HSC Exam Practice

Sample response. Filtration is a physical separation technique that separates an insoluble solid from a liquid using a porous barrier (filter paper), which allows the liquid to pass through but traps the solid. It is based on the difference in particle size between the insoluble solid and the liquid/dissolved substances.

Marking notes. 1 mark for a correct definition of filtration including the idea of a porous barrier/filter paper and insoluble solid; 1 mark for stating the separation basis (particle size).

Sample response. The residue is the insoluble solid that is trapped on the filter paper and does not pass through; it is collected from the filter paper. The filtrate is the liquid (and any dissolved substances) that passes through the filter paper; it is collected in the flask below the funnel.

Marking notes. 1 mark for correctly defining residue (solid on filter paper); 1 mark for correctly defining filtrate (liquid that passes through, collected in flask).

Sample response. Filtration separates mixtures based on particle size — it can only trap particles too large to pass through the filter paper. In a sodium chloride solution, NaCl is fully dissolved; it exists as individual Na⁺ and Cl⁻ ions dispersed at the molecular level throughout the water. These ions are far too small to be retained by the filter paper and pass straight through with the water, meaning no NaCl would be collected as residue. Crystallisation must be used instead.

Marking notes. 1 mark for stating that filtration is based on particle size; 1 mark for explaining that NaCl is dissolved (at the ionic/molecular level) and therefore particle size is too small to be trapped; 1 mark for concluding that NaCl passes through the filter paper with the water and cannot be separated this way.

Sample response. Technique: crystallisation. Steps in order: (1) Heat the KNO₃ solution in an evaporating basin over a Bunsen burner until the solution is concentrated/saturated. (2) Remove the heat and allow the solution to cool slowly; as temperature decreases, solubility decreases and excess KNO₃ crystallises out of solution. (3) Filter off the crystals using filter paper in a funnel; the crystals (residue) are collected on the filter paper and the mother liquor passes through. (4) Air-dry or dry in a low-temperature oven to obtain pure dry KNO₃ crystals.

Marking notes. 1 mark for correctly naming crystallisation; 1 mark for heating/concentrating the solution; 1 mark for slow cooling to form crystals; 1 mark for filtering off crystals and drying. Accept minor variations in step ordering. Award a maximum of 3 marks if the technique is not named.

Sample response. During slow cooling, solute particles have sufficient time to diffuse to a growing crystal face and arrange themselves into an ordered crystal lattice. Because the lattice is highly specific in its geometry, impurity particles (which have different shapes and sizes) are excluded from the structure and remain in solution. This produces large crystals of high purity. During rapid cooling, there is insufficient time for ordered lattice growth; solute particles crystallise out quickly in a disordered fashion, trapping impurity particles within the crystal structure and producing small, impure crystals.

Marking notes. 1 mark for explaining that slow cooling allows time for ordered lattice formation; 1 mark for explaining that impurities are excluded from the ordered lattice (stay in solution); 1 mark for contrasting with rapid cooling (disordered growth, impurities trapped, small crystals).

Sample response. The first question is: “Is the solid dissolved or undissolved (insoluble) in the liquid?” If the solid is undissolved (insoluble), its particles are large enough to be trapped on filter paper, so filtration is used. If the solid is dissolved, it passes through the filter paper with the liquid and cannot be separated by filtration; crystallisation must be used instead, as it exploits the decrease in solubility with decreasing temperature to precipitate the dissolved solid as crystals.

Marking notes. 1 mark for identifying the key question (dissolved or undissolved?); 1 mark for correctly linking “undissolved” to filtration with a reason; 1 mark for correctly linking “dissolved” to crystallisation with a reason.

Sample response (a). KNO₃ has a steep, curved solubility curve that increases dramatically with temperature (from ~13 g at 0 °C to 169 g at 80 °C). CuSO₄ has a moderate upward curve (from ~14 g at 0 °C to ~55 g at 80 °C). NaCl has a nearly flat curve (from ~36 g at 0 °C to ~38 g at 80 °C) [1 mark for comparing curve shapes]. When cooled from 80 °C to 20 °C: KNO₃ mass crystallised = 169 − 31 = 138 g; CuSO₄ mass crystallised = 55 − 20 = 35 g; NaCl mass crystallised = 38 − 36 = 2 g [1 mark for reading correct values from graph for at least two substances]. KNO₃ yields the greatest mass of crystals (138 g) because its solubility changes most steeply with temperature [1 mark for identifying KNO₃ with explanation].

