Qualitative Analysis
CIE A-Level Chemistry 9701 · Cation, Anion & Gas Tests
1 — Cation identification
Aqueous salt sample
Add NaOH(aq) in excess observe precipitate and colour
no ppt
No ppt NH₄⁺ (or very dilute Ba²⁺/Ca²⁺)
Warm with NaOH gas produced?
NH₃ gas
NH₄⁺litmus→blue
no gas
Ba²⁺/Ca²⁺confirm by anion tests
white ppt
White ppt in NaOH
Soluble in excess NaOH? add more NaOH, observe
yes — soluble
Soluble → colourlessAl³⁺, Pb²⁺ or Zn²⁺
+ NH₃(aq) excessppt dissolve?
yes
Zn²⁺sol. in NH₃
no
Pb²⁺/Al³⁺insol. in NH₃
no — insol.
Insoluble in excessMg²⁺ or Ca²⁺
+ NH₃(aq)ppt form?
white ppt
Mg²⁺insol. NH₃
no ppt
Ca²⁺no ppt NH₃
coloured ppt
Red-brown pptFe³⁺insol. in excess — done
Green ppt → brownFe²⁺turns brown in air; insol. excess
Grey-green pptsol. in NaOH→dark green
+ NH₃ excessgrey-green insol.
Cr³⁺confirmed
Pale blue pptinsol. in NaOH
+ NH₃ excessppt dissolves
Cu²⁺dark blue soln
Off-white ppt → brownMn²⁺rapidly turns brown in air; insol.
Note — Ba²⁺ and Ca²⁺ with NaOH: The data booklet states Ba²⁺ gives a faint white ppt unless [Ba²⁺] is very low, and Ca²⁺ gives a white ppt unless [Ca²⁺] is very low. At typical concentrations a faint white ppt may form. These cations give no ppt with NH₃(aq) and are distinguished from other white-ppt cations by this and their anion tests.
Identified / soluble
Insoluble / elimination
Decision / test step
Cu²⁺ pale blue ppt
Fe²⁺ green ppt (→ brown in air)
Mn²⁺ off-white ppt (→ brown in air)
No ppt branch
2 — Anion identification
Ion
Reagent
Observation
Confirmed
Read each row left → right CO₃²⁻
Dilute acid(HCl or HNO₃)
Colourless gas evolvedCO₂ — test with limewater
White ppt in limewater(dissolves with excess CO₂)
CO₃²⁻confirmed
SO₄²⁻
Ba²⁺(aq)e.g. BaCl₂ solution
White ppt formsBaSO₄
Add dilute HCl / HNO₃ppt insoluble in excess acid
SO₄²⁻confirmed
or:
High [Ca²⁺(aq)]e.g. CaCl₂ solution
White ppt CaSO₄ formsonly at high Ca²⁺ concentration
SO₃²⁻
Ba²⁺(aq)e.g. BaCl₂ solution
White ppt formsBaSO₃
Add dilute HCl / HNO₃ppt dissolves in excess acid
SO₃²⁻confirmed
or:
Acidified KMnO₄(aq)
Purple → colourlessdecolourises acidified aq. KMnO₄
SO₃²⁻confirmed
Key distinction — SO₄²⁻ vs SO₃²⁻: Both give white ppt with Ba²⁺(aq). Add dilute HCl or HNO₃ — BaSO₄ stays (SO₄²⁻); BaSO₃ dissolves (SO₃²⁻). Never use H₂SO₄ as the acid. SO₃²⁻ also decolourises acidified aqueous KMnO₄.
NO₃⁻
OH⁻(aq) + Al foilheat gently
NH₃ gas evolvedtest with damp red litmus
Litmus turns blue
NO₃⁻confirmed
NO₂⁻
OH⁻(aq) + Al foilheat gently
NH₃ gas evolvedsame as NO₃⁻
also:
Acidified KMnO₄(aq)
Purple → colourlessdecolourises acidified aq. KMnO₄
NO₂⁻confirmed
Key distinction — NO₃⁻ vs NO₂⁻: Both produce NH₃ with OH⁻ + Al foil. Only NO₂⁻ also decolourises acidified aqueous KMnO₄. Use the KMnO₄ test to distinguish.
Cl⁻
Ag⁺(aq)AgNO₃ solution
White pptAgCl forms
Add NH₃(aq)ppt dissolves fully
Cl⁻confirmed
Br⁻
Ag⁺(aq)AgNO₃ solution
Cream/off-white pptAgBr forms
Add NH₃(aq)ppt partially soluble
Br⁻confirmed
I⁻
Ag⁺(aq)AgNO₃ solution
Pale yellow pptAgI forms
Add NH₃(aq)ppt insoluble
I⁻confirmed
Key distinction — Cl⁻ vs Br⁻ vs I⁻: All give ppt with Ag⁺(aq). Distinguish by colour (white / cream-off-white / pale yellow) and NH₃ solubility: AgCl dissolves fully, AgBr partially, AgI insoluble.
CrO₄²⁻
Yellow solutioncharacteristic colour
Add H⁺(aq)
Turns orange(→ Cr₂O₇²⁻)
or:
Add Ba²⁺(aq)
Yellow ppt BaCrO₄
Add Pb²⁺(aq)
Bright yellow ppt PbCrO₄
S₂O₃²⁻
Dilute acid H⁺(aq)(HCl or HNO₃)
Off-white / pale yellow ppt — slowlysulfur precipitate forms over time
S₂O₃²⁻confirmed
Reagent / test step
Observation (positive)
Confirmed / identified
SO₄²⁻
SO₃²⁻ / Br⁻ / CrO₄²⁻
NO₂⁻
I⁻
3 — Gas tests
Gas
Test
Positive result
NH₃
Hold damp red litmus paper in the gas
Paper turns blue
CO₂
Bubble gas through limewater (Ca(OH)₂ solution)
White ppt forms (CaCO₃) — dissolves with excess CO₂
H₂
Hold a lighted splint at the mouth of the tube
"Pop" sound heard
O₂
Insert a glowing splint into the gas
Splint relights
Additional gas tests (not in official data booklet gas table — for reference only):
• Cl₂: Hold damp litmus paper in gas → paper is bleached / decolourised.
• SO₂: Hold damp potassium dichromate(VI) paper in gas → paper turns orange to green (Cr³⁺ formed). Note: SO₂ also decolourises acidified aqueous KMnO₄.
• Cl₂: Hold damp litmus paper in gas → paper is bleached / decolourised.
• SO₂: Hold damp potassium dichromate(VI) paper in gas → paper turns orange to green (Cr³⁺ formed). Note: SO₂ also decolourises acidified aqueous KMnO₄.
4 — Tests for elements
Element
Test
Positive result
I₂
Add starch solution to the sample
Solution turns blue-black
NH₃ (alkaline)
H₂ (pop)
O₂ (oxidising)
I₂ (starch test)