HSC Exam Practice

Sample response. Miscible describes two liquids that mix in any proportion to form a homogeneous solution; e.g. ethanol and water. Immiscible describes two liquids that do not mix but form two distinct layers; e.g. oil and water (or hexane and water).

Marking notes. 1 mark for a correct definition of miscible; 1 mark for a correct definition of immiscible; 1 mark for a valid miscible example (ethanol + water; any two miscible liquids); 1 mark for a valid immiscible example (oil/hexane + water).

Sample response. Ethanol has a polar –OH group that can form hydrogen bonds with water molecules (O–H···O interactions). These H-bonds between ethanol and water are comparable in strength to those within pure water and pure ethanol, so the molecules mix readily — ethanol is miscible with water. Hexane is a non-polar molecule with only dispersion forces between molecules. When hexane approaches water, it disrupts water’s hydrogen-bond network but forms no compensating interactions — the energy cost is not recovered — so hexane and water do not mix; they are immiscible.

Marking notes. 1 mark for correctly identifying the –OH group in ethanol forming H-bonds with water as the reason for miscibility; 1 mark for explaining that hexane’s non-polar nature means only dispersion forces are available, which cannot compensate for disrupting water’s H-bond network; 1 mark for explicitly linking both cases to the IMF compatibility principle (“like dissolves like”).

Sample response. The error is that not all ionic compounds dissolve in water; the statement is an oversimplification. Solubility depends on whether the hydration energy released when water molecules surround the separated ions exceeds the lattice energy of the ionic solid. BaSO₄ has a very high lattice energy because both Ba²⁺ and SO₄²⁻ carry a 2+ or 2− charge. The hydration energy of these ions is insufficient to overcome the lattice energy, so dissolution is thermodynamically unfavourable and BaSO₄ remains insoluble despite being ionic.

Marking notes. 1 mark for identifying the error (not all ionic compounds are soluble in water; the rule is a tendency not a universal law); 1 mark for introducing lattice energy vs hydration energy as the correct criterion; 1 mark for correctly applying this to BaSO₄ (high lattice energy due to doubly charged ions; hydration energy insufficient).

Sample response. Water molecules approach the surface of the NaCl lattice. The partially negative oxygen atoms (δ− on O) orient toward the positively charged Na⁺ ions, and the partially positive hydrogen atoms (δ+ on H) orient toward the negatively charged Cl⁻ ions. These ion-dipole forces at the lattice surface overcome the interionic electrostatic attractions, pulling Na⁺ and Cl⁻ ions from the lattice. Each ion becomes surrounded by a shell of oriented water molecules (a hydration shell), stabilising the separated ions in solution.

Marking notes. 1 mark for correctly identifying the ion-dipole force and the orientation of water dipoles relative to each ion type; 1 mark for explaining that these ion-dipole forces overcome the interionic attraction in the lattice; 1 mark for describing the resulting hydration shell that stabilises the ions in solution.

Sample response. Hydrophilic substances (“water-loving”) are polar or ionic and interact favourably with water through H-bonding or ion-dipole forces. Example: glucose — dominant IMF with water is hydrogen bonding (via multiple –OH groups). Hydrophobic substances (“water-fearing”) are non-polar and do not interact favourably with water’s hydrogen-bond network. Example: hexane (or any non-polar hydrocarbon) — can only form very weak dispersion forces with water, which are insufficient to compensate for disrupting water’s H-bond network; hexane is therefore excluded from aqueous solution.

Marking notes. 1 mark for correct definition of hydrophilic (polar/ionic; favourable interaction with water); 1 mark for a valid hydrophilic example with the dominant IMF named; 1 mark for correct definition of hydrophobic (non-polar; unfavourable/no net interaction with water); 1 mark for a valid hydrophobic example with the dominant IMF stated (dispersion only) and an explanation of why it is insufficient.

Sample response. Soap molecules are amphiphilic: each molecule has a long hydrophobic non-polar hydrocarbon tail and a hydrophilic polar/ionic head group. The hydrophobic tails dissolve in the grease through dispersion forces (like dissolves like: non-polar tail in non-polar grease), surrounding the grease droplet with tails pointing inward. The hydrophilic heads point outward into the water, interacting with water through ion-dipole or H-bond forces. This arrangement forms a spherical structure called a micelle, with grease trapped inside. The micelle disperses into the water and can be rinsed away.

Marking notes. 1 mark for correctly describing soap as amphiphilic with a hydrophobic tail dissolving in grease and a hydrophilic head interacting with water; 1 mark for describing the correct orientation of the amphiphilic molecules in a micelle (tails in, heads out); 1 mark for explaining that the micelle disperses grease in water, allowing it to be rinsed away.

