Last updated: 7 October 2026
Imagine that the label has fallen off a bottle containing a colourless salt solution. Looking at the solution tells you almost nothing. Many different ionic compounds form colourless solutions, so how could you work out which ions are actually present?
This is where qualitative analysis becomes useful. Instead of measuring how much of a substance is present, we carry out carefully chosen reactions to determine what is present. An unknown gas may relight a glowing splint. An ion may produce a coloured precipitate. Another precipitate may dissolve when more reagent is added. Each result is a chemical clue.
The topic can initially look like a large memory exercise, but there is a much better way to learn it. Organise the tests into three families — gas tests, anion tests and cation tests — and understand what each observation is telling you. Once the patterns become familiar, qualitative analysis becomes a logical identification process rather than a disconnected list of facts.
Try these five questions before reading the chapter. They test the basic ideas that make qualitative analysis much easier to understand.
- Identify common gas tests.
- Recognise important ion-testing reagents.
- Spot common practical misconceptions.
What Does Qualitative Analysis Actually Tell Us?
The word qualitative refers to the identity or nature of a substance. In qualitative analysis, we are interested in questions such as:
- Which gas has been produced?
- Which anion is present in the solution?
- Which cation is present in the solution?
We answer these questions by carrying out reactions that produce characteristic observations. The important idea is that a test is useful only when its positive result allows us to distinguish one substance from another.
Observation first, conclusion second
Suppose aqueous sodium hydroxide is added to an unknown solution and a green precipitate forms. The statement “a green precipitate forms” is the observation.
From the qualitative analysis table, we then infer that Fe²⁺ ions may be present. The ion name is the conclusion, not the observation itself.
Why careful observations matter
In qualitative analysis, small details can change the conclusion. “A precipitate forms” is often not enough. You may need to record:
- whether a precipitate forms;
- the colour of the precipitate;
- whether the precipitate remains or dissolves when excess reagent is added;
- whether a gas is evolved;
- what happens when that gas is tested.
| Weak answer | Better qualitative observation |
|---|---|
| A solid forms. | A white precipitate forms. |
| The gas reacts with litmus. | Damp red litmus paper turns blue. |
| The precipitate disappears. | The white precipitate dissolves in excess reagent to form a colourless solution. |
| There are bubbles. | Effervescence occurs and a colourless gas is evolved. |
Use Fresh Portions of the Unknown Solution
Before carrying out several different tests, divide the unknown solution into separate portions. This is an important practical habit because every reagent you add introduces new substances into the test tube.
If you continue using the same mixture for a second test, ions introduced by the first reagent may interfere with the next reaction.
A reagent can contaminate the next test
Imagine adding hydrochloric acid during one experiment and then using that same mixture to test for chloride ions with silver nitrate.
Hydrochloric acid itself contains Cl⁻ ions. These added chloride ions could form AgCl and give a positive chloride test even if the original sample contained no chloride.
Using a fresh portion prevents this type of false conclusion.
Tests for Gases
Gas tests are often the quickest part of qualitative analysis because each gas has a characteristic positive result. However, seeing bubbles is not enough to identify a gas. Effervescence tells you only that a gas is being produced — you must still carry out the correct test.
| Gas | Test | Positive observation |
|---|---|---|
| Hydrogen, H₂ | Place a lighted splint at the mouth of the test tube. | The gas burns with a “pop” sound. |
| Oxygen, O₂ | Insert a glowing splint into the gas. | The glowing splint relights. |
| Carbon dioxide, CO₂ | Bubble the gas through limewater. | A white precipitate forms and the limewater turns milky. |
| Ammonia, NH₃ | Test with damp red litmus paper. | Damp red litmus paper turns blue. |
| Chlorine, Cl₂ | Test with damp blue litmus paper. | The paper turns red and is then bleached white. |
| Sulfur dioxide, SO₂ | Pass through acidified potassium manganate(VII) solution. | The purple solution is decolourised. |
Hydrogen: the lighted splint test
Hydrogen is a combustible gas. Place a lighted splint at the mouth of the test tube. If hydrogen is present, it burns rapidly and produces the characteristic “pop” sound.
