Chemistry · Year 12 · Module 8 · Lesson 3
HSC Exam Practice
Precipitation Reactions & Qualitative Analysis
Short answer
1.Short answer
Define qualitative analysis and distinguish it from quantitative analysis.
Identify the reagent, observation and net ionic equation for the qualitative test for Cl−(aq).
Outline the observations and net ionic equations for the tests used to distinguish Fe2+(aq) from Fe3+(aq).
Explain why adding dilute acid to a sample before adding BaCl2(aq) improves the reliability of the qualitative test for SO42−(aq).
Describe what is meant by a spectator ion and explain why spectator ions are removed when writing a net ionic equation.
Compare the suitability of a flame test and a precipitation test for confirming the presence of Cu2+(aq) in an unknown solution. Include reference to one limitation of each method.
Data response
2.Data response — systematic qualitative analysis of two mining leachate samples
The table below shows qualitative test results for two water samples (P and Q) collected from bore water sites near a BHP Broken Hill mining lease. The testing protocol followed the NSW EPA systematic order: acid first, then AgNO3, then BaCl2, then NaOH, then flame test.
| Test | Sample P | Sample Q |
|---|---|---|
| Dilute HCl(aq) added | Effervescence (CO2 gas) | No gas |
| AgNO3(aq) | No precipitate | White precipitate |
| BaCl2(aq) after acid step | White precipitate | No precipitate |
| NaOH(aq) | Red-brown precipitate | Pale blue precipitate |
| Flame test | Lilac | Blue-green |
Note: effervescence in step 1 indicates CO32− was present in the original sample P and has been destroyed before step 3. The BaCl2 test result in step 3 therefore refers only to SO42−.
Identify the most likely cation and anion in each sample. For each identification, cite the specific observation that supports it.
Write the net ionic equation for the reaction that confirms the anion in Sample P. Explain why this ion was not detected in Step 2 (AgNO3 test) even though it was present in the original sample.
The lilac flame test result for Sample P suggests K+ may be present. Account for why this conclusion requires a confirmatory precipitation test before being reported as “confirmed.”
Extended response
3.Extended response
Evaluate the use of precipitation reactions and flame tests as first-line methods for qualitative analysis of metal ions in environmental water samples. In your response, discuss the strengths and limitations of each method, explain why no single test is sufficient for a reliable identification, and assess under what conditions the two approaches together provide strong evidence for the identity of a metal ion. Refer to specific ions and named reactions in your answer.
Section 1 — Short answer
Qualitative analysis identifies whether particular substances or ions are present in a sample (1 mark). Quantitative analysis determines how much of a substance is present, expressed as a measured amount or concentration (1 mark).
Reagent: AgNO3(aq) (1 mark). Observation: white precipitate of AgCl(s) (1 mark). Net ionic equation: Ag+(aq) + Cl−(aq) → AgCl(s) (1 mark).
Both Fe2+ and Fe3+ are tested with NaOH(aq). Fe2+ produces a green precipitate: Fe2+(aq) + 2OH−(aq) → Fe(OH)2(s) (1 mark for observation + 1 mark for equation). Fe3+ produces a red-brown precipitate: Fe3+(aq) + 3OH−(aq) → Fe(OH)3(s) (1 mark for observation + 1 mark for equation).
If CO32−(aq) is present, it would react with Ba2+(aq) to form BaCO3(s), a white precipitate that would be mistakenly interpreted as evidence of SO42− (1 mark — naming the interference). Adding dilute acid first destroys CO32−: CO32−(aq) + 2H+(aq) → CO2(g) + H2O(l) (1 mark — equation). This means any subsequent white precipitate with BaCl2 is exclusively BaSO4, making the test specific to SO42− and improving reliability (1 mark — link to reliability).
A spectator ion is an ion that is present in solution and appears on both sides of the complete ionic equation, but does not participate in the chemical change (1 mark). Because spectator ions do not change, including them in an ionic equation adds unnecessary information and obscures the actual reacting species (1 mark). The net ionic equation removes spectators to show only the particles that actually undergo a chemical transformation, making the chemistry of the reaction (and the test) clearer (1 mark).
