1. Tests for Gases
| Gas | Test | Positive result |
|---|---|---|
| Hydrogen (H₂) | Hold burning splint at mouth of tube | Squeaky pop |
| Oxygen (O₂) | Hold glowing splint into gas | Splint relights |
| Carbon dioxide (CO₂) | Bubble through limewater | Limewater turns milky/cloudy |
| Ammonia (NH₃) | Hold damp red litmus paper at mouth | Litmus turns blue |
| Chlorine (Cl₂) | Hold damp blue litmus paper at mouth | Litmus turns red, then bleaches white |
| Sulfur dioxide (SO₂) | Bubble through acidified potassium dichromate solution | Orange → green |
The result must be "turns milky" or "turns cloudy" — not "turns white". The cloudiness is due to fine particles of CaCO₃ forming in suspension.
2. Tests for Cations (Positive Ions)
Using sodium hydroxide solution (NaOH)
| Ion | Observation with NaOH |
|---|---|
| Cu²⁺ | Blue precipitate (insoluble in excess NaOH) |
| Fe²⁺ | Green precipitate (insoluble in excess) |
| Fe³⁺ | Red-brown precipitate (insoluble in excess) |
| Al³⁺ | White precipitate (dissolves in excess NaOH — amphoteric) |
| Zn²⁺ | White precipitate (dissolves in excess NaOH) |
| Ca²⁺ | White precipitate (insoluble in excess) |
| Mg²⁺ | White precipitate (insoluble in excess) |
| NH₄⁺ | No precipitate; on warming, ammonia gas produced (turns damp red litmus blue) |
All four give white precipitates with NaOH. Al³⁺ and Zn²⁺ dissolve in excess NaOH (amphoteric). Ca²⁺ and Mg²⁺ do not. This is the key distinguishing step.
3. Tests for Anions (Negative Ions)
| Ion | Reagent added | Positive result |
|---|---|---|
| Cl⁻ (chloride) | Acidified silver nitrate (AgNO₃) | White precipitate of AgCl (insoluble in dilute HNO₃) |
| Br⁻ (bromide) | Acidified silver nitrate | Cream precipitate of AgBr |
| I⁻ (iodide) | Acidified silver nitrate | Yellow precipitate of AgI |
| SO₄²⁻ (sulfate) | Acidified barium chloride (BaCl₂) | White precipitate of BaSO₄ (insoluble in dilute HCl) |
| CO₃²⁻ (carbonate) | Dilute acid | Effervescence; CO₂ gas turns limewater milky |
| NO₃⁻ (nitrate) | NaOH + aluminium foil, warm | Ammonia gas produced (turns damp red litmus blue) |
Add dilute nitric acid before AgNO₃ (to remove carbonate/sulfate ions that would give false precipitates). Add dilute hydrochloric acid before BaCl₂ (to remove carbonate ions). Without acidification, the test is unreliable.
4. Flame Tests
Clean a platinum or nichrome wire by dipping in concentrated HCl and holding in a Bunsen flame until no colour appears. Dip in the test solution and hold in the flame.
| Ion | Flame colour |
|---|---|
| Lithium (Li⁺) | Crimson red |
| Sodium (Na⁺) | Persistent yellow-orange |
| Potassium (K⁺) | Lilac (pale purple) |
| Calcium (Ca²⁺) | Brick red |
| Copper (Cu²⁺) | Blue-green |
| Barium (Ba²⁺) | Apple green |
Even trace amounts of sodium give a strong yellow-orange flame that can mask other colours. If sodium is suspected, use a blue cobalt glass to filter out the yellow — other colours may then be visible.
- Gas tests: H2 - squeaky pop. O2 - relights glowing splint. CO2 - limewater turns milky. Cl2 - bleaches damp litmus. NH3 - turns damp red litmus blue.
- Flame tests: Li = crimson. Na = yellow (most intense). K = lilac. Ca = brick red. Cu = green.
- NaOH (aq) for cation identification: Cu2+ = blue ppt. Fe2+ = green ppt. Fe3+ = brown ppt. Al3+/Zn2+ = white ppt (dissolves in excess NaOH). NH4+: gas turns damp red litmus blue on warming.
- Anion tests: Cl- (add HNO3 + AgNO3): white ppt. Br- : cream ppt. I-: yellow ppt. SO42- (add HCl + BaCl2): white ppt. CO32- (add HCl): CO2 gas.
- Always add dilute acid first to anion tests to remove carbonate interference.
- Ammonia gas: pungent smell, turns damp red litmus blue.
5. Common Exam Traps
"A white precipitate forms" is an observation. "Sulfate ions are present" is a conclusion. Questions asking for observations want the physical description — colour of precipitate, gas produced, colour change — not the ion name.
Chlorine first turns damp blue litmus red (acidic), then bleaches it white. Both steps must be stated for full marks.
Simply adding NaOH to an ammonium salt at room temperature gives little observable result. You must warm the mixture to drive off ammonia gas — then test with damp red litmus.
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Original study notes for Singapore students. Not affiliated with MOE, SEAB or Cambridge.