Chemistry • Year 12 • Module 8 • Lesson 3
Precipitation Reactions & Qualitative Analysis
Apply the qualitative test framework to real data tables, a graph, and an Australian environmental monitoring scenario.
1. Interpret qualitative test results for unknown solutions
A NSW EPA water-quality chemist tested four separate unknown solutions (A–D) collected from runoff near a former Broken Hill lead-silver-zinc mine leachate site. The table shows which reagent was added and the observed result. Some cells are marked with a dash (—) to indicate the test was not performed on that sample. 9 marks
| Test reagent | Observation (A) | Observation (B) | Observation (C) | Observation (D) |
|---|---|---|---|---|
| AgNO3(aq) | White precipitate | No precipitate | White precipitate | No precipitate |
| BaCl2(aq) | No precipitate | White precipitate | No precipitate | White precipitate |
| NaOH(aq) | Pale blue precipitate | Red-brown precipitate | No precipitate | Pale blue precipitate |
| Dilute HCl(aq) | No gas | No gas | Effervescence | No gas |
| Flame test | Blue-green | — | Yellow | Blue-green |
1.1 For solutions A and B, identify the most likely cation and anion present. Justify each identification by referring to a specific observation in the table. 4 marks
1.2 Write the net ionic equation for the reaction that identifies the anion in solution C. State what observation confirms this identification. 2 marks
1.3 The flame test for solution C shows yellow. A colleague concludes “sodium ion is definitely present.” Evaluate this conclusion. 3 marks
2. Interpret graph — precipitation selectivity and sulfate interference
The graph below is based on laboratory data for the mass of precipitate formed when varying volumes of BaCl2(aq) (0.10 mol L−1) are added to 20.0 mL of a solution containing both SO42−(aq) and CO32−(aq) at different pH conditions. At pH 2 (acidified with HCl), only BaSO4 precipitates; at pH 7 (neutral), both BaSO4 and BaCO3 precipitate. 8 marks
Figure 2.1 — Adapted from laboratory calibration data, NSW EPA Environmental Analytical Method EAM-2b (2022). Conditions: 20.0 mL sample, 0.10 mol L−1 BaCl2(aq) titrant, 25 °C.
2.1 Describe the trend shown in the pH 2 curve from 0 to 20 mL BaCl2 added. Estimate the volume at which the curve plateaus. 2 marks
2.2 Explain, using solubility rules and the “add acid first” rule, why acidifying the sample to pH 2 before adding BaCl2(aq) eliminates the interference from CO32−. Write the relevant net ionic equation as part of your answer. 3 marks
2.3 At pH 7, the plateau mass is approximately double that at pH 2. Using your knowledge of the chemistry involved, explain this observation. 3 marks
3. Case study — AFSL forensic analysis
7 marks
Context. The Australian Federal Scientific Laboratory (AFSL) analysed a white crystalline powder seized at a Sydney port in May 2024. The powder was dissolved in distilled water and tested in sequence. Adding AgNO3(aq) produced a white precipitate. Adding NaOH(aq) to a fresh portion produced no precipitate. Adding BaCl2(aq) to another portion produced no precipitate. A flame test gave a bright yellow colour.
3.1 Use the sequence of test results to identify the most likely anion and cation present in the powder. For each identification, state the specific observation that supports it and write the net ionic equation. 4 marks
3.2 The NaOH(aq) test produced no precipitate, and the BaCl2(aq) test produced no precipitate. Explain what each of these negative results tells you about which cations and anions are absent from the powder. 3 marks
Q1.1 — Ion identification for A and B
Solution A: Cation = Cu2+ (pale blue precipitate Cu(OH)2 with NaOH, supported by blue-green flame); Anion = Cl− (white precipitate AgCl with AgNO3; no precipitate with BaCl2 rules out SO42−).
Solution B: Cation = Fe3+ (red-brown precipitate Fe(OH)3 with NaOH); Anion = SO42− (white precipitate BaSO4 with BaCl2; no precipitate with AgNO3 rules out Cl−).
Q1.2 — Net ionic equation for solution C
CO32−(aq) + 2H+(aq) → CO2(g) + H2O(l). The observation that confirms this is effervescence (CO2 gas released) when dilute HCl was added.
Q1.3 — Evaluate the “sodium definitely present” conclusion
The conclusion is not fully justified. A yellow flame is characteristic of Na+, but sodium contamination of the nichrome wire loop is extremely common and can produce a false positive. The correct conclusion is that Na+ may be present, but the test should be repeated after thoroughly cleaning the wire, and ideally confirmed by a precipitation-based test or AAS. A more cautious, scientifically valid wording is: “the yellow flame is consistent with the presence of Na+, though contamination cannot be excluded without further testing.” (Award: 1 mark for identifying the flame test limitation; 1 mark for naming sodium contamination specifically; 1 mark for providing a more accurate conclusion.)
Q2.1 — Trend description (pH 2 curve)
The mass of precipitate increases steeply as BaCl2 is first added, then the rate of increase slows, and the curve plateaus at approximately 15 mL. Beyond this volume, adding more BaCl2 produces no additional precipitate, indicating that SO42− has been completely consumed.
Q2.2 — Why acid eliminates carbonate interference
Adding acid converts CO32− to CO2(g) and H2O(l) before BaCl2 is added: CO32−(aq) + 2H+(aq) → CO2(g) + H2O(l). Because CO32− has been destroyed, it cannot react with Ba2+ to form BaCO3(s). This means the only precipitate that forms is BaSO4(s), making the test specific for SO42−. This is the “add acid first” rule for anion analysis.
Q2.3 — Why plateau mass is doubled at pH 7
At pH 7, CO32− is still present in solution. Adding Ba2+ precipitates both BaSO4(s) (from SO42−) and BaCO3(s) (from CO32−). The total precipitate mass is therefore the sum of both, approximately double the mass obtained when only BaSO4 forms. The net ionic equations are: Ba2+(aq) + SO42−(aq) → BaSO4(s) and Ba2+(aq) + CO32−(aq) → BaCO3(s).
Q3.1 — AFSL ion identification with net ionic equations
Anion: Cl−. White precipitate with AgNO3(aq) indicates chloride is present. Net ionic equation: Ag+(aq) + Cl−(aq) → AgCl(s).
Cation: Na+ (yellow flame is characteristic of Na+; no precipitate with NaOH rules out Fe2+, Fe3+ and Cu2+). The compound is most consistent with sodium chloride (NaCl).
Q3.2 — Interpreting negative results
No precipitate with NaOH(aq) rules out Fe2+ (which forms a green precipitate), Fe3+ (red-brown) and Cu2+ (pale blue) — none of these cations are present. No precipitate with BaCl2(aq) rules out SO42− — that anion is absent. Together these negative results narrow the identity of the powder, confirming it does not contain those ions and supporting the identification of Na+ and Cl− as the only ions detected.