Chemistry • Year 11 • Module 1 • Lesson 12
Solubility and Like-Dissolves-Like
Build HSC Band 5–6 extended-response technique by analysing a real-world solubility scenario, evaluating student errors with data, and designing an investigation into “like dissolves like”.
1. Data + scenario: dry cleaning and solvent choice (Band 5–6)
8 marks Band 5–6
Scenario. Dry cleaning uses organic solvents (traditionally perchloroethylene, C2Cl4; now often hydrocarbon solvents or liquid CO2) instead of water to remove stains from clothing. The table below compares the solubility of four common fabric stains in water versus a typical dry-cleaning solvent (perchloroethylene).
| Stain substance | Molecular type | Solubility in water | Solubility in perchloroethylene (C2Cl4) |
|---|---|---|---|
| Grease (vegetable oil) | Non-polar lipid | Insoluble | Highly soluble |
| Candle wax | Non-polar long-chain hydrocarbon | Insoluble | Highly soluble |
| Sweat salts (NaCl, KCl) | Ionic | Soluble | Insoluble |
| Red wine (anthocyanins) | Polar organic molecule (multiple –OH groups) | Soluble | Sparingly soluble |
Illustrative data based on industrial dry-cleaning chemistry. Perchloroethylene is a slightly polar chlorinated solvent.
Q1. Analyse and evaluate the data above to explain how the principle of “like dissolves like” accounts for the effectiveness of dry cleaning. In your response you must:
- Classify perchloroethylene as polar or non-polar and justify your classification using its structure (C2Cl4: each carbon bonded to two chlorines; symmetric molecule).
- Use IMF reasoning to explain why grease and candle wax are dissolved by perchloroethylene but not by water.
- Explain why sweat salts dissolve in water but are not removed by dry cleaning, using the concept of ion-dipole forces.
- Predict and explain the behaviour of red wine anthocyanins, which have multiple –OH groups, in both solvents.
- Evaluate whether dry cleaning alone would be sufficient to clean a garment stained with both grease and sweat salts. Justify your evaluation with reference to the data.
2. Source critique — evaluate a student’s claim (Band 5–6)
7 marks Band 5–6
“You can use solubility as a general rule: any time two substances have the same type of bonding, they will dissolve in each other. Covalent compounds dissolve in covalent solvents and ionic compounds dissolve in ionic solvents. The only exception is when temperature is too low.”
Student notes from a Year 11 chemistry class.
Q2. The student’s claim contains multiple scientific flaws. Evaluate the claim by:
- Identifying and explaining at least three specific errors or oversimplifications in the claim, with reference to named examples from this lesson.
- Stating the correct general principle that governs solubility (with reference to IMFs, not bond type).
- Describing how you would demonstrate one of the errors experimentally, with reference to a named example.
3. Experimental design — investigating “like dissolves like” in the laboratory (Band 5–6)
7 marks Band 5–6
Research question: Does the polarity of a solvent determine whether iodine (I2) or sodium chloride (NaCl) preferentially dissolves in it?
Available materials: Solid I2 (purple), solid NaCl (white), water, hexane, ethanol, test tubes, stoppers, ruler, balance (0.01 g precision).
Q3. Design an experiment to investigate the research question. Your design must include:
- Hypothesis: State a testable prediction that links solvent polarity to which solute dissolves.
- Independent, dependent, and controlled variables.
- Method (step-by-step procedure) for testing both solutes in all three solvents (6 combinations).
- Expected results table (blank; columns: solute, solvent, polarity of solvent, expected observation, actual observation).
- One potential source of error in this method and a suggested improvement.
- How the results would confirm or falsify the hypothesis.
Q1 — Dry cleaning scenario (8 marks)
Marking criteria (8 marks).
1 mark — Correctly classifies perchloroethylene as approximately non-polar (symmetric arrangement of C–Cl bonds around each carbon; dipoles cancel; or classifies as weakly polar/non-polar) and links this to dispersion-force dominance.
1 mark — Correctly explains that grease and wax are non-polar: they interact with perchloroethylene via compatible dispersion forces (non-polar + non-polar), so they dissolve in the dry-cleaning solvent.
1 mark — Correctly explains why grease/wax are insoluble in water: water’s H-bond network is disrupted for no compensating gain (grease–water dispersion too weak), citing the “hydrophobic effect” or IMF incompatibility.
1 mark — Correctly explains sweat salts (ionic; NaCl, KCl) dissolve in water via ion-dipole forces (partial charges on water stabilise separated ions).
