HSC Exam Practice

Sample response. A precipitation reaction occurs when two aqueous ionic solutions are mixed and an insoluble solid (precipitate) forms. The precipitate is indicated by the state symbol (s) and a downward arrow (↓) in the equation.

Marking notes. 1 mark for correctly defining precipitation reaction (two aqueous solutions → insoluble solid product); 1 mark for identifying (s) as the state symbol for a precipitate.

Sample response. A full ionic equation shows all soluble ionic compounds written as separate aqueous ions, while the precipitate remains as a complete formula unit. A net ionic equation is obtained by identifying and removing spectator ions — ions that appear identically on both sides of the full ionic equation and take no part in the reaction. The net ionic equation shows only the ions that actually react.

Marking notes. 1 mark — full ionic equation splits soluble aqueous ionic compounds into ions (precipitate kept intact); 1 mark — defines spectator ions correctly (appear unchanged on both sides, take no part in reaction); 1 mark — net ionic equation is obtained by removing spectator ions, showing only the reacting species.

Sample response. Ions present: Fe³⁺, NO₃⁻, K⁺, OH⁻. Possible products: Fe(OH)₃ (Fe³⁺ + OH⁻) and KNO₃ (K⁺ + NO₃⁻). KNO₃: Group 1 (K⁺) and nitrate — both soluble, so no precipitate. Fe(OH)₃: hydroxide rule — insoluble unless Group 1 or Ba²⁺; Fe³⁺ is neither, so Fe(OH)₃ is insoluble. A rust-brown precipitate forms. Net ionic: Fe³⁺(aq) + 3OH⁻(aq) → Fe(OH)₃(s).

Marking notes. 1 mark — correctly identifies all four ions in solution; 1 mark — uses solubility rules to confirm KNO₃ is soluble and Fe(OH)₃ is insoluble; 1 mark — names the precipitate as iron(III) hydroxide and states its colour (rust brown / reddish-brown); 1 mark — correct balanced net ionic equation with state symbols.

Sample response. BaSO₄ is extremely insoluble (Ba²⁺ is a sulfate exception under NAGSAG rules / very low Ksp), so when swallowed it remains as an intact solid in the digestive tract and does not dissociate to release free Ba²⁺ ions into the bloodstream. Because no free Ba²⁺ is absorbed, there is no toxic effect on the patient. The solid BaSO₄ is opaque to X-rays, providing the required contrast imaging of the digestive system.

Marking notes. 1 mark — BaSO₄ is insoluble (exception to sulfate rule / very low Ksp), so does not dissolve; 1 mark — insolubility prevents release of free Ba²⁺ ions that could be absorbed into the bloodstream; 1 mark — the solid BaSO₄ is X-ray opaque and provides contrast imaging. Accept references to Ksp if used correctly.

Sample response. Ions present when mixed: Na⁺(aq), SO₄²⁻(aq), K⁺(aq), NO₃⁻(aq). Possible product combinations: NaNO₃ (Na⁺ + NO₃⁻) and K₂SO₄ (K⁺ + SO₄²⁻). NaNO₃: Na⁺ is Group 1 and NO₃⁻ is nitrate — both NAGSAG “always soluble” categories, so NaNO₃ is soluble. K₂SO₄: K⁺ is Group 1 — soluble with all anions. Neither possible product is insoluble; all ions remain in solution as spectator ions — no precipitate forms.

Marking notes. 1 mark — correctly identifies all four ions present in the mixed solution; 1 mark — identifies both possible product combinations (NaNO₃ and K₂SO₄); 1 mark — applies NAGSAG correctly to both products and concludes no precipitate forms (Group 1 + nitrate / Group 1 + sulfate both soluble).

Sample response. Step 1: Write down all ions present when the two solutions are mixed (one cation and one anion from each compound → four ions total). Step 2: Identify the two possible product combinations by swapping anion partners. Step 3: Apply the NAGSAG solubility rules to each product — if either is insoluble, a precipitate forms. Step 4: Write the full ionic equation (split all soluble aqueous compounds into ions; keep precipitate as a formula unit with (s)); then remove spectator ions (those appearing identically on both sides) to give the net ionic equation.

Marking notes. 1 mark each for any four of: (a) identify all ions in solution; (b) determine the two possible product combinations; (c) apply solubility rules to each product; (d) write the balanced full ionic equation; (e) remove spectator ions to produce the net ionic equation. Accept any well-sequenced four steps.

(a) Sample response. Solution A contains Cl⁻: AgCl(s) is white and dissolves in excess NH₃ (unique to AgCl among silver halides), consistent with the observation. Solution B contains Br⁻: AgBr(s) is pale yellow and does not dissolve in dilute HNO₃, consistent with observations. Solution C contains no halide ions: AgNO₃ produces no precipitate because neither the cation present nor any other ion in C forms an insoluble silver salt. Solution D contains I⁻: AgI(s) is yellow and does not dissolve in dilute HNO₃.