Precipitation test: Cu2+(aq) + 2OH−(aq) → Cu(OH)2(s); pale blue precipitate is a reliable visual indicator; a specific reagent (NaOH) produces a distinctive, colour-coded result (1 mark). Limitation: other ions do not produce blue precipitates, but the pale blue colour can occasionally appear pale green when mixed with other precipitates (1 mark).
Flame test: Cu2+ gives a distinctive blue-green flame that can quickly suggest the ion’s presence (1 mark). Limitation: flame tests give no concentration information; Na+ contamination can dominate and mask the blue-green colour; must be treated as supporting evidence only (1 mark).
Section 2 — Data response
Sample P: Anion = CO32− (effervescence with dilute HCl in Step 1: CO32− + 2H+ → CO2 + H2O); Cation = Fe3+ (red-brown precipitate Fe(OH)3 with NaOH); flame test shows lilac, consistent with K+ also possibly present. (1 mark cation + 1 mark anion with cited evidence.)
Sample Q: Anion = Cl− (white precipitate AgCl with AgNO3; NIE: Ag+(aq) + Cl−(aq) → AgCl(s)); Cation = Cu2+ (pale blue precipitate Cu(OH)2 with NaOH; blue-green flame consistent). (1 mark cation + 1 mark anion with cited evidence.)
The anion in Sample P is CO32−. Net ionic equation: CO32−(aq) + 2H+(aq) → CO2(g) + H2O(l) (1 mark). CO32− was not detected in Step 2 because it had already been destroyed by the addition of dilute HCl in Step 1 before AgNO3 was added (1 mark). Ag2CO3 would have been the expected precipitate if CO32− were still present, but the systematic protocol requires acid-first, so no carbonate remained to react (1 mark for explanation linking the protocol to the outcome).
A lilac flame is consistent with K+ but flame tests are not definitive by themselves: sodium contamination or other flame colours can interfere with or mask the observation. The lesson establishes that flame tests are best treated as supporting evidence only, not as proof of identity. A confirmatory test — such as a second, independent precipitation-based observation or a repeat test with a thoroughly cleaned wire — is needed before K+ can be reported as confirmed (award 1 mark for identifying the limitation of flame tests as qualitative/supporting evidence only; 1 mark for stating that a confirmatory test is required and briefly describing an approach to obtain one).
Section 3 — Extended response (marking criteria)
The following criteria are used. Award marks for the quality of reasoning, specificity of examples, and balance of evidence. No half-marks.
| Criterion | Marks |
|---|---|
| Correctly states at least 2 strengths of precipitation tests (selective reagents, distinctive colour-coded precipitates, net ionic equations can be written, inexpensive) with specific examples (e.g. Cu(OH)2 pale blue, Fe(OH)3 red-brown). | 2 |
| Correctly states at least 1 limitation of precipitation tests (similar precipitate colours for different ions; some ions do not form distinctive precipitates; false positives from interfering ions). | 1 |
| Correctly states at least 1 strength of flame tests (rapid, inexpensive, characteristic wavelengths from specific metal ions, supports cation identification) with a named example (e.g. Cu2+ blue-green, K+ lilac). | 1 |
| Correctly states at least 1 limitation of flame tests (sodium contamination, similar colours for different ions, qualitative only — no concentration information). | 1 |
| Explains why no single test is sufficient: any one test can produce false positives or have ambiguous results; confirmatory or orthogonal tests are needed to build a consistent evidence pattern. | 1 |
| Assesses conditions under which both methods together provide strong evidence: consistent positive results from a precipitation test AND a supporting flame colour for the same ion (e.g. pale blue precipitate AND blue-green flame both indicate Cu2+), or consistent negative results ruling out alternatives. | 2 |
Band 5–6 descriptor: Response integrates evidence coherently, reaches a nuanced conclusion that acknowledges both methods have complementary roles, uses correct chemical terminology throughout, and includes specific named ions and reactions.