Chemistry • Year 12 • Module 5 • Lesson 16
Solubility Rules & Precipitation
Apply the four-ion method, real precipitation data, and Australian water-chemistry contexts to practise Band 4–5 reasoning.
1. Interpret solubility data — Ksp versus temperature
The graph below shows how the solubility product constant (Ksp) of three NESA-specified insoluble salts varies with temperature. Data are plotted on a logarithmic scale. 7 marks
Figure 1.1. Ksp of three NESA precipitation products vs temperature. Adapted from Speight (2005), Lange's Handbook of Chemistry, 16th ed.
1.1 Describe the trend in Ksp for AgCl and PbI2 as temperature increases from 0 °C to 40 °C. Include comparative values from the graph. 2 marks
1.2 At 20 °C, which salt is least soluble? Justify your answer with reference to Ksp and what a lower Ksp means for the equilibrium position. 3 marks
1.3 BaSO4 is used as a contrast agent in medical GI imaging (“barium meal”). Using the graph, explain why warming the patient’s body temperature from 20 °C to 37 °C would have negligible effect on the safety of BaSO4 in the body. 2 marks
2. Cause-and-effect chain — Murray-Darling gypsum addition
The Murray-Darling Basin in southern Australia has been affected by rising soil salinity from irrigation. Adding gypsum (CaSO4·2H2O) to irrigation water or saline soil is one management strategy. Trace the cause-and-effect chain below. Fill in each empty effect box. 5 marks
Note: MgSO4 is actually soluble (check NAGSAG — sulfates are generally soluble, and Mg2+ is not an exception). The benefit of gypsum is primarily through Ca2+ exchange with Na+ on clay surfaces. Use this fact in Effects 3 and 4.
Stuck? Apply NAGSAG carefully: is MgSO4 insoluble? Check the exceptions. Then consider what Ca2+ does to clay compared to Na+.3. Compare and contrast — molecular vs net ionic equation
Complete the comparison table for the reaction KCl(aq) + AgNO3(aq) → AgCl(s) + KNO3(aq). 6 marks
| Feature | Molecular equation | Net ionic equation |
|---|---|---|
| Write the equation here | ||
| Are spectator ions shown? | ||
| How are dissolved species represented? | ||
| How is the precipitate represented? | ||
| Verify charge balance | N/A (not applicable at molecular level) | |
| Best used when… |
4. Case study — lead in old Sydney Water pipes
Some older Sydney Water distribution pipes contain lead solder or lead-lined joints. When water is slightly acidic (pH < 6.5) or low in dissolved minerals, Pb2+ ions dissolve from the pipe surface into the drinking water. Sydney Water’s response has included dosing the distribution system with orthophosphate (PO43−), which forms a protective mineral coating on pipe surfaces. 6 marks
4.1 Using NAGSAG, predict whether Pb3(PO4)2 is soluble or insoluble. Justify your answer. 2 marks
4.2 Write the net ionic equation for the formation of lead(II) phosphate from Pb2+(aq) and PO43−(aq). Verify charge balance. 2 marks
4.3 Predict and justify: would adding sodium sulfate (Na2SO4) be an equally effective precipitant for Pb2+ in this context? Compare with orthophosphate. 2 marks
5. Interpret experimental data — identifying unknowns by precipitation
A student has four unlabelled aqueous solutions, each containing one of: NaCl, BaCl2, Pb(NO3)2, or K2SO4. She systematically mixes pairs and records observations. 6 marks
| Mix | Solution A + Solution B | Observation |
|---|---|---|
| 1 | Unknown 1 + Unknown 2 | White precipitate |
| 2 | Unknown 1 + Unknown 3 | No precipitate |
| 3 | Unknown 1 + Unknown 4 | White precipitate (different from Mix 1) |
| 4 | Unknown 2 + Unknown 3 | White precipitate |
| 5 | Unknown 2 + Unknown 4 | White precipitate |
| 6 | Unknown 3 + Unknown 4 | No precipitate |
5.1 Identify each unknown (1–4). Show your reasoning step by step. 4 marks
5.2 Write the net ionic equation for Mix 5, naming the precipitate and its colour. 2 marks
Q1.1 — Trend in Ksp
Both AgCl and PbI2 show increasing Ksp with temperature, meaning both become more soluble as temperature rises. PbI2 shows a much steeper increase (roughly one order of magnitude from ~4×10−9 at 0°C to ~4×10−8 at 40°C) compared to AgCl, which increases more modestly (approximately 5-fold over the same range). The increase in Ksp indicates that dissolution is endothermic for both salts. [1 mark for correct direction; 1 mark for comparative data including reference values.]
