Identification of ions and gases
Section: Chemistry | Syllabus: Cambridge IGCSE Combined Science (0653)
Tests for Anions
An anion is a negatively charged ion. Each anion below is identified by a specific chemical test.
| Anion | Test | Positive Result |
|---|---|---|
| Carbonate, CO₃²⁻ | React with dilute acid, then test the gas produced | Effervescence; gas turns limewater milky |
| Chloride, Cl⁻ Bromide, Br⁻ Iodide, I⁻ | Acidify with dilute nitric acid, then add aqueous silver nitrate | Cl⁻: white precipitate Br⁻: cream precipitate I⁻: (pale) yellow precipitate |
| Sulfate, SO₄²⁻ | Acidify with dilute nitric acid, then add aqueous barium nitrate | White precipitate |
Dilute hydrochloric acid or sulfuric acid must never be used to acidify the halide test - they would introduce extra chloride or sulfate ions and give a false result. Dilute nitric acid is used because it introduces no ions that would interfere.
Worked Example
- Question (based on a real exam question): describe a test to identify the iodide ions in potassium iodide. [2]
- Answer: acidify with dilute nitric acid, then add aqueous silver nitrate [1]; a (pale) yellow precipitate of silver iodide confirms iodide ions [1]
Common Mistakes
- ❌ Using dilute hydrochloric acid to acidify before the silver nitrate (halide) test
- ✓ HCl itself contains chloride ions, which would react with the silver nitrate and give a false positive - always use dilute nitric acid instead
Testing Cations with Sodium Hydroxide and Ammonia
A cation is a positively charged ion. Metal cations are identified by adding aqueous sodium hydroxide (NaOH) and aqueous ammonia (NH₃) separately, a few drops at a time and then in excess, and observing the colour of any precipitate and whether it dissolves in excess. (Formulas of complex ions are not required.)
| Cation | + NaOH, dropwise | + NaOH, excess | + NH₃(aq), dropwise | + NH₃(aq), excess |
|---|---|---|---|---|
| Calcium, Ca²⁺ | White ppt | Insoluble - stays white ppt | No precipitate | No precipitate |
| Copper(II), Cu²⁺ | Light blue ppt | Insoluble - stays light blue ppt | Light blue ppt | Dissolves - dark/deep blue solution |
| Iron(II), Fe²⁺ | Green ppt | Insoluble - stays green ppt | Green ppt | Insoluble - stays green ppt |
| Iron(III), Fe³⁺ | Red-brown ppt | Insoluble - stays red-brown ppt | Red-brown ppt | Insoluble - stays red-brown ppt |
| Zinc, Zn²⁺ | White ppt | Dissolves - colourless solution | White ppt | Dissolves - colourless solution |
The ammonium ion, NH₄⁺, is different: it gives no precipitate with either reagent. Instead, warming solid ammonium compounds with aqueous sodium hydroxide releases ammonia gas, identified by its pungent smell and by turning damp red litmus paper blue.
Worked Example
- Question (based on a real exam question): aqueous sodium hydroxide is added to solid X and the mixture is heated. A green precipitate forms and an alkaline gas is given off. Which ions are present in X? [2]
- Answer: a green precipitate identifies Fe²⁺ [1] (Fe³⁺ would give red-brown); the alkaline gas is ammonia, released when NH₄⁺ reacts with NaOH on heating [1]
Common Mistakes
- ❌ Mixing up the precipitate colours for Fe²⁺ and Fe³⁺
- ✓ iron(II) gives a green precipitate; iron(III) gives a red-brown precipitate
- ❌ Forgetting that zinc's precipitate dissolves in excess of both NaOH and ammonia
- ✓ zinc hydroxide is unusual in dissolving back to a colourless solution in excess of either reagent - calcium, copper(II), iron(II) and iron(III) precipitates do not dissolve in excess NaOH
Tests for Gases
| Gas | Test | Positive Result |
|---|---|---|
| Ammonia, NH₃ | Hold damp red litmus paper in the gas | Turns blue |
| Carbon dioxide, CO₂ | Bubble through limewater | Limewater turns milky |
| Chlorine, Cl₂ | Hold damp litmus paper in the gas | Litmus is bleached (turns white) |
| Hydrogen, H₂ | Hold a lighted splint at the mouth of the tube | Burns with a squeaky pop |
| Oxygen, O₂ | Hold a glowing splint in the gas | Relights the glowing splint |
Ammonia is alkaline: it turns damp red litmus paper blue, and has a sharp, pungent smell. Chlorine is a pale yellow-green gas that acts as a bleach: damp litmus paper placed in chlorine is bleached white.
Worked Example
- Question (based on a real exam question): a gas produced in a reaction turns limewater milky. Identify the gas. [1]
- Answer: carbon dioxide, CO₂ - this is the standard confirmatory test for the gas.
Common Mistakes
- ❌ Using a dry piece of litmus paper for the chlorine or ammonia test
- ✓ the paper must be damp - both tests rely on the gas first dissolving in the water on the paper
- ❌ Using a flaming splint to test for oxygen
- ✓ the splint must only be glowing (blown out first) - a still-flaming splint already has enough oxygen from the air, so relighting cannot be observed
Flame Tests
Some metal cations can be identified by the colour they give a Bunsen flame. A clean wire is dipped in dilute hydrochloric acid then into the sample, and held in the edge of a roaring (blue) flame.
| Cation | Flame Colour |
|---|---|
| Lithium, Li⁺ | Red |
| Sodium, Na⁺ | Yellow |
| Potassium, K⁺ | Lilac |
| Copper(II), Cu²⁺ | Blue-green |
Worked Example
- Question: a student carries out a flame test on an unknown aqueous metal salt and sees a lilac flame colour. Which cation is present? [1]
- Answer: potassium, K⁺
Common Mistakes
- ❌ Confusing sodium's yellow flame with potassium's lilac flame
- ✓ sodium gives a persistent yellow flame; potassium gives a lilac flame (sometimes briefly masked by any sodium impurity, viewed through blue cobalt glass to filter out yellow)
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