Chemistry • Year 11 • Module 1 • Lesson 3
Separation Techniques — Physical Methods
Build HSC Band 5–6 extended-response technique on justifying technique choice, evaluating experimental evidence, and designing investigations involving filtration and crystallisation.
1. Data + scenario: recovering pure salt from a mine brine sample (Band 5–6)
8 marks Band 5–6
Scenario. A mining company in outback Western Australia extracts brine (salt water) from an underground aquifer. The raw brine contains dissolved NaCl, a small amount of dissolved MgSO4 (also soluble), and suspended fine clay particles (insoluble). The company wants to obtain high-purity NaCl crystals for industrial use. The table below summarises the composition of the raw brine and the solubility data relevant to the separation.
| Component | Concentration in raw brine | State in brine | Solubility at 20 °C (g/100 g water) | Solubility at 80 °C (g/100 g water) |
|---|---|---|---|---|
| NaCl | 28 g / 100 g water | Dissolved | 36 | 38 |
| MgSO4 | 2 g / 100 g water | Dissolved | 26 | 54 |
| Clay particles | 0.5 g / 100 g water | Suspended (insoluble) | Insoluble | Insoluble |
Illustrative data. NaCl solubility changes very little with temperature; MgSO4 solubility increases markedly with temperature.
Q1. Analyse and evaluate the data above to design a multi-step separation procedure that would allow the company to obtain high-purity NaCl crystals from the raw brine. In your response you must:
- Identify which component(s) can be removed by filtration and justify this with reference to the data.
- Explain, using the solubility data, why NaCl is not well-suited to purification by simple crystallisation via temperature change alone.
- Suggest and justify a modification to the crystallisation procedure (e.g. evaporative crystallisation) that would allow NaCl to be recovered despite its flat solubility curve.
- Evaluate whether one cycle of crystallisation would produce chemically pure NaCl, given the presence of MgSO4, and explain how purity could be improved.
- State one limitation of the procedure you have described and suggest one improvement.
2. Experimental design — testing whether filtration affects purity (Band 5–6)
7 marks Band 5–6
Research question. A Year 11 student claims that washing the residue on the filter paper with a small volume of cold distilled water after filtration does not improve the purity of the collected solid. Another student disagrees, arguing that washing removes dissolved impurities trapped in the residue. Design a scientific investigation to test which student is correct, using copper(II) sulfate contaminated sand as your model system. (Sand is insoluble; CuSO4 is dissolved.)
Constraints: Standard Year 11 laboratory equipment is available (balances with 0.001 g precision, filter paper, funnels, beakers, CuSO4 solution, distilled water, sand, drying oven at 80 °C). You have one school lesson.
Q2. Design the investigation and present it in the format below.
- State your hypothesis (a testable prediction including the independent and dependent variables).
- Identify the independent variable, dependent variable, and at least two controlled variables.
- Describe the procedure in at least five numbered steps, including how you will test for the presence of dissolved CuSO4 in or on the sand residue.
- Explain what result would falsify your hypothesis.
- State two limitations of your design and one way to improve reliability.
Q1 — Sample Band 6 response (8 marks), annotated
Step 1 — Filtration to remove clay: The clay particles are listed as “insoluble” in the data table. Because they are insoluble, their particles are large enough to be trapped on filter paper. Filtration is therefore the correct technique to remove clay from the brine: the clay collects as residue on the filter paper, while the NaCl and MgSO4 solutions pass through as filtrate [1 mark — correctly identifies filtration with data-based justification].
Why temperature-change crystallisation is inefficient for NaCl: The solubility data shows that NaCl solubility changes only slightly with temperature (36 g/100 g at 20 °C vs 38 g/100 g at 80 °C — a difference of only 2 g/100 g). Therefore, cooling a hot saturated NaCl solution from 80 °C to 20 °C would yield very little crystallisation [1 mark — uses data to explain why cooling crystallisation is poor for NaCl]. In contrast, MgSO4 solubility changes from 54 g/100 g at 80 °C to 26 g/100 g at 20 °C, so cooling would actually favour MgSO4 crystallisation, potentially increasing contamination of any NaCl that does crystallise.
Evaporative crystallisation modification: Instead of cooling, the filtrate should be heated in an evaporating basin to remove (evaporate) the water. As the volume of water decreases, the NaCl becomes supersaturated and crystallises out. Since both NaCl and MgSO4 are present, the substance that reaches saturation first will crystallise preferentially. By controlling the degree of evaporation (stopping before all water is removed), it is possible to crystallise primarily NaCl while keeping MgSO4 in solution [1 mark — correctly describes and justifies evaporative crystallisation].
