HSC Exam Practice

Sample response. A physical change alters the form or state of a substance without producing any new substance — the chemical identity remains the same (e.g. melting, dissolving). A chemical change produces one or more new substances with different chemical properties from the reactants (e.g. rusting, combustion). The defining criterion is whether a new substance is formed.

Marking notes. 1 mark for a correct definition of physical change (no new substance); 1 mark for a correct definition of chemical change (new substance formed); 1 mark for explicitly stating the single defining criterion (new substance formation) that distinguishes them.

Sample response. (1) Colour change — new substance absorbs different wavelengths of light. (2) Gas evolved — bubbling or fizzing indicating a new gaseous product. (3) Precipitate formed — insoluble solid appearing from solution. (4) Temperature change — energy released (exothermic) or absorbed (endothermic) as bonds are broken and formed. (5) Solid disappearing (consumed in reaction) — reactant solid incorporated into a new product.

Marking notes. 1 mark per correctly named indicator. Accept minor variations in wording (e.g. “effervescence” for gas evolved). Do not award marks for “light produced” or “smell change” unless clearly qualified as indicating a new substance.

Sample response. The Law of Conservation of Mass states that atoms are rearranged but never created or destroyed in a chemical reaction. Because the same atoms must appear on both sides of the equation (just in different arrangements), the number of each type of atom on the reactant side must equal the number on the product side. If the equation is unbalanced, it implies atoms have been gained or lost, which violates conservation of mass.

Marking notes. 1 mark for stating that atoms are rearranged but not created or destroyed; 1 mark for linking this to atom counts being equal on both sides of the equation; 1 mark for explaining that an unbalanced equation implies mass gain or loss (violates the law).

Sample response. A reversible reaction can proceed in both forward and reverse directions under the same conditions, shown with ⇌; for example, the dissolution and crystallisation of sodium chloride: NaCl(s) ⇌ Na⁺(aq) + Cl⁻(aq). An irreversible reaction proceeds essentially to completion in one direction only, shown with →; for example, the combustion of methane: CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(g). The products of an irreversible reaction cannot be converted back to the original reactants under the same conditions.

Marking notes. 1 mark for correct definition of reversible reaction (both directions, same conditions); 1 mark for a valid reversible example with ⇌ notation; 1 mark for correct definition of irreversible reaction (one direction, to completion); 1 mark for a valid irreversible example with → notation.

Sample response. The student is incorrect because a temperature change alone is insufficient evidence for a chemical change. Dissolving ammonium nitrate in water is an endothermic physical change: the NH₄⁺ and NO₃⁻ ions remain chemically ammonium nitrate in solution — no new substance with different chemical properties is formed. The correct classification is a physical change because the defining criterion for a chemical change (formation of a new substance) is not met.

Marking notes. 1 mark for identifying the flaw (temperature change alone is not a defining criterion); 1 mark for correctly classifying dissolution of NH₄NO₃ as a physical change with reasoning (no new substance); 1 mark for explicitly stating the correct defining criterion (new substance formation) and applying it.

Sample response. The colour produced in a firework is generated by a chemical reaction — the combustion of a metal salt — that produces a new oxide substance with different properties from the original salt. During combustion, electrons in the metal atoms absorb energy and fall back to lower energy levels, emitting specific wavelengths of visible light as colour. This colour is therefore the result of new substance formation, not merely a change in physical state. Example: strontium nitrate Sr(NO₃)₂ burns to produce strontium oxide (SrO) and emits red light; the original salt is a white powder, confirming a new substance was formed.

Marking notes. 1 mark for explaining that the colour is produced by a chemical reaction (new substance formation / combustion), not a physical change; 1 mark for correctly explaining the mechanism (electron energy transition / new substance with different properties); 1 mark for naming a specific metal salt and its corresponding colour (strontium/red, barium/green, copper/blue, sodium/yellow — any valid pair).

Sample response (a). The total mass decreases rapidly from 312.5 g at time zero to approximately 310.8 g by 180 s, then levels off to about 310.3 g by 360 s as the reaction slows and approaches completion [describe trend]. The total mass decreases even though the Law of Conservation of Mass holds because CO₂ gas produced in the reaction escapes from the open flask to the atmosphere. The Law of Conservation still holds if the escaped gas mass is included — the system (flask + contents + escaped CO₂) has a constant total mass [explain apparent contradiction].

