Chemistry • Year 12 • Module 8 • Lesson 3
Precipitation Reactions & Qualitative Analysis
Lock in the key vocabulary, the cation and anion test patterns, and the logic of net ionic equations before moving to data-based application.
1. Term–definition match
The ten definitions below are shuffled. In the right-hand column write the matching term from this list: qualitative analysis, quantitative analysis, precipitation reaction, net ionic equation, spectator ions, confirmatory test, flame test, precipitate, reagent, solubility. 10 marks
| # | Definition (shuffled) | Matching term |
|---|---|---|
| 1.1 | Identification of whether particular ions or compounds are present in a sample; does not determine amount. | |
| 1.2 | A double-displacement reaction in which two aqueous ionic solutions combine to form an insoluble solid. | |
| 1.3 | A technique in which a metal ion is identified by the characteristic colour it produces when heated in a gas flame on a nichrome wire loop. | |
| 1.4 | An ionic equation from which spectator ions have been removed, showing only the species that actually change chemically. | |
| 1.5 | Ions that are present in solution but take no part in the chemical change; they appear on both sides of the complete ionic equation and are cancelled. | |
| 1.6 | An insoluble solid that forms and separates from solution when two aqueous solutions are mixed. | |
| 1.7 | A second, independent test that confirms the identity of an ion already suggested by a preliminary test. | |
| 1.8 | The ability of a substance to dissolve in a solvent; used to predict which ionic compounds will form precipitates. | |
| 1.9 | Analysis that determines how much of a substance is present, typically by measuring mass, volume or instrument signal. | |
| 1.10 | A chemical substance added to a sample to produce a detectable reaction that reveals the presence of a target ion. |
2. Cloze paragraph — fill the blanks
Choose the correct word or phrase from the word bank below to complete the paragraph. Each word is used once. 8 marks
Word bank: white • AgCl(s) • net ionic • Na+ • Cl− • spectator • qualitative • BaSO4(s)
When silver nitrate solution is added to a solution containing chloride ions, a __________ precipitate of __________ forms. The sodium and nitrate ions do not take part in this change—they are called __________ ions. Removing them gives the __________ equation: Ag+(aq) + __________(aq) → AgCl(s). This test is an example of __________ analysis because it identifies the presence of an ion, not its concentration. Similarly, adding BaCl2(aq) to a solution containing sulfate ions produces a white precipitate of __________. In that reaction, __________(aq) ions from NaCl or BaCl2 act as spectators and are removed from the net ionic equation.
3. True or false — with correction
Circle T or F. If false, write the corrected version on the line provided. 10 marks (1 T/F + 1 correction per false statement)
3.1 Qualitative analysis can determine whether copper(II) ion is present AND measure its concentration in the same test. T / F
3.2 Adding AgNO3(aq) to a solution containing Cl−(aq) produces a white precipitate of AgCl(s). T / F
3.3 A pale blue precipitate with NaOH(aq) indicates the presence of Fe3+(aq). T / F
3.4 The net ionic equation for the sulfate test is: Ba2+(aq) + SO42−(aq) → BaSO4(s). T / F
3.5 A flame test alone is sufficient to confirm the identity of any metal ion present in an unknown solution. T / F
4. Function recall
Answer each question in 1–2 precise sentences. Use correct chemical terms. 10 marks (2 each)
4.1 What is the purpose of writing a net ionic equation rather than the full molecular equation for a precipitation test?
4.2 What observation (colour and name of precipitate) indicates that Fe3+(aq) is present when NaOH(aq) is added?
4.3 State the net ionic equation for the reaction that identifies carbonate ions in solution using dilute acid.
4.4 Why does sodium contamination reduce the reliability of a flame test for other metal ions?
4.5 Give the flame colour associated with each of these ions: Ba2+, Cu2+, K+.
5. Build a concept map
Draw labelled arrows between the six terms below to show how they connect in qualitative analysis. Each arrow must carry a linking phrase (e.g. “produces”, “identifies”, “confirms”). Aim for at least 6 labelled arrows. 6 marks
Supplied terms: precipitation reaction · reagent · precipitate (colour) · net ionic equation · ion present · confirmatory test.
Q1 — Term–definition matches
1.1 qualitative analysis • 1.2 precipitation reaction • 1.3 flame test • 1.4 net ionic equation • 1.5 spectator ions • 1.6 precipitate • 1.7 confirmatory test • 1.8 solubility • 1.9 quantitative analysis • 1.10 reagent.
Q2 — Cloze answers (in order of blanks)
white / AgCl(s) / spectator / net ionic / Cl− / qualitative / BaSO4(s) / Na+.
Q3 — True/False with corrections
3.1 False. Qualitative analysis identifies the presence of an ion; it does not measure concentration. A separate quantitative method (e.g. titration or AAS) is needed for that.
3.2 True.
3.3 False. A pale blue precipitate with NaOH(aq) indicates Cu2+(aq), not Fe3+. Fe3+ produces a red-brown precipitate (Fe(OH)3).
3.4 True.
3.5 False. A flame test is supporting evidence, not definitive proof. Contamination (especially by sodium) and similar colours for different ions mean a second, confirmatory precipitation test is required.
Q4 — Function recall
4.1 The net ionic equation removes spectator ions that do not change chemically, showing only the actual reacting species. This makes the chemistry of the precipitation test clearer and avoids misleading students into thinking the source reagents matter.
4.2 A red-brown precipitate of Fe(OH)3(s) forms, indicating Fe3+(aq) is present.
4.3 CO32−(aq) + 2H+(aq) → CO2(g) + H2O(l)
4.4 Sodium produces an intense bright yellow flame that can mask the weaker, more subtly coloured flames of other ions, making reliable identification impossible without re-cleaning the loop and repeating the test.
4.5 Ba2+: pale green • Cu2+: blue-green • K+: lilac.
Q5 — Sample concept map
Accept any biologically valid arrows with correct direction and linking phrase. A model path:
- reagent —is added to trigger→ precipitation reaction
- precipitation reaction —produces→ precipitate (colour)
- precipitate (colour) —suggests identity of→ ion present
- precipitation reaction —is described by→ net ionic equation
- net ionic equation —removes spectators to reveal→ ion present
- confirmatory test —provides stronger evidence for→ ion present
Award 1 mark per correctly labelled arrow (causal direction and linking phrase must be plausible).