Chemistry • Year 12 • Module 8 • Lesson 3

Precipitation Reactions & Qualitative Analysis

Build Band 5–6 extended-response technique: synthesise data with chemical knowledge, critique a media claim, and reach defensible evidence-based judgements.

Master · Extended Response

1. Evaluate a systematic identification protocol — NSW EPA water monitoring (Band 5–6)

8 marks   Band 5–6

Scenario. In March 2024, the NSW EPA collected water samples at five sites along the Hunter River near a former industrial site. Each sample was subjected to a standard qualitative screening protocol: (1) add dilute HCl first and observe for gas; (2) add AgNO3(aq) and observe for precipitate; (3) add BaCl2(aq) to a separate portion; (4) add NaOH(aq) and observe colour of any precipitate; (5) conduct a flame test.

The table below shows summarised results for two sites of concern.

Test Site 3 (industrial zone) Site 5 (downstream, residential)
(1) Add dilute HCl No gas No gas
(2) Add AgNO3(aq) White precipitate White precipitate
(3) Add BaCl2(aq) (after acid step) White precipitate No precipitate
(4) Add NaOH(aq) Green precipitate Red-brown precipitate
(5) Flame test Yellow (dominant) with faint brick-red Yellow

Q1. Analyse and evaluate the qualitative test results for Sites 3 and 5. In your response you must:

  • Identify the most likely cation and anion at each site, with reference to specific observations and net ionic equations for at least three of the positive tests.
  • Explain why the “add acid first” step was performed before the BaCl2(aq) test and how it affects the reliability of the SO42− identification.
  • Assess the limitations of the flame test evidence at Site 3, including whether the brick-red signal is meaningful, and recommend one additional test to strengthen the conclusion about the cation at Site 3.
  • Reach an evidence-based judgement about which site poses a greater environmental risk, explaining your reasoning.
Plan: identify ions per site with net ionic equations → explain acid-first logic → assess flame test limitations → compare risk via ion toxicity/concentration implications → use the cation precipitate colour to confirm identity (green = Fe2+, red-brown = Fe3+).

2. Source critique — evaluate a media claim (Band 5–6)

7 marks   Band 5–6

Source. The following paragraph appeared in a community newsletter distributed by an advocacy group near a former smelting site in Broken Hill, NSW (May 2025):

“Scientists tested the local bore water last week. They confirmed the water is safe because the flame test came back as blue-green, which means copper is present but only in tiny amounts. They also found a white powder forming when barium chloride was added, but this is just common salt and is harmless. All the tests showed the water is fine.”

Q2. This statement contains at least three scientific errors or unjustified conclusions. For each error:

  1. Identify and quote the specific claim.
  2. Explain the correct chemistry.
  3. State how a properly designed qualitative analysis protocol would detect or avoid this error.

Then write a scientifically accurate one-paragraph summary that correctly interprets the test results described.

Error 1: flame tests show presence, not amount — they are qualitative, not quantitative. Error 2: white precipitate with BaCl2 = BaSO4, not “common salt” (NaCl is soluble and would not form a precipitate). Error 3: finding ions present does not confirm “safe” — safety requires quantitative measurement against NHMRC/EPA limits.
Answers — Do not peek before attempting

Q1 — Marking criteria (8 marks)

Ion identification (3 marks):

  • Site 3: Anion = SO42− (white precipitate with BaCl2 after acid step; NIE: Ba2+(aq) + SO42−(aq) → BaSO4(s)); white precipitate with AgNO3(aq) also indicates Cl− is possible (NIE: Ag+(aq) + Cl−(aq) → AgCl(s)); Cation = Fe2+ (green precipitate with NaOH; NIE: Fe2+(aq) + 2OH−(aq) → Fe(OH)2(s)).
  • Site 5: Anion = Cl− (white precipitate with AgNO3; NIE: Ag+(aq) + Cl−(aq) → AgCl(s)); no precipitate with BaCl2 rules out SO42−; Cation = Fe3+ (red-brown precipitate with NaOH; NIE: Fe3+(aq) + 3OH−(aq) → Fe(OH)3(s)).

“Add acid first” explanation (2 marks): Adding dilute HCl before BaCl2 destroys any CO32− present (CO32− + 2H+ → CO2 + H2O), preventing BaCO3 from forming and giving a false positive for SO42−. This step ensures that the white precipitate observed with BaCl2 is exclusively BaSO4, improving test specificity and reliability.

Flame test assessment (2 marks): The dominant yellow flame at Site 3 is consistent with Na+ contamination, which is so bright it can mask other flame colours. The faint brick-red signal is consistent with Ca2+ (brick-red) but cannot be confirmed by flame alone when Na+ dominates. A recommended additional test: add Na2CO3(aq) to a separate portion — Ca2+ produces a white precipitate of CaCO3(s), which would confirm or rule out Ca2+.

Environmental risk judgement (1 mark): Site 3 likely poses greater risk because it contains both SO42− (indicator of acid mine drainage) and Fe2+, which may indicate reducing conditions and ongoing corrosion chemistry. Site 5 contains Fe3+ and Cl−, also concerning, but the SO42− + Fe2+ combination at Site 3 more strongly suggests acid mine drainage. Judgement must acknowledge that neither site is safe without quantitative follow-up.

Q2 — Source critique marking criteria (7 marks)

Error 1 (2 marks): Claim: “the flame test came back as blue-green, which means copper is present but only in tiny amounts.” Correct chemistry: a flame test identifies the presence of an ion but gives no information about its concentration. Qualitative analysis cannot determine amount — that requires quantitative analysis such as AAS or complexometric titration. A correct protocol would use the flame test only to suggest Cu2+ is present, then conduct a quantitative method (e.g. AAS) to determine whether the concentration exceeds the NHMRC safe limit (2 mg L−1 for Cu in drinking water).

Error 2 (2 marks): Claim: “a white powder forming when barium chloride was added, but this is just common salt.” Correct chemistry: NaCl (common salt) is soluble in water and does not form a precipitate with BaCl2(aq). A white precipitate with BaCl2(aq) is the test for SO42−, producing insoluble BaSO4(s): Ba2+(aq) + SO42−(aq) → BaSO4(s). A correct protocol would note SO42− as present and recommend quantitative follow-up (e.g. gravimetric analysis or ion chromatography) to determine whether sulfate concentrations indicate acid mine drainage.

Error 3 (2 marks): Claim: “All the tests showed the water is fine.” Correct chemistry: finding that ions are present (qualitative) says nothing about whether their concentrations are within safe limits. Water quality safety requires quantitative analysis compared against regulated standards (e.g. NHMRC Guidelines for Drinking Water Quality; NSW EPA Water Quality Objectives). The presence of Cu2+ and SO42− in bore water near a smelting site is a concern that warrants urgent quantitative investigation, not a conclusion of safety.

Corrected summary (1 mark): The blue-green flame test result indicates that Cu2+ may be present in the bore water. The white precipitate formed with BaCl2(aq) is consistent with SO42− ion, suggesting possible contamination from acid mine drainage. These qualitative results identify which ions are present but cannot determine their concentrations or confirm water safety. Quantitative analysis is required to measure Cu2+ and SO42− concentrations against NHMRC guidelines before any safety assessment can be made.

HSCScience • Chemistry • Year 12 • Module 8 • Lesson 3 • Worksheet 3 (Master)