Qualitative analysis involves performing simple test-tube reactions to observe precipitation, gas evolution, or characteristic flame emissions. This guide details how to identify key metal cations and non-metal anions following the AQA specification.
Core Specification Link
This practical supports topic 3.2.2 Group 2, 3.2.3 Group 7 (Halogens), and 3.2.6 Reactions of Ions in Aqueous Solution of inorganic chemistry.
Aim
To carry out simple test-tube reactions to identify the cations and anions in a set of unknown solutions (including Group 2 metals, ammonium ions, halides, carbonates, sulfates, and hydroxides).
Equipment List
- Nichrome wire loop (flame tests, beyond the AQA specification)
- Concentrated hydrochloric acid (HCl) (flame tests, beyond the AQA specification)
- Unknown solution samples (containing Group 2 or ammonium cations, and halide or other anions)
- Bunsen burner (flame tests, beyond the AQA specification)
- Test tubes and test tube rack
- Dropping pipettes
- Sodium hydroxide solution (NaOH, 1.0 mol dm⁻³)
- Dilute nitric acid (HNO₃, 1.0 mol dm⁻³)
- Silver nitrate solution (AgNO₃, 0.05 mol dm⁻³)
- Dilute and concentrated ammonia solutions (NH₃)
- Barium chloride solution (BaCl₂, 0.1 mol dm⁻³)
- Dilute sulfuric acid (H₂SO₄) and dilute hydrochloric acid (HCl)
- Limewater
- Red litmus paper or pH indicator paper
- Water bath (beaker of hot water or electronic water bath)
Part 1: Identifying Cations (Flame & NaOH Tests)
1. Flame Tests (Beyond the AQA Specification)
Beyond the AQA specification: flame tests are not in AQA 7405, so you will not need them to identify ions. The chemical tests below are what AQA requires.
Metal ions exhibit unique emissions when heated. Electrons absorb heat and are promoted to higher energy levels. When they relax back to their ground state, they emit light of specific wavelengths within the visible spectrum.
- Clean the nichrome wire loop by dipping it in concentrated hydrochloric acid and holding it in the hottest part of the blue Bunsen burner flame . Repeat until the wire produces no colour in the flame.
- Dip the clean wire into the sample solution (or touch it to a tiny amount of the solid sample).
- Hold the wire loop in the edge of the blue Bunsen flame and record the colour observed.
2. Group 2 Cations: Sodium Hydroxide and Dilute Sulfuric Acid
The Group 2 ions are identified from the opposite trends in the solubility of their hydroxides and sulfates: hydroxide solubility increases down the group, and sulfate solubility decreases. Mg(OH)₂ is sparingly soluble and BaSO₄ is insoluble.
- Add a few drops of dilute sodium hydroxide to 1 cm³ of the sample solution.
- To a fresh 1 cm³ sample, add a few drops of dilute sulfuric acid.
| Cation | With dilute NaOH | With dilute H₂SO₄ |
|---|---|---|
| Mg²⁺(aq) | White precipitate (Mg(OH)₂ is sparingly soluble) | No precipitate (MgSO₄ is very soluble) |
| Ca²⁺(aq) | Slight white precipitate | Slight white precipitate |
| Sr²⁺(aq) | Very slight white precipitate | White precipitate |
| Ba²⁺(aq) | No precipitate (colourless solution) | Thick white precipitate (BaSO₄ is insoluble) |
3. Sodium Hydroxide Tests for Metal Aqua Cations
Note: Cu²⁺, Fe²⁺, Fe³⁺ and Al³⁺ are tested in AQA RP11 (3.2.6), not RP4. They are shown here for comparison with the Group 2 ions.
Transition metals and group 3 elements form metal hydroxide precipitates in the presence of hydroxide ions (OH⁻). Some, like aluminium, are amphoteric and redissolve in excess alkali.
- Place 1 cm³ of the sample solution into a clean test tube.
- Add dilute sodium hydroxide dropwise, observing and recording any precipitate that forms.
- Continue adding sodium hydroxide solution until it is in excess, and observe if the precipitate redissolves.
