Qualitative Analysis: Tests for IonsEdexcel GCSE Chemistry: Revision notes
Section 1
Why must a test for an ion be unique?
The test for any ion must be unique — meaning a positive result must only ever be produced by that specific ion, and no other ion should give the same result. If two different ions gave the same test result, the test would be unreliable and could not be used to identify which ion is actually present.
This is why, if a test result is ambiguous, a second confirmatory test is often needed before an ion can be positively identified.
Section 2
How do flame tests identify metal ions in solids?
A flame test identifies certain metal ions in solid compounds by the colour produced when the solid is heated in a flame:
| Ion | Flame colour |
|---|---|
| Lithium, Li⁺ | Red |
| Sodium, Na⁺ | Yellow |
| Potassium, K⁺ | Lilac |
| Calcium, Ca²⁺ | Orange-red |
| Copper, Cu²⁺ | Blue-green |
Method: a clean wire loop (often dipped in dilute hydrochloric acid to clean it first) is used to hold a small sample of the solid in a roaring blue Bunsen flame; the colour of the flame is observed.
Students frequently swap lithium (red) and calcium (orange-red) — learn these as a pair since they are easily confused.
Section 3
How does sodium hydroxide solution identify metal ions?
Adding sodium hydroxide solution to a solution (or dissolved solid) containing certain metal ions produces a coloured precipitate (an insoluble hydroxide), or a gas in the case of ammonium ions:
| Ion | Result with NaOH |
|---|---|
| Aluminium, Al³⁺ | White precipitate, which dissolves in excess NaOH to give a colourless solution |
| Calcium, Ca²⁺ | White precipitate, insoluble in excess NaOH |
| Copper, Cu²⁺ | Blue precipitate |
| Iron(II), Fe²⁺ | Green precipitate |
| Iron(III), Fe³⁺ | Orange-brown/red-brown precipitate |
| Ammonium, NH₄⁺ | No precipitate; ammonia gas released on warming |
Aluminium and calcium both give a white precipitate, so excess NaOH is the confirmatory step that distinguishes them — aluminium hydroxide dissolves in excess, calcium hydroxide does not.
Test for ammonia gas: damp red litmus paper held near the mouth of the test tube turns blue, confirming ammonia (an alkaline gas) is present.
Always mention 'excess NaOH' when distinguishing aluminium from calcium — this is the specific marking point examiners look for.
Section 4
How are carbonate, sulfate and halide ions tested?
Carbonate ion (CO₃²⁻): add dilute acid; if carbonate is present, carbon dioxide gas is released, which turns limewater cloudy/milky.
Sulfate ion (SO₄²⁻): add dilute hydrochloric acid, then barium chloride solution; a white precipitate (barium sulfate) forms if sulfate is present. The dilute HCl is added first to remove any carbonate ions that could also give a false white precipitate with barium chloride.
Halide ions (Cl⁻, Br⁻, I⁻): add dilute nitric acid, then silver nitrate solution:
| Halide ion | Precipitate colour |
|---|---|
| Chloride, Cl⁻ | White |
| Bromide, Br⁻ | Cream |
| Iodide, I⁻ | Yellow |
Dilute nitric acid is added first to remove carbonate ions, which would otherwise also give a false precipitate with silver nitrate.
Always state WHY dilute acid is added before barium chloride/silver nitrate — to remove carbonate ions that would otherwise give a false-positive precipitate — this is a common lost mark.
Section 5
How can unknown salts be identified, and what are instrumental methods?
Core Practical: an unknown salt can be identified by systematically applying the cation tests (flame test or NaOH) and the anion tests (carbonate, sulfate, halide) described above, then combining the results — e.g. a white precipitate with NaOH that dissolves in excess (aluminium) plus a white precipitate with acidified barium chloride (sulfate) identifies the compound as aluminium sulfate.
Instrumental methods of analysis (using machines rather than manual wet-chemistry tests) are also available. These may improve the sensitivity, accuracy and speed of tests compared with traditional methods.
Flame photometry is one such instrumental method: it can be used to identify metal ions and determine their concentration in a dilute solution by comparing readings against a calibration curve built from known concentrations, or by comparing the emission data with reference data for known ions.
Must Know
- A test for an ion must be unique — only that ion gives that specific result
- Flame test colours: Li+ red, Na+ yellow, K+ lilac, Ca2+ orange-red, Cu2+ blue-green
- NaOH precipitate colours: Al3+ white (dissolves in excess), Ca2+ white (insoluble in excess), Cu2+ blue, Fe2+ green, Fe3+ orange-brown; NH4+ releases ammonia gas (turns damp red litmus blue)
- Carbonate test: dilute acid produces CO2 that turns limewater cloudy
- Sulfate test: dilute HCl then barium chloride gives a white precipitate
- Halide test: dilute nitric acid then silver nitrate — Cl- white, Br- cream, I- yellow
- Instrumental methods (e.g. flame photometry) improve sensitivity, accuracy and speed, using calibration curves or reference data
That's the notes covered.
Carry on to the next subtopic.