9.7C - Interpreting ion-test results
Use observations from ion tests to identify the cation and anion in a simple unknown salt. You will combine supported results, balance charges when writing a formula, and recognise when the evidence does not justify a definite conclusion.
Reading the evidence
A salt contains positive ions and negative ions. The positive ion is the cation. The negative ion is the anion. In this lesson, an unknown salt is identified by matching the given test results to the known results from the earlier ion-test lessons.
Qualitative analysis
Qualitative analysis is identifying what substance or ion is present from observations, rather than measuring how much is present.
For 9.7C, the important skill is not writing a long method. It is reading a result and deciding what that result proves.
| Given result | Strong deduction style | Weak deduction style |
|---|---|---|
| "blue-green flame" | copper ion, Cu2+, is present | "copper is there" |
| "white precipitate with silver nitrate after nitric acid" | chloride ion, Cl-, is present | "chlorine is present" |
| "carbon dioxide turns limewater cloudy" | carbonate ion, CO3^2-, is present in the salt tested with acid | "a gas is present" |
The word ion matters. A chloride ion, Cl-, is not chlorine, Cl2. A bromide ion, Br-, is not bromine, Br2. Give a correct ion name or formula unless both are requested.
The safest routine is:
result -> ion identified -> salt name or formula, if asked
For the simple salt deductions, assume one cation type and one anion type from the taught list unless the question says otherwise.
Cation and anion reference results
Cations are positive ions. In this topic, cation evidence usually comes from flame-test colours or from observations with sodium hydroxide solution.
For a flame-test result, match the flame colour to the metal ion.
| Flame-test result | Ion identified |
|---|---|
| red flame | lithium ion, Li+ |
| yellow flame | sodium ion, Na+ |
| lilac flame | potassium ion, K+ |
| orange-red flame | calcium ion, Ca2+ |
| blue-green flame | copper ion, Cu2+ |
For sodium hydroxide results, match the precipitate or gas result to the cation.
| Sodium hydroxide result | Ion identified |
|---|---|
| white precipitate that dissolves in excess sodium hydroxide | aluminium ion, Al3+ |
| white precipitate that does not dissolve in excess sodium hydroxide | calcium ion, Ca2+ |
| blue precipitate | copper ion, Cu2+ |
| green precipitate | iron(II) ion, Fe2+ |
| brown precipitate | iron(III) ion, Fe3+ |
| ammonia gas forms when warmed; damp red litmus paper turns blue | ammonium ion, NH4+ |
If two cation results point to the same ion, that is strong evidence. For example, an orange-red flame and a white precipitate that does not dissolve in excess sodium hydroxide both support calcium ion, Ca2+.
If two results do not fit one cation, do not force them into one neat answer. Say what each result suggests and whether the data may show contamination, a mixture, or an invalid test.
Anion evidence
Anions are negative ions. In the 9.7C style of question, the result table often gives one or more tests for carbonate, sulfate and halide ions.
| Result supplied | Ion identified |
|---|---|
| dilute acid causes bubbling, and the gas turns limewater cloudy | carbonate ion, CO3^2- |
| after dilute hydrochloric acid and barium chloride solution, a white precipitate forms | sulfate ion, SO4^2- |
| after dilute nitric acid and silver nitrate solution, a white precipitate forms | chloride ion, Cl- |
| after dilute nitric acid and silver nitrate solution, a cream precipitate forms | bromide ion, Br- |
| after dilute nitric acid and silver nitrate solution, a yellow precipitate forms | iodide ion, I- |
Ammonia is a gas, not an anion. However, it is still part of this deduction topic because ammonium ions are identified by producing ammonia gas when warmed with sodium hydroxide solution. The positive result for ammonia gas is that damp red litmus paper turns blue.
Negative results can also help, but only inside the test boundary. If there is no bubbling with dilute acid, the carbonate ion has not been shown. If there is no white precipitate in the sulfate test, the sulfate ion has not been shown. If there is no precipitate in a valid halide test, chloride, bromide and iodide have not been shown.
Do not overclaim from a negative result. "No precipitate with silver nitrate" does not prove that the sample contains no anions at all. It only means that the specified halide ions were not detected by that valid halide test.
Combining ions and balancing charges
To identify an unknown salt, first identify the cation and the anion separately. Then combine the names:
cation name + anion name -> salt name
Examples:
| Cation evidence | Anion evidence | Ions identified | Salt identified |
|---|---|---|---|
| yellow flame | white precipitate in the sulfate test | Na+ and SO4^2- | sodium sulfate |
| blue precipitate with sodium hydroxide | white precipitate after nitric acid and silver nitrate | Cu2+ and Cl- | copper(II) chloride |
| ammonia gas when warmed with sodium hydroxide | cream precipitate in the halide test | NH4+ and Br- | ammonium bromide |
| brown precipitate with sodium hydroxide | white precipitate in the halide test | Fe3+ and Cl- | iron(III) chloride |
If a question asks for the formula of the salt, balance the ion charges so the compound is neutral.
Continue this lesson.
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