Flame Tests, Precipitate Tests for Cations & Anion Tests
In Triple Science GCSE Chemistry, unknown ionic compounds are identified by testing separately for the positive metal ion (cation) and the negative non-metal ion (anion). Metal cations are identified either by the characteristic colour they produce in a roaring blue Bunsen flame (flame tests and flame emission spectroscopy) or by the coloured insoluble metal hydroxide precipitates they form when aqueous sodium hydroxide is added. Anions (carbonates, halides, and sulfates) are identified using dilute acids, acidified silver nitrate, and acidified barium chloride. Examiners regularly set 6-mark 'identify the unknown salts' detective questions requiring exact reagents and precipitate colours.
- Flame Tests for Metal Cations
- Clean a nichrome or platinum wire loop by dipping in concentrated HCl and heating in a roaring blue Bunsen flame until no colour is seen; dip into sample and hold in the edge of the blue flame. Colours: Lithium (Li⁺) = Crimson; Sodium (Na⁺) = Yellow; Potassium (K⁺) = Lilac; Calcium (Ca²⁺) = Orange-red; Copper(II) (Cu²⁺) = Green. Note: If a mixture of metal ions is tested, one flame colour masks the others.
- Sodium Hydroxide (NaOH) Precipitate Tests for Cations
- Add a few drops of aqueous sodium hydroxide (NaOH) to solution of metal compound: 1. Copper(II), Cu²⁺ -> Blue precipitate (Cu(OH)₂). 2. Iron(II), Fe²⁺ -> Green precipitate (Fe(OH)₂). 3. Iron(III), Fe³⁺ -> Brown (reddish-brown) precipitate (Fe(OH)₃). 4. Calcium (Ca²⁺), Magnesium (Mg²⁺), and Aluminium (Al³⁺) all form White precipitates; Only the Aluminium (Al³⁺) white precipitate re-dissolves when excess NaOH is added to form a colourless solution.
- Test for Carbonate Ions (CO₃²⁻)
- Add any dilute acid (e.g., dilute hydrochloric acid or nitric acid) to the sample. If carbonate ions are present, effervescence (fizzing) occurs as carbon dioxide gas is produced: CO₃²⁻(aq) + 2H⁺(aq) -> CO₂(g) + H₂O(l). Bubble the gas through limewater; it turns cloudy white.
Flame Tests, Precipitate Tests for Cations & Anion Tests: Reaction Profile (ΔH & E_a) + Le Chatelier Yield Simulator
Toggle a catalyst on the enthalpy profile diagram or adjust temperature and pressure to see how dynamic equilibrium shifts.
Modelled on N₂(g) + 3H₂(g) ⇌ 2NH₃(g) (ΔH = −92 kJ/mol) (4 moles gas → 2 moles gas):
Higher pressure shifts equilibrium to the side with fewer gas moles (right, 4 → 2). Lower temperature favours the exothermic forward reaction.
Key Concepts & Exam Definitions (6)
- Flame Tests for Metal Cations
- Clean a nichrome or platinum wire loop by dipping in concentrated HCl and heating in a roaring blue Bunsen flame until no colour is seen; dip into sample and hold in the edge of the blue flame. Colours: Lithium (Li⁺) = Crimson; Sodium (Na⁺) = Yellow; Potassium (K⁺) = Lilac; Calcium (Ca²⁺) = Orange-red; Copper(II) (Cu²⁺) = Green. Note: If a mixture of metal ions is tested, one flame colour masks the others.
- Sodium Hydroxide (NaOH) Precipitate Tests for Cations
- Add a few drops of aqueous sodium hydroxide (NaOH) to solution of metal compound: 1. Copper(II), Cu²⁺ -> Blue precipitate (Cu(OH)₂). 2. Iron(II), Fe²⁺ -> Green precipitate (Fe(OH)₂). 3. Iron(III), Fe³⁺ -> Brown (reddish-brown) precipitate (Fe(OH)₃). 4. Calcium (Ca²⁺), Magnesium (Mg²⁺), and Aluminium (Al³⁺) all form White precipitates; Only the Aluminium (Al³⁺) white precipitate re-dissolves when excess NaOH is added to form a colourless solution.
