HSC Exam Practice

Sample response. Coagulation is the addition of a coagulant (typically alum, Al₂(SO₄)₃) to water, which provides Al²⁺ ions that hydrolyse to form colloidal Al(OH)₃. This destabilises the surface charges of fine suspended particles, allowing them to begin aggregating into larger clusters.

Marking notes. 1 mark for addition of alum/coagulant with resultant Al(OH)₃ or Al²⁺ hydrolysis; 1 mark for the outcome (destabilises particles / neutralises charge / promotes aggregation).

Sample response. The conventional drinking-water treatment train proceeds: (1) raw water intake/screening (remove large debris); (2) coagulation and flocculation (destabilise and aggregate fine particles); (3) sedimentation (flocs settle); (4) filtration through sand/gravel/activated carbon (remove remaining particles and dissolved organics); (5) disinfection (chlorination, UV or ozone to kill pathogens); (6) distribution to consumers (with residual disinfectant maintained).

Marking notes. 1 mark per correct stage in logical order, up to 3 marks. Accept any five stages correctly named in sequence; deduct if order is clearly illogical.

Sample response. Cl₂(g) + H₂O(l) → HOCl(aq) + HCl(aq). HOCl (hypochlorous acid) is the more effective disinfectant species. The second equilibrium HOCl ⇌ H⁺ + OCl⁻ means at lower pH more HOCl is present.

Marking notes. 1 mark for correct balanced equation; 1 mark for identifying HOCl as the active species; 1 mark for second equilibrium or pH dependence.

Sample response. When chlorine is added to water, the equilibrium HOCl ⇌ H⁺ + OCl⁻ is established. HOCl is the more active disinfecting species. As pH increases, the equilibrium shifts to the right (Le Chatelier’s principle — higher [H⁺] is consumed by hydroxide); a greater proportion of chlorine exists as OCl⁻, which is a much weaker disinfectant. At pH 7.5 (approximately the pKa of HOCl), HOCl and OCl⁻ are present in equal proportions. Above pH 8 the dominant species is OCl⁻, meaning disinfection effectiveness decreases substantially. Water treatment plants therefore aim to keep treated water at pH 7–7.5 to maintain HOCl as the dominant species.

Marking notes. 1 mark for correct equilibrium HOCl ⇌ H⁺ + OCl⁻; 1 mark for HOCl as more active species; 1 mark for increase in pH shifts equilibrium to produce more OCl⁻; 1 mark for implication — disinfection effectiveness decreases at higher pH.

Sample response. Chloramines form fewer DBPs than free chlorine because they react less extensively with natural organic matter (NOM), producing substantially fewer trihalomethanes. They also provide a useful residual disinfectant in the distribution network. Their main limitation is that they disinfect more slowly than free chlorine. UV radiation avoids chlorine-based DBPs entirely, but it leaves no residual disinfectant in the distribution system, making it unsuitable as the sole disinfectant for a large pipe network.

Marking notes. 1 mark for chloramines having fewer DBPs than free chlorine; 1 mark for chloramines providing a residual; 1 mark for UV having no DBPs; 1 mark for UV having no residual disinfectant in distribution.

Sample response. Fluoridation adds fluoride ions to drinking water at a concentration of approximately 0.6–1.1 mg L⁻¹ to reduce dental caries (tooth decay) in the community. The two reagents commonly used are sodium fluoride (NaF) and sodium hexafluorosilicate (Na₂SiF₆).

Marking notes. 1 mark for purpose (reduce dental caries); 1 mark for concentration range (~0.6–1.1 mg L⁻¹); 1 mark for naming NaF or Na₂SiF₆ (accept either one or both).

Part (a) — 1 mark. Chloramines (62 μg L⁻¹) and UV + chloramines (38 μg L⁻¹) both fall below the WHO guideline of 100 μg L⁻¹. Free Cl₂ (standard: 240; low dose: 148) both exceed the WHO guideline.

Part (b) — 4 marks. Free chlorine (HOCl) is a potent electrophile that reacts readily with the C=C bonds and aromatic rings present in natural organic matter (NOM), substituting chlorine atoms and producing halogenated compounds including trihalomethanes (CHCl₃, CHBrCl₂, etc.). At standard dose, THM = 240 μg L⁻¹ because HOCl reacts extensively with NOM. Chloramines (NH₂Cl and related species) are weaker electrophiles and react more slowly and less extensively with NOM, producing far fewer chlorinated by-products — THM = 62 μg L⁻¹, a 74% reduction. Chloramines are also in lower concentration when they are the primary disinfectant, and the lower reactivity means less halogenation of organic precursors. 1 mark for reactivity of HOCl with NOM forming THMs; 1 mark for comparison of THM values from the graph (240 vs 62); 1 mark for chloramines being weaker oxidants/electrophiles; 1 mark for linking weaker reactivity to lower THM yield.

Part (c) — 1 mark. UV leaves no residual disinfectant in the water. If microbial contamination occurs after treatment (e.g. in distribution pipes), there is no ongoing protection. UV cannot provide the residual disinfectant protection required throughout a large distribution network.

Part (d) — 3 marks. THM concentrations would decrease. Activated carbon adsorbs dissolved organic matter (NOM) before chlorination, reducing the concentration of THM precursors available to react with HOCl. With less NOM available, the same Cl₂ dose produces fewer halogenated by-products. This is why removing organic matter before the disinfection step is a critical part of managing DBP formation. 1 mark for predicting a decrease; 1 mark for mechanism (less NOM = fewer precursors for reaction with HOCl); 1 mark for explicitly connecting activated carbon removal of NOM to reduced THM formation.

Sample response. Reverse osmosis (RO) uses a semi-permeable membrane to separate water from dissolved ions under high pressure. Water molecules pass through the membrane while dissolved Na⁺, Cl⁻, Ca²⁺, Mg²⁺ and other ions are rejected, reducing TDS from seawater levels (~35,000 mg L⁻¹) to drinking-water quality (<100 mg L⁻¹). This makes RO the only practical technology for treating seawater. However, the process requires 3.5–4.2 kWh per kilolitre produced — far more than conventional surface-water treatment (0.3–0.6 kWh/kL). Perth’s Binningup Seawater Desalination Plant (Water Corporation) demonstrates this trade-off: it supplies ~100 GL/yr and is fully climate-independent, ensuring Perth’s water security during the declining rainfall regime of south-west WA. However, it produces ~1.5 L of concentrated brine per litre of product water, which must be managed to avoid harm to marine environments near the outlet. The claim that RO is always preferable is rejected. For cities with reliable catchments and low-TDS surface water (such as Sydney drawing from Warragamba Dam in normal years), conventional coagulation/filtration/chlorination is far cheaper and produces minimal brine waste. RO is only preferable when the source is saline or semi-arid climate makes surface water unreliable; the trade-off is higher energy cost and brine management. A context-dependent approach — using RO for climate security while maintaining surface-water treatment as a primary low-cost supply — is the most defensible position.

Marking criteria. 1 mark — Correct explanation of RO chemistry (semi-permeable membrane, applied pressure, ion rejection). 1 mark — Energy trade-off quantified or referenced (RO energy-intensive vs conventional treatment). 1 mark — Named Australian example with correct detail (Perth/Binningup or Kwinana SWDP, Water Corporation). 1 mark — Identifies the brine waste issue and its environmental implication. 1 mark — Identifies when RO is superior (saline source or climate-unreliable catchment) and when conventional is superior (reliable low-TDS source). 1 mark — Explicit rejection of “always preferable” with a context-dependent evaluative conclusion.