1 mark — Correctly explains sweat salts are insoluble in perchloroethylene because it cannot provide sufficient ion-dipole forces to overcome the ionic lattice energy (non-polar/weakly polar solvent ≠ compatible with ions).
1 mark — Correctly predicts that red wine anthocyanins dissolve well in water (multiple –OH groups form H-bonds with water) but only sparingly in perchloroethylene (polar groups incompatible with a largely non-polar solvent).
1 mark — Correctly evaluates that dry cleaning alone is insufficient for a garment with both grease and sweat salt stains, because the dry-cleaning solvent removes the non-polar grease but cannot remove ionic sweat salts, which require an aqueous (polar) wash.
1 mark — Reaches an explicit evaluative judgement integrating both solvents as complementary, with reference to IMF compatibility as the underlying principle throughout.
Q2 — Source critique (7 marks)
Sample response. The student’s claim contains several significant errors. Error 1: “Same type of bonding means dissolve in each other” is incorrect. Solubility is governed by IMF compatibility, not bond type. Ethanol (covalent) is miscible with water (covalent), but hexane (also covalent) and water are immiscible — covalent bonding is shared, yet they do not mix because their IMFs differ (H-bonding vs dispersion). Error 2: “Ionic compounds dissolve in ionic solvents” is factually wrong in the direction it implies. Ionic compounds dissolve in polar solvents (like water), not ionic solvents. Water is covalently bonded but highly polar — it dissolves NaCl through ion-dipole forces, not because it is ionic. Error 3: “Any covalent compound dissolves in any covalent solvent” is false. I2 (covalent, non-polar) does not dissolve in water (covalent, polar). Hexane (covalent, non-polar) and water (covalent, polar) are immiscible. The polarity of the covalent molecule determines compatibility, not the mere fact of covalent bonding. Error 4: Temperature is not the “only exception” — the claim ignores lattice energy effects (BaSO4 and AgCl are ionic but insoluble even at elevated temperatures because lattice energy exceeds hydration energy). Correct principle: Dissolution depends on the compatibility of the intermolecular forces between solute and solvent. Polar/ionic solutes dissolve in polar solvents; non-polar solutes dissolve in non-polar solvents. Experimental demonstration: Add I2 to separate test tubes of water and hexane. I2 will produce little colour in water but a clear brown/violet solution in hexane, demonstrating that two covalent substances (I2 in hexane) dissolve while two other covalent substances (I2 in water) do not — refuting the “same bonding type = dissolves” rule.
Marking criteria (7 marks): 2 marks — identifies and explains at least two specific errors with named examples (1 mark per error); 1 mark — identifies a third error; 1 mark — states the correct IMF-based principle; 1 mark — names a specific experimental demonstration; 1 mark — links the experimental observation to refuting the claim; 1 mark — overall quality and coherence of the critique.
Q3 — Experimental design (7 marks)
Marking criteria (7 marks).
1 mark — Testable hypothesis: e.g. “I2 (non-polar) will dissolve in the non-polar solvent (hexane) and produce a brown/violet colour, while NaCl (ionic) will dissolve in the polar solvent (water) producing a clear solution; neither will dissolve in the incompatible solvent.”
1 mark — Variables correctly identified: Independent = solvent type (water, hexane, ethanol); Dependent = whether solute dissolves (colour change / clarity observed); Controlled = mass of solute added, volume of solvent, temperature, time of mixing.
1 mark — Method includes all 6 combinations (I2 + water; I2 + hexane; I2 + ethanol; NaCl + water; NaCl + hexane; NaCl + ethanol) with consistent procedure (e.g. add a small equal mass of each solute to equal volume of each solvent in a stoppered test tube; shake 10 times; observe and record).
1 mark — Results table with correct columns (solute, solvent, polarity of solvent, expected observation, actual observation) and all 6 rows filled with expected results (I2: violet in hexane/ethanol; pale/no colour in water; NaCl: clear in water; undissolved in hexane; dissolves in ethanol to some extent).
1 mark — One valid source of error (e.g. different amounts of solute added; temperature variation; contamination between solvents) and a specific improvement (e.g. use a calibrated balance to measure 0.05 g of each solute; use separate spatulas).
1 mark — States how results confirm the hypothesis (I2 colour in hexane / NaCl clear in water) vs falsify it (I2 colour in water / NaCl dissolves in hexane).
1 mark — Correctly predicts ethanol results as intermediate: I2 dissolves in ethanol (dispersion from ethyl group) and NaCl partially dissolves in ethanol (weak H-bonding from –OH), demonstrating polarity spectrum rather than binary polar/non-polar.