Q1.2 — Least soluble at 20 °C
BaSO4 has the lowest Ksp at 20°C (approximately 1.1×10−10), making it the least soluble of the three. [1 mark] A lower Ksp means the equilibrium constant for BaSO4(s) ⇌ Ba2+(aq) + SO42−(aq) lies further to the left, so a smaller concentration of ions is in equilibrium with the solid. [1 mark] This extreme insolubility is why BaSO4 can be ingested safely as a medical contrast agent. [1 mark for application/implication]
Q1.3 — BaSO4 safety and temperature
The graph shows that BaSO4’s Ksp is essentially flat between 0°C and 60°C (~1.1×10−10 throughout). [1 mark] Therefore, warming from 20°C to 37°C causes negligible change in solubility, and the concentration of dissolved (toxic) Ba2+ ions released remains far below harmful levels regardless of body temperature. [1 mark]
Q2 — Murray-Darling cause-and-effect chain
Effect 1: MgSO4 is actually soluble (Mg2+ is not an exception to the sulfate rule), so no precipitation of Mg2+ occurs. SO42− remains dissolved alongside Mg2+.
Effect 2: No precipitation: Mg2+(aq) + SO42−(aq) → no precipitate (MgSO4 is soluble). Accept: student correctly applies NAGSAG and states no reaction occurs.
Effect 3: The dissolved Ca2+ ions from gypsum displace Na+ from clay exchange sites (Ca2+ has higher charge density and binds clay more strongly). This reduces clay dispersion.
Effect 4: Flocculation of clay particles improves soil structure, drainage, and permeability, reducing waterlogging and surface crusting.
Overall outcome: Gypsum addition improves soil structure by exchanging Na+ for Ca2+ on clay surfaces, causing flocculation and improving drainage — not primarily through precipitation of Mg2+.
Mark note: Award 1 mark per correct effect. The key insight is recognising MgSO4 is soluble and the benefit of gypsum is Ca2+ exchange, not precipitation.
Q3 — Compare and contrast table
Molecular equation: KCl(aq) + AgNO3(aq) → AgCl(s) + KNO3(aq). Shows spectator ions: yes. Dissolved species shown as formula units. Precipitate: AgCl(s). Best used: communicating overall reactants and products including counter-ions, balancing for stoichiometry.
Net ionic equation: Ag+(aq) + Cl−(aq) → AgCl(s). Spectator ions: not shown (K+ and NO3− cancelled). Dissolved species shown as separated ions. Precipitate: AgCl(s). Charge balance: +1 + (−1) = 0 = 0 ✓. Best used: showing the essential chemistry, identifying spectator ions, writing equations that apply to any combination producing the same precipitate.
Q4.1 — Is Pb3(PO4)2 insoluble?
Yes, Pb3(PO4)2 is insoluble. Under NAGSAG, phosphates (PO43−) fall under “G — generally insoluble” unless paired with Group 1 cations or NH4+. Pb2+ is not Group 1, so lead(II) phosphate is insoluble. [1 mark rule + 1 mark correct application]
Q4.2 — Net ionic equation for Pb3(PO4)2
3Pb2+(aq) + 2PO43−(aq) → Pb3(PO4)2(s). Charge check: left = 3(+2) + 2(−3) = +6 − 6 = 0; right = 0 (neutral solid) ✓. [1 mark equation, 1 mark verified charge balance]
Q4.3 — Na2SO4 vs orthophosphate
PbSO4 is insoluble (Pb2+ is an exception to the sulfate rule), so Na2SO4 would also precipitate Pb2+. However, orthophosphate is preferred because Pb3(PO4)2 has a much lower Ksp (~8×10−43) than PbSO4 (~2×10−8), meaning phosphate precipitation is far more complete and reduces dissolved Pb2+ to much lower concentrations. Also, the phosphate mineral forms a coherent protective coating on pipe walls. [1 mark for noting both precipitate Pb2+; 1 mark for comparing effectiveness/Ksp or noting coating function]
Q5.1 — Identifying unknowns
Available ions: Na+, Cl− (NaCl); Ba2+, Cl− (BaCl2); Pb2+, NO3− (Pb(NO3)2); K+, SO42− (K2SO4). Expected precipitates: BaSO4 (white), PbSO4 (white), PbCl2 (white). Key deductions: Mix 5 = white precipitate. Unknown 2 reacts with Unknowns 1 (white), 3 (white), and 4 (white). Pb2+ reacts with SO42− (white PbSO4) and with Cl− (white PbCl2). Ba2+ reacts with SO42− (white BaSO4) but not Cl−. Mix 2 (Unknown 1 + Unknown 3 = no precipitate) and Mix 6 (Unknown 3 + Unknown 4 = no precipitate) constrain the assignments. Conclude: Unknown 2 = Pb(NO3)2 (reacts with all three others); Unknown 4 = K2SO4 (Mix 5: PbSO4 white; Mix 3: BaSO4 white; Mix 6: no precipitate with NaCl); Unknown 1 = BaCl2 (reacts with SO42− giving BaSO4, no reaction with NaCl); Unknown 3 = NaCl. [1 mark per correct identification, 4 marks total]
Q5.2 — Net ionic equation for Mix 5
Mix 5 = Pb(NO3)2 + K2SO4. Precipitate: PbSO4 — white (lead(II) sulfate is a white solid; Pb2+ is a named exception to the general sulfate solubility rule). Net ionic equation: Pb2+(aq) + SO42−(aq) → PbSO4(s). Spectator ions: K+ and NO3−. Charge check: +2 + (−2) = 0 ✓. [1 mark correct equation + state symbols; 1 mark precipitate named as PbSO4 with colour stated as white and charge check shown]