2H₂(g) + O₂(g) → 2H₂O(l)
Be careful with the wording. A lighted splint is used for hydrogen. A glowing splint is not the hydrogen test.
Oxygen: the glowing splint test
Oxygen does not itself burn, but it supports combustion. A glowing splint contains a hot ember without an obvious flame. When placed in oxygen, combustion becomes vigorous enough for the splint to relight.
O₂ → glowing splint relights
Carbon dioxide: the limewater test
Carbon dioxide is identified by bubbling the gas through limewater, which is aqueous calcium hydroxide.
Ca(OH)₂(aq) + CO₂(g) → CaCO₃(s) + H₂O(l)
The tiny particles of white CaCO₃ make the mixture appear cloudy or milky. The observation is therefore better described as “a white precipitate forms and limewater turns milky” rather than simply saying that limewater changes colour.
Ammonia: damp red litmus turns blue
Ammonia is an alkaline gas. Hold damp red litmus paper near the mouth of the test tube. The paper turns blue.
The paper must be damp because ammonia needs to dissolve in water before its alkaline behaviour can affect the indicator.
Chlorine: acidic first, then bleaching
Chlorine has a two-stage effect on damp blue litmus paper. The paper first turns red and is then bleached white.
This sequence is useful because it shows two effects. Chlorine in water first produces acidic substances, causing the blue indicator to turn red. Chlorine then destroys the coloured indicator molecules, so the paper becomes white.
Do not write only “litmus is bleached”
For chlorine, give the complete observation: damp blue litmus paper turns red and is then bleached white.
Sulfur dioxide: decolourising acidified potassium manganate(VII)
Sulfur dioxide is a reducing agent. Pass the gas through acidified potassium manganate(VII) solution.
A positive result is:
purple solution → colourless
State that the solution is decolourised. Do not describe a colourless precipitate because no precipitate is formed in this test.
Tests for Anions
Anions are negatively charged ions. The tests in this section do not all work in the same way. Carbonate ions release a gas, chloride and iodide ions form silver salts, sulfate ions form barium sulfate, and nitrate ions are converted into ammonia before identification.
The order in which reagents are added can affect what you observe. In the standard O-Level procedure for the silver nitrate and barium nitrate tests, the sample is acidified before the precipitating reagent is added.
Test for carbonate ions, CO₃²⁻
Add a dilute acid to the sample. If carbonate ions are present, effervescence occurs because carbon dioxide gas is produced.
CO₃²⁻(aq) + 2H⁺(aq) → CO₂(g) + H₂O(l)
Do not stop after observing bubbles. Bubble the gas through limewater. If the limewater turns milky, carbon dioxide has been confirmed and the original sample contains carbonate ions.
Carbonate identification requires two stages
Dilute acid is added → effervescence occurs → the gas turns limewater milky → the gas is CO₂ → CO₃²⁻ ions are present in the original sample.
Tests for chloride and iodide ions
Chloride and iodide ions are identified using silver nitrate solution.
The standard O-Level procedure is:
- Acidify a fresh portion with dilute nitric acid.
- Add aqueous silver nitrate.
- Observe the colour of the precipitate.
| Anion | Observation | Precipitate |
|---|---|---|
| Cl⁻ | White precipitate | AgCl |
| I⁻ | Yellow precipitate | AgI |
Ag⁺(aq) + Cl⁻(aq) → AgCl(s)
Ag⁺(aq) + I⁻(aq) → AgI(s)
Why is dilute nitric acid used in the silver nitrate test?
Silver ions do not react only with halide ions. If carbonate ions are present, CO₃²⁻ ions can also react with Ag⁺ ions and produce a precipitate. This creates an additional observation that could interfere with interpretation of the halide test.
In the standard O-Level procedure, dilute nitric acid is added before silver nitrate. The H⁺ ions remove carbonate ions:
CO₃²⁻(aq) + 2H⁺(aq) → CO₂(g) + H₂O(l)
The carbonate therefore leaves the mixture as carbon dioxide before Ag⁺ ions are introduced.