| Cation | Equation for Precipitate Formation | Precipitate Colour | With Excess NaOH |
|---|---|---|---|
| Cu²⁺(aq) | Cu²⁺(aq) + 2OH⁻(aq) → Cu(OH)₂(s) | Blue | Insoluble (remains blue precipitate) |
| Fe²⁺(aq) | Fe²⁺(aq) + 2OH⁻(aq) → Fe(OH)₂(s) | Green (darkens on air contact) | Insoluble (remains green precipitate) |
| Fe³⁺(aq) | Fe³⁺(aq) + 3OH⁻(aq) → Fe(OH)₃(s) | Brown / Rust | Insoluble (remains brown precipitate) |
| Al³⁺(aq) | Al³⁺(aq) + 3OH⁻(aq) → Al(OH)₃(s) | White | Redissolves to form a colourless solution: Al(OH)₃(s) + OH⁻(aq) → [Al(OH)₄]⁻(aq) |
| Ca²⁺(aq) | Ca²⁺(aq) + 2OH⁻(aq) → Ca(OH)₂(s) | White | Insoluble (remains white precipitate) |
| Mg²⁺(aq) | Mg²⁺(aq) + 2OH⁻(aq) → Mg(OH)₂(s) | White | Insoluble (remains white precipitate) |
4. Test for Ammonium Ion (NH₄⁺)
Ammonium ions react with warm hydroxide ions to release volatile ammonia gas (NH₃):
\[ \text{NH}_4^+(\text{aq}) + \text{OH}^-(\text{aq}) \rightarrow \text{NH}_3(\text{g}) + \text{H}_2\text{O}(\text{l}) \]- Add dilute sodium hydroxide solution to the sample in a test tube.
- Warm the mixture gently in a water bath.
- Hold a piece of damp red litmus paper over the mouth of the test tube.
- Observation: The litmus paper turns blue due to alkaline ammonia gas dissolving in the dampness. Ammonia also has a characteristic sharp, pungent odour.
Part 2: Identifying Anions (Carbonate, Sulfate, Hydroxide & Halides)
| Anion | Reagents & Procedure | Observation | Ionic Equation |
|---|---|---|---|
| Carbonate (CO₃²⁻) | Add dilute hydrochloric acid. Bubble any gas evolved through limewater. | Effervescence (fizzing). Limewater turns cloudy/milky. | CO₃²⁻(aq) + 2H⁺(aq) → CO₂(g) + H₂O(l) |
| Sulfate (SO₄²⁻) | Add dilute hydrochloric acid (to clear carbonates), then add barium chloride solution (BaCl₂). | Thick white precipitate. | Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s) |
| Hydroxide (OH⁻) | Touch a drop of the solution on red litmus paper (or add universal indicator). | Red litmus paper turns blue (universal indicator turns blue or purple): the solution is alkaline. | No precipitate; the test detects OH⁻ ions in the solution. |
| Chloride (Cl⁻) | Acidify with dilute nitric acid (HNO₃), then add silver nitrate solution (AgNO₃). Add dilute ammonia. | White precipitate. Redissolves in dilute ammonia. | Ag⁺(aq) + Cl⁻(aq) → AgCl(s) |
| Bromide (Br⁻) | Acidify with dilute nitric acid, then add silver nitrate solution. Add concentrated ammonia. | Cream precipitate. Redissolves only in concentrated ammonia. | Ag⁺(aq) + Br⁻(aq) → AgBr(s) |
| Iodide (I⁻) | Acidify with dilute nitric acid, then add silver nitrate solution. Add concentrated ammonia. | Yellow precipitate. Insoluble in both dilute and concentrated ammonia. | Ag⁺(aq) + I⁻(aq) → AgI(s) |
In the ammonium test the damp litmus paper only meets the gas above the tube; in the hydroxide test it touches the solution.
Why these reagents?
- Barium chloride is used because barium sulfate is insoluble, so sulfate ions give a white precipitate.
- It is acidified with dilute hydrochloric acid to remove carbonate ions, which would otherwise give a white precipitate of barium carbonate. Hydrochloric acid is fine because chloride ions give no precipitate with barium ions. Sulfuric acid must not be used because it adds sulfate ions.
- Silver nitrate is used because the silver halides are insoluble and each has its own colour.
- It is acidified with dilute nitric acid to remove carbonate and hydroxide ions, which would also form precipitates with silver ions. Hydrochloric acid must not be used because it adds chloride ions.
- Ammonia is added because the white, cream and yellow precipitates can be hard to tell apart; their solubility in dilute and concentrated ammonia confirms the halide. Solubility in ammonia: AgCl > AgBr > AgI.
Step 1: Test for the ammonium ion using sodium hydroxide solution and heat
Add dilute sodium hydroxide solution to a sample of each solution and warm gently in a water bath. Hold damp red litmus paper over the mouth of each tube.
- The solution containing NH₄Cl will release a gas (ammonia) that turns the damp red litmus paper blue: \[ \text{NH}_4^+(\text{aq}) + \text{OH}^-(\text{aq}) \rightarrow \text{NH}_3(\text{g}) + \text{H}_2\text{O}(\text{l}) \]
- The other three solutions (BaCl₂, NaCl, and Al₂(SO₄)₃) will produce no alkaline gas.
