HSC Exam Practice

Sample response. Leaching is the removal of a soluble substance from a solid by sustained contact with water (or other solvent). It is a physical change: the solute (cycasin) dissolves in water — a change of state from solid/gel to aqueous — but is not chemically transformed. No new substances are formed; cycasin retains its chemical identity and formula in solution, and could theoretically be recovered by evaporation of the water.

Marking notes. 1 mark for a correct definition of leaching (removal of soluble substance from solid by water); 1 mark for correctly classifying as physical change; 1 mark for justification (no new substance formed / toxin retains chemical identity / could be recovered by evaporation — any one valid justification).

Sample response. Cycasin is water-soluble (polar), so it dissolves readily in water and can diffuse out of the seed tissue into the surrounding water, driven by the concentration gradient. Water leaching is effective because the “like dissolves like” principle applies: a polar solute dissolves in a polar solvent (water). A non-polar, fat-soluble toxin does not interact favourably with the polar water molecules and therefore does not dissolve in water; it cannot diffuse into the water and cannot be leached. A non-polar solvent or a different treatment (e.g. heat decomposition) would be required.

Marking notes. 1 mark for identifying cycasin’s water solubility (polar/water-soluble) as the enabling property; 1 mark for explaining why a non-polar/fat-soluble toxin cannot dissolve in water (“like dissolves like” / insufficient interaction with polar water molecules); 1 mark for concluding that a different method would be needed (non-polar solvent or heat).

Sample response. As cycasin diffuses from the seed into the surrounding water, the concentration of cycasin in the water increases. In still water, this concentration continues to rise until the concentration gradient between seed interior and surrounding water approaches zero — at this point diffusion essentially stops and leaching ceases, even though cycasin may remain inside the seed. Changing the water (or using flowing water) removes the toxin-saturated water and replaces it with fresh water of near-zero cycasin concentration, restoring a steep concentration gradient. This steep gradient (high [cycasin] inside, low outside) drives continued rapid diffusion and sustained leaching until more cycasin is removed.

Marking notes. 1 mark for identifying that cycasin concentration in still water rises over time; 1 mark for stating that this reduces the concentration gradient until leaching slows/stops; 1 mark for explaining that water changes restore the gradient to maintain rapid leaching.

Sample response. A physical change is one in which no new substances are formed; the composition and chemical identity of the substances involved are unchanged (e.g. changes of state, dissolution). A chemical change is one in which new substances with different chemical identities are formed, involving the breaking and forming of chemical bonds. (a) Grinding seeds between stones is a physical change: the chemical composition of the seed material is unchanged; only the particle size and surface area change. (b) Microbial decomposition of cycasin during burial is a chemical change: microbial enzymes catalyse the chemical breakdown of cycasin molecules into new, structurally different, less toxic compounds. New substances are formed and the change cannot be reversed by simple physical means.

Marking notes. 1 mark for correct distinction (physical: no new substance; chemical: new substance formed with bonds broken/made); 1 mark for correctly classifying grinding as physical with a valid reason; 1 mark for correctly classifying burial/microbial as chemical; 1 mark for a valid reason for the chemical classification (enzymes catalyse decomposition; new substances formed).

Sample response. The law of conservation of mass (attributed to Lavoisier) states that in a chemical reaction the total mass of the reactants equals the total mass of the products: atoms are neither created nor destroyed, only rearranged. This requires that the number of atoms of each element must be equal on both sides of a balanced equation. Reaction type: precipitation (two aqueous solutions produce an insoluble precipitate — BaSO₄ is insoluble). Balanced equation: Na₂SO₄(aq) + BaCl₂(aq) → BaSO₄(s) + 2NaCl(aq). Atom check: 2Na, 1S, 4O, 1Ba, 2Cl each side. ✓

Marking notes. 1 mark for naming the law of conservation of mass and stating that atoms are neither created nor destroyed; 1 mark for correctly classifying as precipitation; 1 mark for the correctly balanced equation with state symbols (Na₂SO₄(aq) + BaCl₂(aq) → BaSO₄(s) + 2NaCl(aq)).

Sample response. Traditional ecological knowledge (TEK) meets the defining criteria of a scientific knowledge system: it is based on systematic and repeated observation, tested against outcomes (edibility, toxicity), refined and improved across generations, and transmitted as a coherent body of reliable knowledge. The specific practices embedded in cycad detoxification — choosing running water over still water to maintain concentration gradients, sequencing steps in a specific order, using grinding to increase surface area — reflect operational understanding of chemical principles. The validity of a knowledge system is determined by its reliability and internal logical structure, not by the equipment used. The absence of Western laboratory instruments does not disqualify TEK as scientific knowledge.

Marking notes. 1 mark for identifying that TEK is based on systematic observation and testing refined over generations; 1 mark for providing a specific example of chemical reasoning embedded in the practices (e.g. running water maintains concentration gradient; grinding increases surface area; sequential steps target different mechanisms); 1 mark for a clear statement that the validity of a knowledge system is determined by reliability and logical structure, not equipment used.

Sample response (a). Residual cycasin decreases rapidly from 245 mg/100 g at day 0 to 80 mg/100 g at day 3 (a reduction of 165 mg/100 g, approximately 67%). The rate of decrease slows substantially: from 80 mg at day 3 to 28 mg at day 7, then more slowly to 14 mg at day 14. The curve shows a steep initial decline that progressively flattens, approaching but not reaching zero.