The order of addition affects the observations rather than changing the underlying principle. If silver nitrate were added first to a sample containing carbonate ions, a silver carbonate precipitate could initially form. Adding dilute nitric acid afterwards would dissolve the carbonate precipitate, while silver chloride would remain.
Why nitric acid and not hydrochloric acid?
The acid must remove carbonate interference without introducing an ion that interferes with the test itself.
Hydrochloric acid contains Cl⁻ ions. If HCl were used, these added chloride ions could react with Ag⁺ ions:
Ag⁺(aq) + Cl⁻(aq) → AgCl(s)
A white AgCl precipitate could then form even if the original sample contained no chloride ions. This would give a false positive chloride result.
Dilute nitric acid is therefore used because it removes carbonate interference without introducing Cl⁻ ions.
Test for sulfate ions, SO₄²⁻
To test for sulfate ions:
- Acidify a fresh portion with dilute nitric acid.
- Add aqueous barium nitrate.
A white precipitate indicates sulfate ions because insoluble barium sulfate is formed.
Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s)
Carbonate ions can also form a precipitate with Ba²⁺ ions. Acidification removes carbonate ions as CO₂ and H₂O before barium ions are introduced, allowing the sulfate result to be interpreted more reliably.
Test for nitrate ions, NO₃⁻
Nitrate ions are tested differently because the nitrate ion itself does not give a distinctive precipitate in this procedure.
- Add aqueous sodium hydroxide.
- Add a piece of aluminium foil.
- Warm the mixture gently.
- Test the gas evolved using damp red litmus paper.
If nitrate ions are present, ammonia gas is produced. The ammonia turns damp red litmus paper blue.
Nitrate ions and ammonium ions can both lead to ammonia
| Ion being tested | Reagents and conditions | Positive result |
|---|---|---|
| NH₄⁺ | Add NaOH(aq) and warm. | NH₃ is evolved. |
| NO₃⁻ | Add NaOH(aq) and aluminium foil, then warm. | NH₃ is evolved. |
Aluminium foil is therefore the key additional reagent in the nitrate-ion test.
Summary of the anion tests
| Anion | Test | Positive observation |
|---|---|---|
| CO₃²⁻ | Add dilute acid and test the gas with limewater. | Effervescence; gas turns limewater milky. |
| Cl⁻ | Acidify with dilute nitric acid, then add aqueous silver nitrate. | White precipitate. |
| I⁻ | Acidify with dilute nitric acid, then add aqueous silver nitrate. | Yellow precipitate. |
| SO₄²⁻ | Acidify with dilute nitric acid, then add aqueous barium nitrate. | White precipitate. |
| NO₃⁻ | Add NaOH(aq) and aluminium foil, then warm. | NH₃ evolved; damp red litmus turns blue. |
Tests for Cations
Most of the cations in O-Level qualitative analysis are identified using two reagents:
- aqueous sodium hydroxide, NaOH(aq);
- aqueous ammonia, NH₃(aq).
The testing method has two stages. First add the reagent drop by drop and observe whether a precipitate forms. Then add the reagent in excess and observe whether that precipitate remains or dissolves.
This second stage is particularly important because several ions form precipitates of the same colour.
Why do metal ions form precipitates?
Hydroxide ions can react with many metal ions to form insoluble metal hydroxides. These insoluble substances appear as precipitates.
For example, Cu²⁺ ions form light-blue copper(II) hydroxide:
Cu²⁺(aq) + 2OH⁻(aq) → Cu(OH)₂(s)
Different metal hydroxides have different colours and different behaviour in excess reagent, allowing the original metal ions to be distinguished.
The coloured cations: Cu²⁺, Fe²⁺ and Fe³⁺
| Cation | Few drops of NaOH(aq) | Excess NaOH(aq) | Few drops of NH₃(aq) | Excess NH₃(aq) |
|---|---|---|---|---|
| Cu²⁺ | Light-blue precipitate | Insoluble | Light-blue precipitate | Dissolves to form a deep-blue solution |
| Fe²⁺ | Green precipitate | Insoluble | Green precipitate | Insoluble |
| Fe³⁺ | Red-brown precipitate | Insoluble | Red-brown precipitate | Insoluble |
Learn the three coloured precipitates as a set
Cu²⁺ → light blue
Fe²⁺ → green
Fe³⁺ → red-brown
Copper(II) has an additional distinctive result: its light-blue precipitate dissolves in excess aqueous ammonia to give a deep-blue solution.
