reagent and acidification, warming or excess condition.
Describe precipitate, gas, colour, dissolution or flame result.
Use the complete evidence to identify or exclude an ion or gas.
Qualitative analysis identifies substances from observable evidence. In the examination, recognising a memorable colour is not enough. You need to know which reagent was added, whether the sample was acidified or warmed, whether a precipitate or gas formed, whether the precipitate dissolved in excess reagent and what follows from those observations.
This is part of the AO3 reasoning assessed in Papers 5 and 6. Paper 5 may involve a real sample; Paper 6 presents a described test and result. The official qualitative-analysis notes are provided with both papers.
How to use the official Notes for use in qualitative analysis
Pages 47–48 of the 2026–2028 0620 syllabus contain the official Notes for use in qualitative analysis: tables for anion tests, aqueous cations, gases and flame tests. They are provided in both Paper 5 and Paper 6. This article does not reproduce the complete table; it teaches a reading and inference strategy.
Use this sequence:
- identify whether the target is an anion, aqueous cation, gas or flame colour;
- find the exact reagent and condition in the table;
- compare the stated observation with the official test result;
- check whether another ion gives a similar first result;
- use solubility, excess reagent or a confirmatory gas test to distinguish them.
Observation and inference are not the same
Observation
A light-blue precipitate formed.
Inference
The evidence is consistent with copper(II) ions under the stated test conditions.
Observation: “a light-blue precipitate formed; it dissolved in excess aqueous ammonia to give a dark-blue solution.” Inference: under the stated test conditions, this evidence is consistent with the presence of Cu²⁺.
A weak answer is “the copper ion became blue”. It omits the reagent, confuses the observation with the ion name and hides the solubility evidence. The reliable sequence is visible change first, identification second.
The same distinction applies to instrumental data. “The pH increased from 3 to 7” is data; “the solution became neutral” is an inference. They connect, but are not interchangeable.
Anion tests: acidification and reagent are part of the answer
These are selected examples, not the complete official table:
| Ion tested | Reagent and condition | Observation | Inference key |
|---|---|---|---|
| carbonate, CO₃²⁻ | dilute acid, then test the gas produced for CO₂ | effervescence; carbon dioxide forms | the gas turns limewater milky |
| aqueous chloride, Cl⁻ | acidify with dilute nitric acid, then add AgNO₃(aq) | white precipitate | acidification and AgNO₃ are both required |
| aqueous bromide, Br⁻ | same pretreatment, then AgNO₃(aq) | cream precipitate | distinguish it from white and yellow |
| aqueous iodide, I⁻ | same pretreatment, then AgNO₃(aq) | yellow precipitate | precise colour + correct reagent |
| aqueous sulfate, SO₄²⁻ | dilute nitric acid, then Ba(NO₃)₂(aq) | white precipitate | do not substitute another barium-salt method |
Why does acidification matter? It helps remove or exclude interfering ions that could produce a similar precipitate with the reagent. Do not write only “add silver nitrate”; the official sequence is part of the method.
Aqueous cations: two reagents, precipitate colour and solubility
The official Notes compare the effects of aqueous sodium hydroxide and aqueous ammonia. The key is not only the initial precipitate colour but what happens in excess.
Selected examples:
- Al³⁺: a white precipitate with NaOH(aq) that gives a colourless solution in excess; with NH₃(aq), a white precipitate insoluble in excess.
- Cu²⁺: a light-blue precipitate with either reagent; insoluble in excess NaOH but soluble in excess NH₃ to give a dark-blue solution.
- Fe²⁺: a green precipitate insoluble in excess; its surface may turn brown on standing.
- Fe³⁺: a red-brown precipitate insoluble in excess of either listed reagent.
- Zn²⁺: a white precipitate that gives a colourless solution in excess of either NaOH or NH₃.
An unknown white precipitate can therefore indicate more than one cation. Solubility and behaviour with the other reagent provide the next decision point. A single colour association such as “white = aluminium” is unreliable.
A decision tree for an unknown precipitate
A white precipitate is not yet a unique identification.
