A White Solid: When the Chloride Test Proves Nothing
A white solid was recovered from a laboratory bench. A student carried out the following tests.
(1) A flame test on the solid produced a lilac flame. (2) A portion of the solid was added to dilute hydrochloric acid. Effervescence occurred, and the gas turned limewater milky. (3) Another portion of the solid was dissolved in distilled water. Dilute hydrochloric acid was added until effervescence stopped. Silver nitrate solution was then added to the resulting solution, giving a white precipitate. The precipitate dissolved in dilute ammonia solution.
The student concluded that the solid was a mixture of potassium chloride and potassium carbonate. The teacher said that conclusion (3) was not justified. The solid may be pure potassium carbonate, pure potassium chloride, or a mixture of the two.
1. Explain why the silver nitrate test as carried out cannot prove that chloride ions were present in the original solid.
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In test (3), dilute hydrochloric acid was added to the solid before silver nitrate. That acid contains chloride ions, so it introduces Cl- into the solution. When silver nitrate is added, any white precipitate of AgCl could come entirely from the chloride added in the acid, even if the original solid contained no chloride at all. The test therefore gives a positive result for pure potassium carbonate as well as for a mixture containing potassium chloride. To test for chloride in the original solid, the solid should be dissolved in distilled water and acidified with dilute nitric acid, not hydrochloric acid, to remove carbonate interference. Silver nitrate is then added to a portion of that solution. A white precipitate that dissolves in dilute ammonia indicates chloride; the nitric acid does not introduce Cl-.
2. Write the ionic equation for the reaction that produced the gas in test (2), and for the reaction that turned limewater milky. State what each observation shows about the solid.
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The gas-producing reaction is CO3^2-(aq) + 2H+(aq) → CO2(g) + H2O(l). The limewater reaction is CO2(g) + Ca(OH)2(aq) → CaCO3(s) + H2O(l); the white precipitate of calcium carbonate makes the limewater milky. Effervescence with dilute acid and a gas that turns limewater milky are characteristic of carbonate, so the solid contains carbonate ions. The lilac flame in test (1) is characteristic of potassium ions. Together, tests (1) and (2) show that the solid contains potassium and carbonate, but they do not establish whether chloride is also present.
3. Describe how the student could modify the tests to decide whether the original solid contains potassium chloride as well as potassium carbonate. State the expected observations for pure potassium carbonate and for a mixture.
Read the worked answer
Dissolve a fresh portion of the original solid in distilled water. Add dilute nitric acid in excess until no more effervescence occurs, so that carbonate is destroyed and carbon dioxide is expelled. Warm gently to remove dissolved CO2, cool, and then add silver nitrate solution. For pure potassium carbonate, the acidified solution contains potassium nitrate and no chloride; adding silver nitrate gives no white precipitate of AgCl (or at most a transient cloudiness that does not persist). For a mixture containing potassium chloride, a white precipitate of AgCl forms and dissolves in dilute ammonia solution. Nitric acid must be used rather than hydrochloric acid because hydrochloric acid would itself introduce chloride ions and invalidate the test.