Qualitative analysis of ions is a set of chemical tests used to identify which ions are present in an unknown solution. It matters because many ions are colorless in water, so chemists use reactions that create visible changes such as precipitates, color changes, or gas formation. In classic cation analysis, ions are separated into groups by adding reagents in a careful order.
Each step narrows the possibilities until a confirmatory test identifies a specific ion.
Key Facts
- Selective precipitation separates ions because different ionic compounds have different solubilities.
- A precipitate forms when the ion product exceeds Ksp for an insoluble salt.
- For a salt AB, Ksp = [A+][B-] at equilibrium in a saturated solution.
- Group I cations such as Ag+, Pb2+, and Hg2 2+ form insoluble chlorides with dilute HCl.
- Group II cations often form insoluble sulfides in acidic solution when H2S supplies S2- at low concentration.
- Confirmatory tests should be performed on separated fractions, not on the original mixture.
Vocabulary
- Qualitative analysis
- A laboratory method used to identify the substances present in a sample without measuring their exact amounts.
- Selective precipitation
- The process of causing some ions to form an insoluble solid while other ions remain dissolved.
- Precipitate
- An insoluble solid that forms when ions in solution react to make a compound with very low solubility.
- Confirmatory test
- A specific follow-up reaction that provides strong evidence for the identity of one ion.
- Solubility product
- The equilibrium constant Ksp that describes how much of a slightly soluble ionic compound dissolves in water.
Common Mistakes to Avoid
- Adding reagents in the wrong order. This is wrong because the separation scheme depends on removing ion groups step by step before later reagents are added.
- Calling any cloudy mixture a positive test. This is wrong because cloudiness can come from contamination, incomplete mixing, or pH effects, so the expected color and reagent conditions must match the test.
- Skipping filtration before the next step. This is wrong because leftover precipitate can react with later reagents and give false positives.
- Using confirmatory tests on the original unknown solution. This is wrong because several ions can interfere with the same color change or precipitate unless the target ion has first been separated.
Practice Questions
- 1 A solution contains Ag+ and Na+. Dilute HCl is added and a white precipitate forms. Write the net ionic equation for the precipitation reaction and name the precipitate.
- 2 For AgCl, Ksp = 1.8 x 10^-10. If [Ag+] = 0.010 M, what minimum [Cl-] is needed to begin precipitation of AgCl?
- 3 An unknown cation mixture gives a white precipitate with dilute HCl. After filtration, the precipitate dissolves in aqueous NH3 and reappears when dilute HNO3 is added. Explain which ion is indicated and why these steps are more convincing than the first precipitate alone.