Dissolving or remaining insoluble narrows the candidate set.
A separate portion tests a different response.
State the ion only when the combined evidence supports it.
Gas and flame tests: state the positive result
Selected official gas tests:
| Gas | Test and positive result |
|---|---|
| NH₃ | turns damp red litmus paper blue |
| CO₂ | turns limewater milky |
| Cl₂ | bleaches damp litmus paper |
| H₂ | produces a “pop” with a lighted splint |
| O₂ | relights a glowing splint |
| SO₂ | changes acidified aqueous potassium manganate(VII) from purple to colourless |
Do not write “test with a splint”. State whether the splint is lighted or glowing and give the positive result. For safety, test only the small quantity of gas specified by the task or approved laboratory protocol, under supervision.
In official flame-test descriptions, for example, Li⁺ gives red, Na⁺ yellow, K⁺ lilac, Ca²⁺ orange-red, Ba²⁺ light green and Cu²⁺ blue-green. Learn the colour together with the ion and flame-test condition.
Unknown samples: plan exclusion and confirmation
Unknown-sample worksheet
Reagent, condition and exact observable result.
A separate confirmatory test and its expected observation.
Record which ions remain possible after each result.
Explain why the combined evidence supports one candidate.
Original exam-style scenario: a colourless aqueous sample gives a white precipitate with NaOH(aq) that dissolves in excess. A separate portion gives a white precipitate with NH₃(aq) that does not dissolve in excess.
Observation 1: white precipitate, giving a colourless solution in excess NaOH. This could indicate Al³⁺ or Zn²⁺. Observation 2: precipitate insoluble in excess NH₃. Under the official Notes, this distinguishes towards Al³⁺ because a Zn²⁺ precipitate would dissolve in excess NH₃.
The inference is strengthened by two complementary results. The first test narrows the possibilities; the second separates them.
Precise colour language: the label matters more than the screen shade
Colour is one part of the evidence
Use the reagent and precipitate context, not colour alone.
Record whether the precipitate dissolves in excess reagent.
Name the test conditions that make the observation meaningful.
Connect the observation to reagent, solubility and inference.
Laboratory colour can depend on concentration, lighting, background and sample quantity. A colour displayed on screen is therefore not an authoritative measurement standard. Use the official verbal descriptions—light blue, red-brown, cream, yellow—and connect each to the reagent and solubility.
If uncertain among white, cream and yellow, do not guess from visual memory. Check the acidification, silver nitrate reagent and exact wording in the Notes provided in the examination.
Common errors and targeted repairs
| Error | Why is it weak? | Repair |
|---|---|---|
| writing only the ion name | no visible evidence | reagent + condition + observation + ion |
| “it turned blue” | precipitate or solution is unclear | name the phase and exact colour |
| omitting the excess-reagent test | several cations remain indistinguishable | state whether it dissolves or remains insoluble |
| using the wrong acid for pretreatment | a new interfering ion may be introduced | follow the reagent in the official Notes |
| naming a gas without a test | a claim, not identification | test apparatus + positive result |
| smelling an unknown gas | unsafe and not an official identification | use only an approved targeted gas test |
The Paper 5 and 6 practical guide shows how this connects to the complete AO3 system of observation, data handling and experimental planning.
Practice system: unknown samples instead of reciting tables
A weekly 25-minute block:
- 5 minutes: locate three rows quickly in the official Notes.
- 8 minutes: complete the worksheet for one unknown sample.
- 5 minutes: write observation and inference in two separate sentences.
- 4 minutes: exclude one ion with a similar first result.
- 3 minutes: update the error log and schedule a 48–72-hour reattempt.
The aim is fast, accurate table use and a complete evidence chain, not word-for-word recitation of the page. The 8-week study plan returns to Paper 5/6 skills every week so qualitative analysis is not left until the final days.
ChemistryTutor.vn
Build an evidence chain from the ion name
Use an unknown-sample question to find where the reagent–observation–inference chain breaks.
Request a qualitative-analysis diagnostic