Sample response (b). Mass change = 312.5 − 310.3 = 2.2 g. The 2.2 g of CO₂ gas escaped from the open flask. This is consistent with the Law of Conservation of Mass because the 2.2 g of gas came from the reaction of marble chips and HCl — atoms were rearranged into CO₂ but were not destroyed. If the flask were sealed, the total mass recorded would remain constant at 312.5 g.

Sample response (c). The graph of total mass vs time would be approximately horizontal — the total mass would remain constant at 312.5 g throughout the reaction. With a sealed balloon trapping the CO₂, no gas escapes; all atoms (and therefore all mass) remain within the system. This directly demonstrates conservation of mass in a closed system.

Marking notes. Part (a): 1 mark for correctly describing the trend (rapid initial decrease levelling off); 1 mark for identifying CO₂ escaping as the cause; 1 mark for clarifying that conservation of mass still holds in a closed system. Part (b): 1 mark for correctly reading the mass change (2.2 g); 1 mark for identifying 2.2 g as the escaped CO₂ mass; 1 mark for linking to conservation of mass (atoms rearranged, not destroyed). Part (c): 1 mark for predicting a flat/constant mass line; 1 mark for explaining that no gas escapes and all mass remains within the closed system.

Sample response. Observable indicators of chemical change — such as colour change, gas evolved, precipitate formation, temperature change, and solid disappearing — are highly useful practical tools because they can be detected immediately during an experiment without specialist equipment. They provide real-time, qualitative evidence that a new substance may have formed. For example, when iron ore is reduced to molten iron at the Port Kembla steelworks, the colour change from red iron oxide to silver-grey molten iron, combined with the production of CO₂ and CO gases, strongly suggests a chemical change is occurring. Similarly, when calcium carbonate marble chips react with hydrochloric acid, the vigorous CO₂ bubbling and dissolution of the solid are immediately visible indicators. However, indicators have significant limitations as a sole diagnostic tool. A temperature change, for instance, can accompany both physical changes (endothermic dissolution of NH₄NO₃) and chemical changes (exothermic reaction of Zn with H₂SO₄); temperature alone cannot distinguish between the two. Colour changes can occur during physical processes such as the colour-sorting of coloured glass (heating changes the glass shape but not its chemical identity). Indicators are also qualitative — they cannot confirm the identity or quantity of the new substance formed. The Law of Conservation of Mass provides a more rigorous test: in a sealed system, if the total mass of products equals the total mass of reactants, the atom count is conserved, confirming atoms were rearranged. If the sealed-system mass changes unexpectedly, there is evidence of a systematic error. Applied to the iron smelting example: weighing the sealed blast furnace input (iron ore + coke + oxygen) and output (iron + CO₂) quantitatively demonstrates conservation — no atoms were created or destroyed, only rearranged into new products. In summary, indicators are valuable as fast, accessible, practical evidence, but they are most reliable when multiple indicators are observed together and confirmed by conservation of mass measurements. Relying on a single indicator risks misclassification, as the ammonium nitrate example shows.

Marking criteria (7 marks). 1 = identifies and explains at least two strengths of using observable indicators (e.g. fast, no specialist equipment, real-time evidence). 1 = identifies and explains at least two limitations of relying on indicators alone (e.g. temperature change is ambiguous; single indicators can mislead). 1 = named Australian industrial or everyday example used correctly to illustrate a strength of indicators (Port Kembla steelworks, fireworks, acid + marble chips, bushfire). 1 = second named example (can be same context applied to a limitation, or a different context). 1 = explains the Law of Conservation of Mass as a more rigorous test — with reference to atoms being rearranged and mass being conserved in a closed system. 1 = correctly contrasts a scenario where indicators suffice with one where conservation of mass is needed to confirm (e.g. NH₄NO₃ dissolving where temperature change is misleading). 1 = reaches an explicit evaluative judgement integrating both indicators and conservation of mass as complementary tools, not independent alternatives.