Step 2: Add excess sodium hydroxide solution to the remaining three solutions
Add sodium hydroxide dropwise until in excess to fresh samples of the remaining three solutions:
- The solution containing Al₂(SO₄)₃ will form a white precipitate of aluminium hydroxide, which dissolves in excess sodium hydroxide to form a colourless solution: \[ \text{Al}^{3+}(\text{aq}) + 3\text{OH}^-(\text{aq}) \rightarrow \text{Al(OH)}_3(\text{s}) \quad \text{(white ppt)} \] \[ \text{Al(OH)}_3(\text{s}) + \text{OH}^-(\text{aq}) \rightarrow [\text{Al(OH)}_4]^-(\text{aq}) \quad \text{(colourless solution)} \]
- The solutions containing BaCl₂ and NaCl will show no change (sodium does not precipitate, and barium hydroxide is soluble at this concentration).
Step 3: Distinguish between the two remaining chloride solutions
To distinguish between the remaining BaCl₂ and NaCl solutions:
- Add acidified silver nitrate to both. Both will form a white precipitate of silver chloride: \[ \text{Ag}^+(\text{aq}) + \text{Cl}^-(\text{aq}) \rightarrow \text{AgCl}(\text{s}) \quad \text{(white ppt)} \]
- To confirm BaCl₂ from NaCl, add dilute sulfuric acid to a fresh sample. Barium ions (Ba²⁺) give a white precipitate of barium sulfate (a flame test, beyond the AQA specification, would also show an apple-green flame): \[ \text{Ba}^{2+}(\text{aq}) + \text{SO}_4^{2-}(\text{aq}) \rightarrow \text{BaSO}_4(\text{s}) \quad \text{(white ppt)} \] Sodium ions (Na⁺) give no precipitate with sulfuric acid (and a yellow flame).
Safety & Risk Assessment
| Hazard | Risk | Precaution |
|---|---|---|
| Barium chloride (BaCl₂) | Highly toxic by ingestion and acts as a skin irritant. | Wear protective nitrile gloves. Wash hands immediately after use. Dispose of waste in a designated heavy metal residue bottle. |
| Silver nitrate (AgNO₃) | Corrosive and causes permanent brown-black stains on skin and clothing. | Wear gloves and a lab coat. Handle with care to prevent spills. |
| Concentrated Hydrochloric acid | Corrosive; releases toxic, choking hydrogen chloride gas. | Only needed for flame tests (beyond the AQA specification). Clean the wire in a fume cupboard. Wear safety goggles and gloves. |
Sources of Error & Improvements
- Contamination in flame tests (beyond the AQA specification): Sodium traces are extremely common and emit an intense yellow light that easily masks other ions. Improvement: Dip the wire loop in concentrated acid and burn off repeatedly until zero background colour remains.
- Halide false positives: If the sample contains carbonate (CO₃²⁻) or hydroxide (OH⁻) ions, adding silver nitrate without acidifying will give precipitates of silver carbonate or silver oxide that could be mistaken for a silver halide. Improvement: Always acidify the sample with dilute nitric acid (HNO₃) first. The nitric acid removes carbonate ions by converting them to carbon dioxide and water, and neutralises hydroxide ions. Never use hydrochloric acid (HCl) to acidify, as it adds chloride ions, which would react with silver ions and give a false positive white precipitate.
Exam questions, marked instantly
Write your answer and the AI examiner checks it against the mark scheme, showing exactly which marks you earned and what to add.
Exam-style question
2 marksExplain why aluminium hydroxide dissolves in excess sodium hydroxide solution but magnesium hydroxide does not.
Show Mark Scheme
- Aluminium hydroxide is amphoteric and reacts with excess hydroxide ions to form a soluble complex, [Al(OH)₄]⁻ [1]
- Magnesium hydroxide is basic, not amphoteric, so it does not react with excess hydroxide ions and remains as a precipitate [1]
Exam-style question
3 marksDescribe tests you could use to distinguish between aqueous potassium chloride and aqueous potassium iodide. Give the results.
Show Mark Scheme
- Add dilute nitric acid, then silver nitrate solution, to each sample [1]
- Potassium chloride gives a white precipitate; potassium iodide gives a yellow precipitate [1]
- The white silver chloride precipitate dissolves in dilute ammonia; the yellow silver iodide does not dissolve, even in concentrated ammonia [1]
CPAC Skills Assessed
- CPAC 1: Correctly follows written procedures to identify multiple unknown ions.
- CPAC 3: Safely handles toxic barium and staining silver nitrate solutions.
- CPAC 4: Accurately observes and records precipitate colour changes and gas evolution.
When asked to describe the halide test, you must state that you acidify with nitric acid (HNO₃). Simply stating "acidify" or using "HCl" or "H₂SO₄" will lose marks. Also, distinguish halide precipitate colours carefully: white (Cl⁻), cream (Br⁻), yellow (I⁻).