- Test for Carbonate Ions (CO₃²⁻)
- Add any dilute acid (e.g., dilute hydrochloric acid or nitric acid) to the sample. If carbonate ions are present, effervescence (fizzing) occurs as carbon dioxide gas is produced: CO₃²⁻(aq) + 2H⁺(aq) -> CO₂(g) + H₂O(l). Bubble the gas through limewater; it turns cloudy white.
- Test for Halide Ions (Cl⁻, Br⁻, I⁻)
- Add dilute nitric acid (HNO₃), followed by aqueous silver nitrate solution (AgNO₃). Halide ions react with Ag⁺(aq) to form insoluble silver halide precipitates: 1. Chloride (Cl⁻) -> White precipitate of AgCl. 2. Bromide (Br⁻) -> Cream precipitate of AgBr. 3. Iodide (I⁻) -> Yellow precipitate of AgI. Ionic equation: Ag⁺(aq) + X⁻(aq) -> AgX(s).
- Test for Sulfate Ions (SO₄²⁻)
- Add dilute hydrochloric acid (HCl), followed by aqueous barium chloride solution (BaCl₂). (Alternatively, dilute nitric acid + barium nitrate). If sulfate ions are present, a White precipitate of insoluble barium sulfate forms: Ba²⁺(aq) + SO₄²⁻(aq) -> BaSO₄(s).
- Flame Emission Spectroscopy
- An instrumental method used to analyse metal ions in solution. A sample is heated in a flame; thermal energy excites electrons to higher energy levels, and as they fall back down, they emit light at specific wavelengths. The light passes through a spectroscope to produce a line spectrum. The combination of wavelengths identifies the metal ion, and the intensity of the lines measures the ion's concentration.
Quick-Fire Flashcards (6)
Click any card to check the answer, then mark it as Known to track your progress.
State the flame test colours for Li⁺, Na⁺, K⁺, Ca²⁺, and Cu²⁺.
State the precipitate colours when aqueous NaOH is added to solutions containing Cu²⁺, Fe²⁺, and Fe³⁺.
State the reagents and precipitate colours for testing Cl⁻, Br⁻, and I⁻ ions.
State the reagents and positive result for testing sulfate (SO₄²⁻) ions, and write the ionic equation.
Why can you not use dilute hydrochloric acid or sulfuric acid to acidify a sample before testing for halide ions with silver nitrate?
In flame emission spectroscopy, what two pieces of information can be obtained from the line spectrum?
Exam-Style Questions & Mark Schemes (4)
Write or think through your answer first, then click Check Mark Scheme to see the exact points examiners award marks for.
- Q1
An unknown white crystalline solid X gives a lilac flame in a flame test. When dissolved in water and acidified with dilute nitric acid, adding silver nitrate solution produces a yellow precipitate. Identify the cation, the anion, and the chemical name and formula of solid X, and write the ionic equation for the formation of the yellow precipitate. (Triple Science) [5 marks]
Click “Check Mark Scheme” to compare your answer, 1. Cation: The lilac flame test identifies potassium ions, K⁺ [1 mark]. 2. Anion: The yellow precipitate with acidified silver nitrate identifies iodide ions, I⁻ [1 mark]. 3. Name of solid X: Potassium iodide [1 mark]. 4. Formula of solid X: KI [1 mark]. 5. Ionic equation: Ag⁺(aq) + I⁻(aq) -> AgI(s) [1 mark].
- Q2
Three unlabelled bottles contain colourless aqueous solutions of aluminium sulfate, calcium chloride, and magnesium carbonate. Describe a sequence of chemical tests (not flame tests) that a student could perform to positively identify which solution is in each bottle. (Triple Science) [6 marks]
Click “Check Mark Scheme” to compare your answer, 1. Take a small sample from each bottle and add dilute nitric acid: the bottle that fizzes (effervesces) and produces a gas that turns limewater cloudy contains carbonate ions, identifying it as magnesium carbonate [1 mark]. 2. To fresh samples of the remaining two solutions, add aqueous sodium hydroxide (NaOH) dropwise: both will initially form a white precipitate (of Al(OH)₃ and Ca(OH)₂) [1 mark]. 3. Add excess sodium hydroxide solution to both tubes: the tube where the white precipitate dissolves to form a colourless solution contains Al³⁺ ions (identifying aluminium sulfate), whereas the Ca(OH)₂ precipitate remains insoluble [1 mark]. 4. To confirm the anions in the two bottles, take fresh samples and add dilute nitric acid followed by silver nitrate solution: the calcium chloride solution will form a white precipitate of silver chloride (AgCl) [1 mark]. 5. To fresh samples, add dilute hydrochloric acid followed by barium chloride solution [1 mark]. 6. The aluminium sulfate solution will form a white precipitate of barium sulfate (BaSO₄) [1 mark].