Sample response (b). Initially the concentration of cycasin inside the seed is very high (245 mg equivalent in the seed tissue) and the flowing water provides near-zero external concentration, creating a very steep concentration gradient. This steep gradient drives rapid diffusion of cycasin out of the seed, resulting in fast leaching (67% removed in 3 days). As leaching progresses, the concentration of cycasin remaining in the seed decreases. The concentration gradient between seed interior and surrounding water therefore becomes shallower, so the net rate of diffusion slows. Between day 7 and day 14 so little cycasin remains in the seed that the gradient is very small, and the leaching rate approaches but does not reach zero.

Sample response (c). Based on the graph, between day 7 (28 mg) and day 14 (14 mg), the cycasin level crosses the 20 mg/100 g threshold. An interpolated reading suggests the threshold is crossed at approximately 8–9 days of soaking. The traditional 12-day protocol is consistent with this: at day 14 the level (14 mg/100 g) is below the 20 mg threshold, and a 12-day protocol would also produce a product below 20 mg/100 g. The community’s practice of 12 days provides a reasonable safety margin above the minimum required and is consistent with the data.

Marking notes. Part (a) — 1 mark for describing the steep initial decline with specific data values; 1 mark for noting that the rate decreases over time (curve flattens). Part (b) — 1 mark for stating the initial gradient is steep (high [cycasin] inside, low outside); 1 mark for explaining that the gradient decreases as cycasin is removed; 1 mark for linking reduced gradient to slower diffusion rate. Part (c) — 1 mark for correctly identifying that the threshold is crossed between day 7 and day 14; 1 mark for evaluating the 12-day protocol as consistent with and providing safety margin beyond the minimum.

Equation 1. Acid–base (neutralisation). Fe₂O₃(s) + 6HCl(aq) → 2FeCl₃(aq) + 3H₂O(l). Check: 2Fe, 6H, 6Cl, 3O each side. ✓

Equation 2. Precipitation (PbI₂ is insoluble). Pb(NO₃)₂(aq) + 2KI(aq) → PbI₂(s) + 2KNO₃(aq). Check: 1Pb, 2N, 6O, 2K, 2I each side. ✓

Equation 3. Acid–carbonate. CaCO₃(s) + 2HCl(aq) → CaCl₂(aq) + H₂O(l) + CO₂(g). Check: 1Ca, 1C, 2H, 2Cl, 3O each side. ✓

Marking notes. 2 marks per row: 1 mark for correct reaction type, 1 mark for correctly balanced equation with state symbols. Deduct the equation mark if state symbols are absent or incorrect but the balancing is correct (award 1/2 only). Do not award the equation mark for an unbalanced equation regardless of stated type.

Sample response. The claim that Aboriginal cycad detoxification methods are merely trial-and-error cooking steps is not defensible on chemical grounds. Each specific practice in the traditional protocol is chemically purposeful and cannot be explained as random trial and error.

The use of running water rather than still water is chemically significant because of the concentration gradient principle. As cycasin (a water-soluble glycoside) diffuses from seed tissue into the surrounding water, the external concentration rises. In still water, this rise reduces the gradient and eventually stops leaching when equilibrium is reached. Running water removes the toxin-saturated water continuously, maintaining a steep concentration gradient (high [cycasin] inside the seed, near-zero outside) and therefore sustaining rapid diffusion and leaching. The selection of running water specifically for this task — not still ponds — demonstrates that the knowledge system had refined the optimal process condition for gradient management.

Grinding seeds before soaking exploits surface area principles. A larger surface area increases the number of seed-water contact points per unit time, increasing the rate of diffusion and therefore the rate of leaching. A community that randomly applied cooking steps would have no systematic reason to grind seeds before water treatment; the inclusion of grinding as a preparatory step is only chemically justified in the context of surface area maximisation for leaching.

The sequential nature of the process — grinding, then extended water soaking, then roasting — is also not random. Water soaking is targeted at the water-soluble cycasin (physical leaching); roasting at moderate heat accelerates diffusion of residual toxin. Different steps use different chemical mechanisms, and the ordering reflects an applied understanding that the water step is the primary removal mechanism and must come before the heat step.

Traditional ecological knowledge is a knowledge system defined by systematic observation, testing, refinement, and reliable transmission of outcomes across generations. The cycad detoxification protocol precisely meets these criteria: the outcome (safe food) is reliably produced by the method, the method is refined (choosing running over still water, grinding before soaking), and it is transmitted as specific procedural knowledge. The absence of Western terminology for “concentration gradient” or “surface area” does not mean the underlying chemical logic is absent — it means it is expressed in a different but equally valid framework. The claim that these are cooking steps without chemical understanding is therefore incorrect.

Marking criteria.

• 1 mark — States a clear evaluative judgement that the “trial-and-error cooking steps” claim is not defensible.
• 1 mark — Explains the chemical principle (concentration gradient) underlying the running water practice and shows why running water is specifically superior to still water.
• 1 mark — Explains the chemical principle (surface area) underlying grinding and connects it specifically to increased leaching rate via diffusion.
• 1 mark — Identifies and explains a third chemical feature showing purposeful design (e.g. sequential ordering of steps, combination of physical and chemical processes targeting different toxin-removal mechanisms, or the application of heat to accelerate diffusion of residual toxin).
• 1 mark — Correctly defines or applies traditional ecological knowledge as a systematic, tested, refined knowledge system, and distinguishes it from the claim of random trial-and-error.
• 1 mark — Reaches an explicit final evaluation that integrates chemical evidence: the specific practices are chemically justified and constitute an applied chemistry knowledge system, not random steps.