A green Fe²⁺ precipitate may slowly become brown when left standing because it is oxidised by oxygen in the air. In a qualitative analysis question, record the initial green precipitate.
The white precipitate cations: Al³⁺, Ca²⁺ and Zn²⁺
Aluminium, calcium and zinc ions are more easily confused because they can all give white precipitates with aqueous sodium hydroxide.
The colour alone therefore cannot identify the ion. You must examine what happens when the reagent is added in excess.
| Cation | Few drops of NaOH(aq) | Excess NaOH(aq) | Few drops of NH₃(aq) | Excess NH₃(aq) |
|---|---|---|---|---|
| Al³⁺ | White precipitate | Dissolves to form a colourless solution | White precipitate | Insoluble |
| Ca²⁺ | White precipitate | Insoluble | No precipitate or only a very slight white precipitate | No further change |
| Zn²⁺ | White precipitate | Dissolves to form a colourless solution | White precipitate | Dissolves to form a colourless solution |
How to distinguish Al³⁺ from Zn²⁺
Suppose an unknown solution forms a white precipitate with aqueous sodium hydroxide and that precipitate dissolves when excess NaOH(aq) is added.
Calcium has now been ruled out, but the ion could still be Al³⁺ or Zn²⁺.
Use a fresh portion and add aqueous ammonia:
- Al³⁺ forms a white precipitate that is insoluble in excess NH₃(aq).
- Zn²⁺ forms a white precipitate that dissolves in excess NH₃(aq) to give a colourless solution.
Follow the evidence instead of guessing from one observation
An unknown solution gives a white precipitate with a few drops of NaOH(aq). The precipitate dissolves in excess NaOH(aq).
This eliminates Ca²⁺ but leaves Al³⁺ and Zn²⁺ as possibilities.
A fresh portion gives a white precipitate with NH₃(aq), which also dissolves when excess NH₃(aq) is added.
The ion is therefore Zn²⁺.
Test for ammonium ions, NH₄⁺
The ammonium ion is different from the metal cations because it is not identified by forming a coloured metal hydroxide precipitate.
Add aqueous sodium hydroxide and warm gently. If NH₄⁺ ions are present, ammonia gas is released.
NH₄⁺(aq) + OH⁻(aq) → NH₃(g) + H₂O(l)
Test the gas with damp red litmus paper. The paper turns blue.
The positive result is ammonia gas, not a precipitate
When testing NH₄⁺ ions, the important observation is that ammonia gas is evolved on warming with aqueous sodium hydroxide.
Confirm the gas using damp red litmus paper.
Summary of the cation tests
| Cation | Key identifying observations |
|---|---|
| Al³⁺ | White precipitate; soluble in excess NaOH but insoluble in excess NH₃. |
| Ca²⁺ | White precipitate with NaOH, insoluble in excess; little or no precipitate with NH₃. |
| Cu²⁺ | Light-blue precipitate; dissolves in excess NH₃ to form a deep-blue solution. |
| Fe²⁺ | Green precipitate, insoluble in excess NaOH and NH₃. |
| Fe³⁺ | Red-brown precipitate, insoluble in excess NaOH and NH₃. |
| Zn²⁺ | White precipitate; soluble in excess NaOH and excess NH₃. |
| NH₄⁺ | NH₃ evolved when warmed with NaOH; damp red litmus turns blue. |
How to Identify an Unknown Substance Systematically
In examination questions, you may be given several observations from an unknown solution and asked to identify the ions present. Do not jump to a conclusion after the first clue. Work through the evidence systematically.
Step 1: Observe the original sample
Record its physical state and colour. A blue solution may suggest Cu²⁺, while a pale-green solution may suggest Fe²⁺. Treat these as clues rather than final proof.
