Chemical Tests for Cations and Anions - Comprehensive guide based on the IGCSE syllabus
This content covers Section 9.2: Qualitative Analysis from the IGCSE Coordinated Sciences (0654) syllabus, focusing on tests for cations (positive ions) and anions (negative ions) using flame tests, sodium hydroxide, and other reagents.
Cations are positively charged ions. Two main tests are used: flame tests and sodium hydroxide tests.
Clean a nichrome or platinum wire loop by dipping in concentrated HCl and heating until no color is observed. Then dip in the sample and heat in a Bunsen flame.
| Cation | Flame Color | Observation |
|---|---|---|
| Lithium (Li⁺) | Crimson red | Bright crimson-red flame |
| Sodium (Na⁺) | Yellow-orange | Persistent yellow-orange flame |
| Potassium (K⁺) | Lilac | Lilac (pale purple) flame, often masked by sodium |
| Calcium (Ca²⁺) | Brick red | Brick-red/orange-red flame |
| Copper (Cu²⁺) | Blue-green | Blue-green with green flashes |
Always wear safety goggles during flame tests. The wire loop should be cleaned between tests to avoid contamination of results.
Add a few drops of sodium hydroxide (NaOH) solution to the unknown solution and observe any precipitate formed.
| Cation | Observation with NaOH | Precipitate Color |
|---|---|---|
| Aluminium (Al³⁺) | White precipitate, soluble in excess NaOH | White |
| Calcium (Ca²⁺) | White precipitate, insoluble in excess | White |
| Copper(II) (Cu²⁺) | Blue precipitate, insoluble in excess | Blue |
| Iron(II) (Fe²⁺) | Green precipitate, insoluble in excess | Green |
| Iron(III) (Fe³⁺) | Brown precipitate, insoluble in excess | Brown |
| Zinc (Zn²⁺) | White precipitate, soluble in excess NaOH | White |
Anions are negatively charged ions. Different tests are used depending on the anion suspected.
| Anion | Test | Observation |
|---|---|---|
| Carbonate (CO₃²⁻) | Add dilute acid | Effervescence (bubbles) of carbon dioxide gas which turns limewater milky |
| Chloride (Cl⁻) | Acidify with dilute HNO₃, then add AgNO₃ | White precipitate of silver chloride (soluble in dilute NH₃) |
| Bromide (Br⁻) | Acidify with dilute HNO₃, then add AgNO₃ | Cream precipitate of silver bromide (partially soluble in dilute NH₃) |
| Iodide (I⁻) | Acidify with dilute HNO₃, then add AgNO₃ | Yellow precipitate of silver iodide (insoluble in dilute NH₃) |
| Nitrate (NO₃⁻) | Add NaOH and aluminium foil, warm gently | Ammonia gas produced (turns damp red litmus paper blue) |
| Sulfate (SO₄²⁻) | Acidify with dilute HCl, then add BaCl₂ | White precipitate of barium sulfate |
| Sulfite (SO₃²⁻) | Add dilute HCl, warm gently | Sulfur dioxide gas produced (turns acidified K₂Cr₂O₇ from orange to green) |
When testing for carbonates with acid, be cautious of the gas produced. For nitrate tests with aluminium, do not heat strongly as hydrogen gas is produced which is flammable.
Some anion tests produce gases that can be identified:
| Gas | Test | Observation |
|---|---|---|
| Carbon dioxide (CO₂) | Bubble through limewater | Turns limewater (calcium hydroxide) milky/cloudy |
| Ammonia (NH₃) | Damp red litmus paper | Turns damp red litmus paper blue |
| Chlorine (Cl₂) | Damp blue litmus paper | Bleaches damp blue litmus paper (turns it white) |
| Hydrogen (H₂) | Lighted splint | Burns with a 'pop' sound |
| Oxygen (O₂) | Glowing splint | Relights a glowing splint |
| Sulfur dioxide (SO₂) | Acidified potassium dichromate(VI) | Turns from orange to green |