Revision notes · Chemical analysis
Identification of ions by chemical and spectroscopic means
Flame tests4.8.3.1
Definition: A flame test is an analytical technique used to identify metal ions based on the characteristic colour of light they emit when heated in a flame.
Flame tests are used to identify specific metal cations by heating a sample in a Bunsen burner flame.
| Metal ion | Flame colour |
|---|---|
| Lithium | Crimson |
| Sodium | Yellow |
| Potassium | Lilac |
| Calcium | Orange-red |
| Copper | Green |
- •If a sample contains a mixture of metal ions, some flame colours can be masked by others.
⚠️ Common mistake: Confusing the flame colours of calcium (orange-red) and lithium (crimson) or writing 'red' without specifying orange-red or crimson.
🧠 Remember: Li-Crimson, Na-Yellow, K-Lilac, Ca-Orange-red, Cu-Green.
Metal hydroxides4.8.3.2
Definition: A metal hydroxide precipitate test involves adding sodium hydroxide solution to an unknown solution to identify metal cations by the colour of the solid formed.
Sodium hydroxide solution reacts with dissolved metal ions to form insoluble metal hydroxide precipitates.
| Metal ion | Precipitate colour | Dissolves in excess NaOH? |
|---|---|---|
| Aluminium | White | Yes — forms a colourless solution |
| Calcium | White | No |
| Magnesium | White | No |
| Copper(II) | Blue | No |
| Iron(II) | Green | No |
| Iron(III) | Brown | No |
- •The ionic equation for copper hydroxide formation is Cu²⁺(aq) + 2OH⁻(aq) → Cu(OH)₂(s).
⚠️ Common mistake: Forgetting that aluminium hydroxide redissolves in excess sodium hydroxide, which is the key test to distinguish it from calcium and magnesium.
🧠 Remember: Blue Cu, Green Fe2+, Brown Fe3+, White Al/Ca/Mg (only Al redissolves).
Carbonates4.8.3.3
Definition: A carbonate test detects carbonate ions in a compound by reacting them with dilute acid to generate carbon dioxide gas.
Carbonate ions react with dilute acids to produce effervescence.
- •Adding dilute acid to a compound containing carbonate ions causes effervescence (fizzing).
- •The gas produced is carbon dioxide, which turns limewater cloudy when bubbled through it.
- •The general equation is: carbonate + acid → salt + water + carbon dioxide.
- •The ionic equation is CO₃²⁻(aq) + 2H⁺(aq) → CO₂(g) + H₂O(l).
⚠️ Common mistake: Writing that limewater turns 'white' instead of 'cloudy' or 'milky' when carbon dioxide is present.
🧠 Remember: Acid + Carbonate = CO₂ bubbles that turn limewater cloudy.
Halides4.8.3.4
Definition: A halide test identifies chloride, bromide, or iodide ions in solution using dilute nitric acid followed by silver nitrate solution.
Halide ions form distinctly coloured silver halide precipitates when mixed with silver nitrate.
- •The solution is first acidified with dilute nitric acid to remove any carbonate impurities.
- •Adding silver nitrate solution produces insoluble silver halide precipitates.
| Halide ion | AgNO₃ precipitate colour |
|---|---|
| Chloride | White |
| Bromide | Cream |
| Iodide | Yellow |
⚠️ Common mistake: Acidifying the sample with hydrochloric acid instead of nitric acid, which introduces chloride ions and causes a false positive.
🧠 Remember: Milk, Cream, Butter: Chloride (white), Bromide (cream), Iodide (yellow).
Sulfates4.8.3.5
Definition: A sulfate test identifies sulfate ions in solution using dilute hydrochloric acid followed by barium chloride solution.
Sulfate ions react with barium ions to form an insoluble white precipitate.
- •The test solution is first acidified with dilute hydrochloric acid to react with and remove any carbonate ions.
- •Adding barium chloride solution produces a dense white precipitate of barium sulfate if sulfate ions are present.
- •The ionic equation is Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s).
- •Sulfuric acid cannot be used to acidify the solution because it contains sulfate ions.
⚠️ Common mistake: Acidifying the solution with sulfuric acid, which adds sulfate ions to the test and guarantees a false positive.
🧠 Remember: HCl then Barium Chloride = White precipitate for Sulfate.
Instrumental methods4.8.3.6
Definition: An instrumental method is an automated analytical technique that uses specialized machinery to identify and quantify chemical substances.
Modern industrial chemical analysis relies heavily on automated instrumental methods rather than manual wet chemical tests.
- •Instrumental methods are highly accurate, sensitive, and rapid.
- •They can operate effectively on very small samples.
- •Main drawbacks include the high cost of purchasing equipment and the requirement for specialised training.
- •Examples of instrumental methods include flame emission spectroscopy, gas chromatography, and mass spectrometry.
⚠️ Common mistake: Claiming that instrumental methods are cheap or easy to use without training.
🧠 Remember: Instruments are Fast, Accurate, and Sensitive, but Expensive.
Flame emission spectroscopy4.8.3.7
Definition: Flame emission spectroscopy is an instrumental technique used to analyze metal ions in solution by measuring the light spectrum emitted when heated in a flame.
Flame emission spectroscopy identifies metal ions and determines their exact concentration in a sample.
- •The sample is passed into a flame, exciting the metal ions so they emit light of specific wavelengths.
- •The emitted light is passed through a spectrometer to produce a distinctive line spectrum.
- •Each metal ion has a unique line spectrum, allowing individual ions to be identified even in mixtures.
- •The intensity of the light emitted is directly proportional to the concentration of the metal ion in solution.
⚠️ Common mistake: Believing flame emission spectroscopy cannot identify ions in mixtures; unlike simple flame tests, its unique spectral lines easily resolve mixtures.
🧠 Remember: Line spectrum pattern = Ion ID; Line intensity = Concentration.
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