Step 2: Divide the sample into fresh portions
Use separate portions for different tests so that reagents added during one test do not contaminate another.
Step 3: Identify any gas produced
If you observe effervescence or a gas is evolved during warming, carry out the appropriate gas test immediately.
Step 4: Test for the anion
Follow the standard reagent sequence. For example, the O-Level halide test uses dilute nitric acid followed by aqueous silver nitrate.
Step 5: Test for the cation
Add NaOH(aq) or NH₃(aq) drop by drop, record the precipitate colour and then add the reagent in excess.
Step 6: Check whether all observations agree
Your proposed identity should explain every important observation. If one result contradicts your conclusion, reconsider the identification.
Identifying both ions in an unknown salt
An unknown solution is blue. When aqueous ammonia is added drop by drop, a light-blue precipitate forms. The precipitate dissolves in excess aqueous ammonia to form a deep-blue solution.
These observations identify the cation as Cu²⁺.
A fresh portion is acidified using dilute nitric acid. Aqueous silver nitrate is then added and a white precipitate forms.
This identifies the anion as Cl⁻.
The unknown salt is therefore consistent with copper(II) chloride, CuCl₂.
Common Mistakes in Qualitative Analysis
Writing an inference instead of an observation
“Cu²⁺ ions are present” is an inference. “A light-blue precipitate forms” is an observation. If the question asks for observations, describe what you actually see.
Forgetting to add the reagent in excess
Several ions form white precipitates initially. The behaviour in excess reagent is often what distinguishes them. Stopping after a few drops can leave the identification incomplete.
Thinking nitric acid is used only because the instructions say to acidify
The acid has a chemical purpose. It removes interfering carbonate ions. In the standard O-Level procedure this is done before silver nitrate or barium nitrate is added.
Assuming silver nitrate must always be added after nitric acid for the chemistry to work
Adding silver nitrate first would change the observations because carbonate ions could initially form an additional precipitate. Dilute nitric acid added afterwards would dissolve the carbonate precipitate. For O-Level questions, however, use the standard syllabus sequence: acidify first, then add silver nitrate.
Using hydrochloric acid before silver nitrate
Hydrochloric acid introduces Cl⁻ ions. These added chloride ions can react with Ag⁺ ions to form white AgCl, creating a false positive result for chloride. Use dilute nitric acid instead.
Testing everything in the same test tube
Earlier reagents may interfere with later tests. Use a fresh portion of the original solution for each separate test unless a continuous sequence is specifically required.
Identifying a gas simply because bubbles appear
Effervescence proves only that a gas is being evolved. The gas must still be identified using the appropriate chemical test.
Using dry litmus paper
Ammonia and chlorine require water before their effect on litmus can be observed. Use damp litmus paper.
Mixing up lighted and glowing splints
A lighted splint is used for hydrogen. A glowing splint is used for oxygen.
Confusing nitrate and ammonium tests
Both positive tests can produce ammonia. The nitrate test requires NaOH(aq), aluminium foil and warming. The ammonium test requires NaOH(aq) and warming without aluminium.
Writing only “precipitate dissolves”
Give the complete observation where possible. For example: “The white precipitate dissolves in excess aqueous sodium hydroxide to form a colourless solution.”
Qualitative Analysis Exam Checklist
Identify the type of test
Decide whether you are identifying a gas, an anion or a cation before choosing the reagent.
Use fresh portions
Separate the original sample before carrying out unrelated tests to avoid contamination.
Name the reagent precisely
State nitric acid, silver nitrate, barium nitrate, sodium hydroxide or ammonia rather than writing only “add reagent”.
Include important conditions
Mention warming, aluminium foil, damp litmus paper and acidification whenever they are required.
Record the precipitate colour
White, light blue, green, red-brown and yellow observations can distinguish different ions.
Check the excess stage
State whether the precipitate remains insoluble or dissolves when excess reagent is added.
Confirm any gas produced
Effervescence alone is not identification. Carry out the appropriate gas test.
Separate observation from conclusion
First state what happens. Then use that evidence to identify the ion or gas.
These ten questions move beyond simple recall. Use the reagents, observations and test sequence to decide what the chemical evidence actually proves.