- Q3
When aqueous sodium hydroxide is added to a green solution of iron(II) sulfate, a green precipitate is formed. Write the balanced symbol equation and the ionic equation (including state symbols) for this precipitation reaction. (Triple Science) [4 marks]
Click “Check Mark Scheme” to compare your answer, 1. Reactants and products: Iron(II) sulfate + sodium hydroxide -> iron(II) hydroxide + sodium sulfate [1 mark]. 2. Balanced symbol equation: FeSO₄(aq) + 2NaOH(aq) -> Fe(OH)₂(s) + Na₂SO₄(aq) [1 mark]. 3. Correct ionic species removing Na⁺ and SO₄²⁻ spectator ions: Fe²⁺ + 2OH⁻ -> Fe(OH)₂ [1 mark]. 4. Correct state symbols in ionic equation: Fe²⁺(aq) + 2OH⁻(aq) -> Fe(OH)₂(s) [1 mark].
- Q4
A mineral water sample contains a mixture of sodium ions (Na⁺) and potassium ions (K⁺). Explain why a manual Bunsen flame test is unsuitable for identifying both ions in the water, and explain how flame emission spectroscopy overcomes this problem and provides additional quantitative data. (Triple Science) [4 marks]
Click “Check Mark Scheme” to compare your answer, 1. In a manual flame test of a mixture, the bright yellow flame colour of the sodium ions (Na⁺) masks (obscures) the paler lilac flame colour of the potassium ions (K⁺), so only sodium would be seen [1 mark]. 2. In flame emission spectroscopy, the light emitted by the excited ions passes through a prism/spectroscope, which separates the light into individual wavelengths to produce a line spectrum [1 mark]. 3. Both Na⁺ and K⁺ produce their own unique pattern of spectral lines at distinct wavelengths, allowing both ions to be identified simultaneously by comparing against reference spectra [1 mark]. 4. Also, flame emission spectroscopy is quantitative: measuring the intensity of the spectral lines allows the chemist to determine the exact concentration of each metal ion in the mineral water [1 mark].
Common Exam Questions (4)
What are the 5 GCSE Chemistry flame test colours to memorise?
1. Lithium (Li⁺) -> Crimson flame. 2. Sodium (Na⁺) -> Yellow flame. 3. Potassium (K⁺) -> Lilac flame. 4. Calcium (Ca²⁺) -> Orange-red (brick-red) flame. 5. Copper(II) (Cu²⁺) -> Green (blue-green) flame.
How do you distinguish between Ca²⁺, Mg²⁺, and Al³⁺ ions when all three form a white precipitate with sodium hydroxide?
First, add excess sodium hydroxide (NaOH) solution: the aluminium hydroxide (Al³⁺) white precipitate dissolves in excess NaOH to form a colourless solution, whereas Ca²⁺ and Mg²⁺ precipitates do not dissolve. Second, perform a flame test to distinguish Ca²⁺ and Mg²⁺: Ca²⁺ gives an orange-red flame, whereas Mg²⁺ gives no flame colour.
Why must you add dilute nitric acid before adding silver nitrate in the halide test, and dilute hydrochloric acid before barium chloride in the sulfate test?
The dilute acid reacts with and removes any carbonate ions (or sulfite ions) that might be present as impurities in the sample. If carbonate ions were not removed first, they would react with Ag⁺ or Ba²⁺ ions to form a white precipitate of silver carbonate or barium carbonate, giving a false positive result.
What are the three advantages of instrumental methods (like flame emission spectroscopy) over chemical tests?
Memorise the three 'S' words: Instrumental methods are 1. More Sensitive (can detect tiny trace amounts of substances), 2. More Specific / Accurate (can identify mixtures of metal ions where one bright flame colour would mask another in a manual flame test), and 3. Faster (rapid automated analysis).
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