Evaluation of purity after one cycle: One cycle of crystallisation will not produce chemically pure NaCl. Because MgSO4 is also dissolved in the filtrate, some MgSO4 will be incorporated into or adsorbed onto the NaCl crystal surface during crystallisation [1 mark — identifies MgSO4 as a persistent impurity]. To improve purity, the student should perform recrystallisation: redissolve the NaCl crystals in a minimum volume of hot water, then allow to crystallise again via evaporation; each cycle reduces MgSO4 contamination because NaCl crystallises preferentially when conditions are optimised [1 mark — describes recrystallisation and explains why it improves purity].
Limitation and improvement: One limitation is that evaporating all the water risks incorporating MgSO4 into the crystals if too much water is evaporated. Stopping the evaporation at the right point requires careful monitoring [1 mark]. Improvement: use a conductivity meter or refractometer to monitor concentration during evaporation; alternatively, test a drop of the concentrated solution to detect MgSO4 before crystallisation is complete [1 mark].
Overall structure and terminology: Award 1 additional mark if the response uses precise chemical language throughout (solubility, saturated, crystallisation, evaporative crystallisation, recrystallisation, residue, filtrate, mother liquor) and the procedure is presented in a logical, multi-step sequence.
Marking criteria summary (8 marks): 1 = identifies filtration for clay with data-based justification; 1 = explains why temperature-change crystallisation is inefficient for NaCl using solubility data (only 2 g difference); 1 = correctly describes and justifies evaporative crystallisation; 1 = identifies MgSO4 as a persistent impurity after one cycle; 1 = describes recrystallisation to improve purity; 1 = states one specific limitation; 1 = suggests a practical improvement; 1 = precise terminology throughout.
Q2 — Sample Band 6 response (7 marks), annotated
Hypothesis: If washing the residue with cold distilled water improves purity, then the washed sand residue will contain less CuSO4 (shown by lower mass of recovered CuSO4 after evaporating the wash water) compared to an unwashed control. Independent variable: whether the residue is washed with cold distilled water. Dependent variable: mass of CuSO4 remaining in/on the sand residue. Controlled variables: mass of sand (5 g each), volume and concentration of CuSO4 solution used (50 mL of 0.5 mol/L), grade of filter paper, volume of distilled water wash (10 mL) [1 mark — testable hypothesis with IV, DV, and at least two controlled variables].
Procedure: (1) Prepare two identical mixtures: mix 5 g sand with 50 mL 0.5 mol/L CuSO4 solution in separate beakers (Sample A and Sample B). (2) Filter both mixtures through identical filter paper in a funnel into a conical flask. Collect the filtrate from each in separate, labelled flasks. (3) For Sample B (washed): pour 10 mL cold distilled water slowly over the residue on the filter paper. Collect the wash liquid in a separate labelled flask. For Sample A (unwashed): do not add any water to the residue. (4) Allow both residues to dry in an oven at 80 °C for 30 minutes. (5) Evaporate the wash liquid from Sample B to dryness in an evaporating basin; weigh the remaining solid to determine the mass of CuSO4 that was washed off. Also observe whether the washed residue (Sample B) has less blue colour than the unwashed residue (Sample A) [1 mark — five clear steps including a method to detect remaining CuSO4].
Falsification: If the mass of CuSO4 recovered from evaporating the wash liquid (Sample B) is zero, or if both residues contain the same amount of CuSO4, the hypothesis would be falsified — washing would have no measurable purifying effect [1 mark].
Limitations: (1) Drying in the oven may be incomplete in 30 minutes; residual moisture could affect mass readings [1 mark]. (2) The blue colour comparison is qualitative and subjective; it may not detect small differences in CuSO4 concentration [1 mark].
Improvement: Use a conductivity meter to measure the electrical conductivity of the wash liquid; higher conductivity indicates more dissolved CuSO4 was removed by washing, providing a quantitative measure of purification. Repeat each condition three times and calculate a mean [1 mark].
Expected result: Washing the residue will remove dissolved CuSO4 trapped in the water film around the sand grains, reducing contamination. The wash liquid from Sample B will yield measurable CuSO4 on evaporation, and Sample B residue will be visibly less blue than Sample A, confirming that washing improves purity [1 mark].
Marking criteria summary (7 marks): 1 = testable hypothesis with IV, DV, and at least two controls; 1 = five-step procedure with a method to detect CuSO4 in the residue; 1 = states what would falsify the hypothesis; 1 = first valid limitation; 1 = second valid limitation; 1 = specific improvement to reliability; 1 = precise chemical terminology (solubility, filtrate, residue, CuSO4, evaporation, independent/dependent/controlled variable).