Marking notes (a). 0.5 + 0.5 mark per correctly identified solution with justification from observations + solubility rules, max 4 marks. Accept any combination where identification and justification are consistent with the data.

(b) Net ionic equation for solution A: Ag⁺(aq) + Cl⁻(aq) → AgCl(s). [1 mark — correct ions with state symbols]

(c) Spectator ions in experiment B (AgNO₃ + source of Br⁻): NO₃⁻(aq) and the cation from the bromide salt (e.g. Na⁺ or K⁺). [1 mark — both spectator species identified. Accept any Group 1 or ammonium cation depending on the unknown compound assumed.]

(a) Ag⁺(aq) + Cl⁻(aq) → AgCl(s). [1 mark]

(b) n(AgCl) = 0.574 / 143.3 = 4.006 × 10⁻³ mol [1 mark]. Since Ag⁺ : Cl⁻ : AgCl = 1:1:1, n(Cl⁻) = 4.006 × 10⁻³ mol. Mass Cl⁻ = 4.006 × 10⁻³ × 35.45 = 0.1420 g. [1 mark]

(c) Volume of sample = 200.0 mL = 0.2000 L. Mass Cl⁻ = 0.1420 g = 142.0 mg. Concentration = 142.0 / 0.2000 = 710 mg/L. [1 mark for correct unit conversion; 1 mark for correct final answer]

(d) The sample (710 mg/L) exceeds the NSW guideline of 250 mg/L. [1 mark]. Assumption: all AgCl precipitate was captured during filtration (none lost through the filter paper); OR that all Cl⁻ was precipitated (excess AgNO₃ ensured). [1 mark for any valid stated assumption]

Marking notes. Award marks at each step only if working is shown. Consequential error allowed: if wrong n(AgCl) but correctly applied to subsequent steps, award subsequent marks.

Sample response. Precipitation reactions provide an effective and relatively inexpensive method for removing dissolved heavy-metal ions from contaminated water. When a reagent is added whose anion forms an insoluble compound with the target cation, the metal ion is converted from an invisible dissolved form into a filterable solid. For example, mercury(II) ions (Hg²⁺) are removed by sodium sulfide: Hg²⁺(aq) + S²⁻(aq) → HgS(s) — a black precipitate. Lead(II) ions (Pb²⁺) are removed by sodium carbonate: Pb²⁺(aq) + CO₃²⁻(aq) → PbCO₃(s) — a white precipitate. Both reactions are confirmed by NAGSAG: sulfide with a non-Group-1 cation is insoluble; carbonate with a non-Group-1 cation is insoluble. The spectator ions in both reactions (Na⁺, NO₃⁻ or equivalent) remain dissolved and are harmless at the concentrations used. The effectiveness is governed by Ksp: a lower Ksp means the equilibrium is further toward the precipitate side, reducing [Mⁿ⁺] to a lower residual value. PbS (Ksp ≈ 10⁻²⁸) removes Pb²⁺ more completely than PbCO₃ (Ksp ≈ 10⁻¹⁴). However, even the most insoluble precipitate leaves a tiny residual concentration of the metal ion governed by Ksp — precipitation can never achieve [Mⁿ⁺] = 0. A significant limitation is secondary contamination: excess sulfide reagent introduces toxic S²⁻ ions; excess carbonate raises pH. Stoichiometric dosing is therefore critical. Precipitation also produces solid waste (the filtered precipitate) that requires appropriate disposal. Despite these limitations, precipitation remains a cost-effective first-line treatment strategy when combined with filtration and careful dose monitoring. Its strengths are simplicity, low cost, and the ability to convert dissolved toxic ions into insoluble, filterable solids — exactly the mechanism used in Sydney Water's treatment of heavy-metal contamination from old pipe infrastructure.

Marking criteria.

• 1 mark — Explains the principle: adding a reagent converts dissolved Mⁿ⁺ into an insoluble precipitate that can be filtered.
• 1 mark — Names at least two specific heavy-metal ions (e.g. Pb²⁺, Hg²⁺, Cu²⁺, Fe³⁺) with appropriate reagents.
• 1 mark — Writes a correct net ionic equation with state symbols for one example.
• 1 mark — Writes a correct net ionic equation with state symbols for a second example.
• 1 mark — Uses NAGSAG rules to justify why the precipitate forms (cation not Group 1, sulfide/hydroxide/carbonate rule applies).
• 1 mark — Explains spectator ions: the counter-ions from the reagent remain dissolved and do not contribute to contamination at typical doses.
• 1 mark — Applies Ksp correctly: lower Ksp → more complete precipitation; very small but non-zero [Mⁿ⁺] remains at equilibrium.
• 1 mark — Identifies at least one specific limitation (secondary contamination from excess reagent / toxic counter-ion; residual metal concentration due to Ksp; solid waste disposal; precise stoichiometry required).
• 1 mark — Reaches an evaluative judgement about the overall usefulness / limitations of precipitation as a water-treatment strategy, using Australian context or a named example. Response uses precise chemical language (net ionic equation, state symbols, Ksp, spectator ions) throughout.