- Interpret unfamiliar test results.
- Distinguish ions with similar observations.
- Apply qualitative analysis as a logical evidence chain.
Frequently Asked Questions
Understanding Qualitative Analysis More Deeply
Once the individual tests are familiar, these questions help connect them into a more reliable way of thinking about unknown substances.
Do I really have to memorise every qualitative analysis test?
Some reagent–observation combinations do need to be remembered, but the topic becomes much easier when you organise them into patterns.
Gas tests use characteristic splint, litmus or solution tests. Anion tests generally involve gas formation or precipitates. Cation tests mainly use NaOH(aq) and NH₃(aq), where you compare precipitate colour and solubility in excess reagent.
Why is the word “excess” so important in cation tests?
The first few drops of reagent tell you whether a precipitate forms. Adding excess reagent gives a second observation: does that precipitate remain or dissolve?
This is essential for distinguishing ions such as Al³⁺, Ca²⁺ and Zn²⁺.
Why can I not identify a gas just from effervescence?
Effervescence tells you only that gas bubbles are being formed. Different reactions can produce different gases.
You must test the gas itself. CO₂ turns limewater milky, H₂ gives a “pop” with a lighted splint and O₂ relights a glowing splint.
Why must litmus paper be damp for ammonia and chlorine?
The gas needs to dissolve in water before it can affect the indicator. Damp litmus provides the thin layer of water required.
Why do we acidify before some precipitation tests?
Acidification removes interfering ions before the main test reagent is added.
Carbonate ions are particularly important because they can form precipitates with ions such as Ag⁺ and Ba²⁺. H⁺ ions convert CO₃²⁻ into CO₂ and H₂O.
Must nitric acid always be added before silver nitrate?
The standard O-Level procedure is to acidify with dilute nitric acid first and then add aqueous silver nitrate.
Chemically, reversing the order changes what you observe. If AgNO₃ is added first to a sample containing carbonate ions, a carbonate precipitate may form initially. Dilute nitric acid added afterwards can dissolve this carbonate precipitate while AgCl remains.
For examination procedures, use the standard stated sequence unless the question specifically asks you to interpret a different experimental order.
Why is nitric acid used instead of hydrochloric acid in the chloride test?
Hydrochloric acid introduces Cl⁻ ions. These added chloride ions could react with Ag⁺ ions to produce white AgCl even when chloride was absent from the original sample.
Nitric acid removes carbonate interference without introducing extra Cl⁻ ions.
How do I remember the difference between Al³⁺ and Zn²⁺?
Both produce white precipitates that dissolve in excess NaOH(aq). Aqueous ammonia provides the useful distinction.
The Al³⁺ precipitate remains insoluble in excess NH₃(aq), while the Zn²⁺ precipitate dissolves to form a colourless solution.
Why can the colour of the original solution be useful but not enough on its own?
Some ions give characteristic solution colours, so colour can provide an early clue. For example, Cu²⁺ solutions are commonly blue.
However, colour depends on concentration and other substances present. A chemical test gives stronger evidence.
How can both nitrate ions and ammonium ions produce ammonia in their tests?
Ammonium ions react directly with hydroxide ions on warming to release ammonia.
Nitrate ions require an additional reaction involving aluminium in alkaline conditions. This is why aluminium foil is included in the nitrate test but not in the ammonium test.
How detailed should an observation be in an examination?
Include every visible feature that helps distinguish the substance: whether a precipitate forms, its colour, whether it dissolves in excess reagent, whether a gas is evolved and what happens during the gas test.
For example, “white precipitate dissolves in excess NaOH(aq) to form a colourless solution” is more useful than simply writing “precipitate disappears”.
Bringing Qualitative Analysis Together
Qualitative analysis becomes much easier when you stop treating every result as an isolated fact. Begin with the correct reagent, observe exactly what happens and then use the evidence to identify the substance. Gas tests, anion tests and cation tests are all built on this same sequence: test → observation → conclusion. Once that reasoning becomes automatic, even an unfamiliar unknown can be approached systematically rather than by guesswork.