Sample response (a). I₂ is a non-polar covalent molecule: the two identical iodine atoms share electrons equally, so there is no net dipole [1 mark]. I₂ therefore has only dispersion forces (London/van der Waals) between molecules. Hexane is also non-polar with only dispersion forces — I₂–hexane interactions are compatible (both dispersion), so I₂ dissolves readily in hexane [1 mark]. Water is a polar solvent with a strong hydrogen-bond network. For I₂ to dissolve in water, this H-bond network would need to be disrupted; however, I₂ cannot form H-bonds and provides only weak dispersion interactions with water molecules — the energy cost is not compensated [1 mark]. By “like dissolves like” (non-polar solute in non-polar solvent), I₂ preferentially partitions into the hexane layer [1 mark].

Sample response (b). NaCl would be found entirely in the lower water layer [1 mark]. NaCl consists of Na⁺ and Cl⁻ ions that require ion-dipole forces with a polar solvent to remain dispersed. Water provides these forces through its permanent dipole. Hexane is non-polar and cannot provide ion-dipole forces; it cannot overcome the ionic lattice energy of NaCl, so NaCl remains in the polar water layer [1 mark].

Sample response (c). The claim is incorrect [1 mark]. I₂ does interact with water molecules through weak dispersion forces. The correct statement is that I₂–water dispersion interactions are too weak to compensate for the disruption of water’s hydrogen-bond network required to accommodate the I₂ molecule — so dissolution is thermodynamically unfavourable. I₂ is not completely without interaction with water; rather, the interactions are insufficient for significant dissolution to occur [1 mark].

Marking notes. Part (a): 1 mark each for: I₂ is non-polar; dispersion compatibility with hexane; H-bond disruption cost in water not compensated; “like dissolves like” conclusion. Part (b): 1 mark for predicting NaCl in water layer; 1 mark for explaining ion-dipole forces with water vs inability of hexane to provide them. Part (c): 1 mark for identifying the claim as incorrect; 1 mark for providing the accurate statement (weak dispersion forces exist but are insufficient to compensate for H-bond network disruption).

Sample response. The principle “like dissolves like” states that dissolution is favoured when the solute and solvent have compatible types of intermolecular forces. At the molecular level, dissolution requires three steps: breaking solute–solute interactions, disrupting solvent–solvent interactions, and forming solute–solvent interactions. Dissolution is thermodynamically favourable only when the energy released in step 3 compensates for steps 1 and 2 — which occurs when the IMF types are compatible. As a predictive tool, the principle has considerable strengths. For ionic/polar solutes in polar solvents: NaCl (ionic) dissolves in water (polar) because the strong ion-dipole forces between the partially charged water dipole and the ions compensate for both the lattice energy and the disruption of water’s H-bond network — correctly predicted by “like dissolves like”. For non-polar solutes in non-polar solvents: iodine (I₂, non-polar) dissolves in hexane (non-polar) because compatible dispersion forces require minimal energy trade-off — again correctly predicted. The principle is fast, requires no numerical data, and explains a wide range of everyday phenomena including oil spills, dry cleaning, and biological membrane structure. However, the principle has significant limitations that require qualification. Limitation 1: Not all ionic compounds dissolve in polar solvents. BaSO₄ and AgCl are ionic but insoluble in water because their lattice energies are so high that the hydration energy of their doubly or strongly charged ions is insufficient to overcome the lattice — “like dissolves like” (polar solvent + ionic solute) incorrectly predicts solubility without considering the lattice/hydration energy balance. Limitation 2: Some “like-type” pairs are only partially miscible. Butanol has a non-polar four-carbon chain and a polar –OH group; it is partially soluble in water but not fully miscible, because the hydrophobic tail increasingly dominates as chain length grows. Simple polarity matching does not predict partial miscibility. Limitation 3: The principle cannot predict the degree of solubility (sparingly vs highly soluble) — it is qualitative only. In conclusion, “like dissolves like” is a powerful first-pass predictive tool grounded in IMF compatibility, but it must be applied with knowledge of its limitations: high lattice energies can override polarity matching for ionic compounds, and amphiphilic or partially polar molecules may not fit a simple binary prediction.

Marking criteria (7 marks). 1 = Correctly states the molecular basis of the principle with reference to IMF compatibility and the three-step energy model of dissolution. 1 = Correctly applies the principle as a strength for a polar/ionic example with specific IMF reasoning (ion-dipole for NaCl in water or H-bonding for ethanol in water). 1 = Correctly applies the principle as a strength for a non-polar example with specific IMF reasoning (dispersion for I₂ in hexane or wax in oil). 1 = Identifies and explains Limitation 1: not all ionic compounds are water-soluble; uses a named example (BaSO₄, AgCl) with lattice energy vs hydration energy reasoning. 1 = Identifies and explains Limitation 2: amphiphilic or partially polar molecules do not fit simple binary predictions; uses a named example (butanol, or any amphiphilic molecule). 1 = Makes a clear evaluative judgement that integrates both strengths and limitations, positioning “like dissolves like” as a useful but simplified heuristic. 1 = Sustained and accurate use of IMF terminology throughout (ion-dipole, H-bond, dispersion, hydrophilic, hydrophobic) with no factual errors in the response.