Sample response (b). KNO₃ will crystallise out predominantly [1 mark]. This is because KNO₃ has a much steeper solubility curve than NaCl; when the solution cools from 80 °C to 20 °C, the solubility of KNO₃ decreases from 169 g to 31 g per 100 g water — a drop of 138 g — whereas NaCl solubility barely changes (from 38 g to 36 g, a drop of only 2 g) [1 mark for explanation using curves]. Maximum mass of KNO₃ crystallising = 169 − 31 = 138 g per 100 g water [1 mark for correct value]. NaCl remains largely in solution because its solubility is almost unchanged by the temperature decrease, so it does not reach saturation during the cooling [1 mark for explaining why NaCl stays dissolved].

Sample response (c). NaCl solubility changes only ~2 g/100 g water between 0 °C and 80 °C, so cooling a NaCl solution produces virtually no crystals [1 mark]. Evaporative crystallisation works by removing the solvent (water) through heating; as the volume of water decreases, the NaCl concentration increases until it exceeds the saturation point and NaCl crystallises out, regardless of the flat solubility curve [1 mark].

Marking notes. Part (a): 1 + 1 + 1 marks as above. Part (b): 1 + 1 + 1 + 1 marks as above. Part (c): 1 + 1 marks as above. Graphs must be read to within ±3 g; accept estimates in this range.

Sample response. Filtration and crystallisation are complementary physical separation techniques, each exploiting a different physical property of a mixture. Filtration works on the basis of particle size: insoluble solid particles are too large to pass through filter paper, while the liquid and dissolved substances pass through as filtrate. It is most appropriate when the target substance is insoluble — for example, collecting dry chalk (CaCO₃) from chalk-in-water, or removing undissolved sand from a brine sample. Filtration is fast and requires minimal equipment, making it highly useful in industry and laboratories. However, its key limitation is that it cannot separate two dissolved substances from each other, nor can it recover a dissolved solid from solution. If the target substance is soluble, it passes straight through the filter paper and is lost in the filtrate. Crystallisation exploits the decrease in solubility with temperature (or the reduction in solvent volume via evaporation) to force a dissolved substance out of solution as pure crystals. It is most appropriate when the target substance is dissolved — for example, obtaining KNO₃ crystals from a hot saturated solution by slow cooling, or producing NaCl crystals by evaporation. Crystallisation also functions as a purification technique, not just a separation technique: the ordered crystal lattice preferentially incorporates the target substance while excluding impurities, which remain in the mother liquor. Each recrystallisation cycle further improves purity. However, crystallisation has limitations: substances with flat solubility curves (like NaCl) cannot be purified efficiently by temperature-change crystallisation alone and require evaporative methods. Furthermore, a single crystallisation cycle does not produce chemically pure product if the impurities are also soluble in the same solvent. In many real-world scenarios, both techniques must be used sequentially. For example, to purify salt from a mixture of sand, grit, and dissolved salt: filtration is performed first to remove insoluble particles, followed by crystallisation of the filtrate to recover pure NaCl. Neither technique alone achieves both steps. In conclusion, filtration and crystallisation are highly useful physical methods with complementary strengths: filtration is fast and effective for insoluble solids; crystallisation provides purification for dissolved solids but may require multiple cycles and method adjustments for substances with low solubility-temperature sensitivity.

Marking criteria (7 marks). 1 = correctly identifies the physical property exploited by filtration (particle size) with a clear explanation. 1 = correctly identifies the physical property exploited by crystallisation (solubility change with temperature / solvent reduction) with a clear explanation. 1 = discusses conditions under which filtration is most appropriate, with a named real-world or laboratory example. 1 = discusses conditions under which crystallisation is most appropriate, with a named example. 1 = evaluates whether either technique alone is sufficient to purify a substance (crystallisation can purify but requires multiple cycles; filtration alone cannot remove dissolved impurities). 1 = identifies at least one significant limitation of each technique or of using them in isolation (e.g. filtration misses dissolved impurities; single crystallisation inadequate for soluble impurities). 1 = reaches an explicit evaluative judgement integrating both techniques as complementary tools, referencing at least two named contexts.