Chemistry • Year 11 • Module 1 • Lesson 5
IQ1 Consolidation — Classification and Separation
Apply your understanding of technique selection, gravimetric analysis data, and error sources to real scenarios and quantitative data.
1. Interpret experimental data — selecting separation techniques at a water treatment plant
A water treatment plant receives raw water from a river. The table below lists six components present in the raw water, along with key observations. Complete the table by identifying the most appropriate separation technique and the key property that technique exploits. 8 marks
| Component to remove | Key observation | Technique | Property exploited |
|---|---|---|---|
| Fine sand and silt | Visible suspended solid particles; do not dissolve in water | ||
| Dissolved chloride ions (Cl−) | Fully dissolved in water; react with Ag+ to form insoluble AgCl | ||
| Dissolved organic dyes | Multiple coloured compounds dissolved in water; different polarities | ||
| Dissolved NaCl (for salt recovery) | NaCl solubility decreases sharply on cooling; no other dissolved salts present | ||
| Ethanol contaminant in water | Ethanol and water are miscible; b.p. ethanol = 78 °C, water = 100 °C | ||
| Exact sulfate concentration (SO42−) | Need a quantitative result (mass in mg/L); sulfate reacts with Ba2+ to form insoluble BaSO4 |
1.1 Complete the “Technique” and “Property exploited” columns in the table above. 6 marks (1 per correct technique–property pair)
1.2 Explain why filtration alone is not sufficient to remove dissolved chloride ions from the water, and state what additional technique must be used. 2 marks
2. Interpret graph — gravimetric analysis: drying the precipitate to constant mass
A student collected a BaSO4 precipitate during a gravimetric analysis. She dried the precipitate in an oven at 120 °C and measured its mass every 20 minutes. The graph below shows the data. 7 marks
Figure 2. Mass of BaSO4 precipitate during drying at 120 °C. Illustrative data. Reaction: Ba2+(aq) + SO42−(aq) → BaSO4(s).
2.1 Describe the trend in precipitate mass over time and identify when constant mass was first reached. 2 marks
2.2 Calculate the total mass of water removed during drying and explain what would happen to the calculated sulfate result if the student had stopped drying after only 40 minutes and used the mass at that point. 2 marks
2.3 A second student claims: “I stopped drying when the mass stopped decreasing between two consecutive weighings, so I have definitely reached constant mass.” Evaluate whether this is a valid procedure and explain your reasoning. 3 marks
3. Compare gravimetric analysis and chromatography across five criteria
Complete the two-column table below. For each criterion, write a concise description that contrasts the two techniques. 10 marks (1 per cell)
| Criterion | Gravimetric analysis | Chromatography |
|---|---|---|
| Type of result (qualitative or quantitative) | ||
| Property exploited | ||
| Type of mixture it suits best | ||
| What is measured / recorded | ||
| One Australian context example |
4. Predict and justify — a mining scenario
A gold mine near Kalgoorlie uses a cyanide leach process to dissolve gold from crushed ore. The resulting solution contains dissolved gold ions (Au+), dissolved cyanide, and fine rock particles. A metallurgist needs to (i) remove the fine rock particles from the solution and (ii) determine the exact mass of gold ions in a 1 L sample. 5 marks
4.1 Identify the most appropriate technique for step (i) and justify your choice by referencing the key property being exploited. 2 marks
4.2 The metallurgist adds a reagent that precipitates the gold ions as an insoluble gold compound. Predict the full procedure she should follow from this point to obtain a quantitative result. Include all steps and explain why each is necessary. 3 marks
Q1.1 — Technique selection table (6 marks)
Fine sand and silt: Filtration • Particle size (insoluble solid; particles retained by filter paper). Dissolved Cl−: Gravimetric analysis • Forms insoluble AgCl precipitate with Ag+; precipitate mass used with stoichiometry for quantification. Dissolved organic dyes: Chromatography • Differential affinity for stationary/mobile phases (different polarities). NaCl: Crystallisation • Solubility changes with temperature (NaCl crystallises out on cooling). Ethanol/water: Simple distillation • