Chemistry • Year 12 • Module 5 • Lesson 15
Dissolution & ATSI Knowledge
Apply dissolution thermodynamics, solubility equilibrium and ATSI knowledge to data graphs, experimental scenarios and real-world case studies.
1. Interpret a solubility–temperature graph
The graph below shows the solubility of four ionic compounds as a function of temperature. Use it to answer questions 1.1–1.4. 8 marks
1.1 Describe the trend in solubility with increasing temperature for KNO3 and for Ce2(SO4)3. Quote at least one data value for each. 2 marks
1.2 Using Le Chatelier's Principle and the sign of ΔHsoln, explain why KNO3 becomes more soluble as temperature increases. 3 marks
1.3 Ce2(SO4)3 shows ‘retrograde solubility’. Predict the sign of ΔHsoln for Ce2(SO4)3 and justify your prediction using Le Chatelier's Principle. 2 marks
1.4 A student adds excess KNO3 to water at 40 °C until the solution is saturated, then heats the solution to 80 °C. Predict and explain the effect on the amount of undissolved solid present. 1 mark
2. Data table — factors affecting cycasin extraction rate
A research team studying traditional cycad detoxification measured the percentage of cycasin removed from seed samples after 24 hours under different experimental conditions. Each trial used the same mass of seed material. 7 marks
| Trial | Seed preparation | Water condition | Temperature | % cycasin removed after 24 h |
|---|---|---|---|---|
| 1 | Whole seeds | Still water, unchanged | 20 °C | 12% |
| 2 | Crushed seeds | Still water, unchanged | 20 °C | 31% |
| 3 | Whole seeds | Water changed every 6 h | 20 °C | 48% |
| 4 | Crushed seeds | Water changed every 6 h | 20 °C | 74% |
| 5 | Crushed seeds | Running water | 20 °C | 91% |
| 6 | Crushed seeds | Running water | 35 °C | 97% |
Hypothetical research data adapted to reflect the chemical mechanisms described in the lesson.
2.1 Identify the controlled variable(s) across Trials 1–5. 1 mark
2.2 Compare Trials 1 and 2. Using the lesson's chemistry, explain the difference in % cycasin removed. 2 marks
2.3 Compare Trials 2, 3 and 4. What does this comparison reveal about the combined effect of surface area and water replacement on cycasin extraction? 2 marks
2.4 Compare Trials 5 and 6. Using dissolution equilibrium and kinetics, explain why the higher temperature increases cycasin removal. 2 marks
3. Cause-and-effect chain — adding water to a saturated NaCl solution
The cause boxes are filled in. Complete the effect boxes and the overall outcome. 5 marks
| Cause | Effect (write your answer) |
|---|---|
| Water is added to a saturated NaCl(aq) solution containing excess NaCl(s). | |
| [Na+] and [Cl−] in solution both decrease (dilution effect). | |
| The system is no longer at equilibrium — product concentration has decreased. | |
| LCP: the equilibrium shifts to partially counteract the disturbance. | |
| More NaCl(s) dissolves. |
Overall outcome (so…):
4. Case study — Great Artesian Basin bore water
5 marks
Scenario. The Great Artesian Basin (GAB) is one of the world's largest underground freshwater aquifer systems, spanning 22% of Australia. Groundwater travels through sandstone and limestone aquifers for thousands of years before reaching the surface at natural springs or drilled bores. GAB bore water is notably high in dissolved ionic species including Na+, Cl−, HCO3−, SO42−, and Ca2+. Traditional custodians of the semi-arid regions overlying the GAB have used these springs as reliable water and mineral sources for tens of thousands of years, developing knowledge of their chemical properties, their seasonal variability, and their safe use. Modern geochemical analysis confirms that the high ionic content results from the slow dissolution of minerals in the rock matrix over geological timescales — an application of solubility equilibrium operating under high pressure and temperature deep underground.
Q4. Using the concepts of dissolution equilibrium, Le Chatelier's Principle and factors affecting dissolution rate, explain why GAB bore water accumulates such high concentrations of dissolved ions over geological timescales. In your response, discuss the role of: (i) contact time between water and rock; (ii) temperature and pressure at depth; (iii) the principle that groundwater is continually replenished by rain recharge. 5 marks
Q1.1 — Trend description (2 marks)
KNO3: solubility increases steeply with temperature, e.g. approximately 13 g/100 g at 0 °C rising to approximately 130 g/100 g at 80 °C. Ce2(SO4)3: solubility decreases with temperature, e.g. approximately 21 g/100 g at 0 °C falling to approximately 5 g/100 g at 80 °C (retrograde solubility). [1 mark each, must include at least one data value per compound]
Q1.2 — LCP + endothermic dissolution (3 marks)
KNO3 dissolution is endothermic (ΔHsoln > 0) [1]. The dissolution equilibrium KNO3(s) ⇌ K+(aq) + NO3−(aq) is endothermic in the forward direction [1]. By LCP, increasing temperature adds heat to the system; the system shifts to absorb this extra energy by favouring the forward endothermic reaction (dissolving more solid), increasing Ksp and therefore solubility [1].
Q1.3 — Retrograde solubility prediction (2 marks)
ΔHsoln for Ce2(SO4)3 is negative (exothermic) [1]. By LCP, increasing temperature shifts the exothermic dissolution equilibrium to the left (reverse direction) to absorb the added heat, decreasing the amount of dissolved solute and hence solubility [1].
Q1.4 — Heating saturated KNO3 (1 mark)
The amount of undissolved solid decreases (or disappears entirely). Increasing temperature increases the solubility of KNO3 (endothermic dissolution), so the equilibrium shifts right, dissolving more solid until a new saturated equilibrium is established at the higher temperature. [1]
Q2.1 — Controlled variable (1 mark)
Temperature (held at 20 °C across Trials 1–5) and the duration of soaking (24 h). Accept: mass of seed material (stated as same in the scenario). [1]
Q2.2 — Trials 1 vs 2: surface area effect (2 marks)
Crushed seeds (Trial 2) have a greater surface area in contact with water than whole seeds (Trial 1) [1]. Greater surface area provides more contact sites where cycasin can enter solution simultaneously, increasing the rate of dissolution and thus removing more cycasin (31% vs 12%) in the same time period [1].
Q2.3 — Trials 2, 3, 4: combined effects (2 marks)
Comparing Trial 2 (31%) and Trial 3 (48%) shows that water replacement alone (with whole seeds) increases extraction more than crushing alone (12% vs 31%): the concentration gradient maintained by regular water changes has a large effect [1]. Trial 4 (74%) shows that combining both surface area increase (crushing) and concentration gradient maintenance (water changes) produces a synergistic effect much greater than either factor alone — the two factors together maximise the rate of dissolution by simultaneously maximising contact sites and maintaining the driving concentration gradient [1].
Q2.4 — Trials 5 vs 6: temperature effect (2 marks)
Higher temperature (35 °C vs 20 °C) increases the kinetic energy of cycasin molecules within the seed tissue, increasing their rate of diffusion through the seed matrix to the surface [1]. Higher temperature also increases the rate of both dissolution and hydration of cycasin at the seed-water interface, shifting the dissolution equilibrium further right and increasing the overall extraction to 97% [1]. Accept also: increase in cycasin solubility at higher temperature (endothermic dissolution of cycasin).
Q3 — Cause-and-effect chain (5 marks)
Effect 1: The concentration of Na+(aq) and Cl−(aq) decreases (dilution). [1]
Effect 2: The solution is no longer at equilibrium; the ion product [Na+][Cl−] falls below Ksp. [1]
Effect 3: LCP shifts the equilibrium to the right (forward direction) to partially counteract the decrease in ion concentrations. [1]
Effect 4: More NaCl(s) dissolves, releasing Na+(aq) and Cl−(aq) until equilibrium is re-established. [1]
Overall outcome: Adding water causes more NaCl(s) to dissolve; the amount of undissolved solid decreases. Concentration of ions returns to the saturation value once equilibrium is re-established. [1]
Q4 — GAB bore water accumulation (5 marks)
(i) Contact time. Groundwater travels through aquifer rock over thousands to millions of years. Extended contact time allows slow dissolution of minerals (e.g. CaCO3, NaCl-bearing rock) to continue until the groundwater approaches saturation. Unlike a classroom experiment, there is no time limit on achieving equilibrium [1].
(ii) Temperature and pressure. Deep underground, temperature and pressure are significantly higher than at the surface. Higher temperature increases the rate of dissolution (kinetic effect: greater molecular energy at grain surfaces) and, for endothermic dissolution reactions, shifts the equilibrium further right (LCP), increasing solubility [1]. Increased pressure can enhance dissolution of certain minerals by compressing ion-dipole interactions and raising effective concentrations [1].
(iii) Water replenishment. The GAB is continuously recharged by rainfall infiltrating through recharge zones. Fresh, low-ionic water entering the aquifer re-establishes a concentration gradient between the water and mineral surfaces [1]. By LCP, this shifts the mineral dissolution equilibrium to the right — the same mechanism as running water in cycad detoxification. Over geological timescales, this continuous replenishment maximises mineral dissolution across the entire aquifer system [1].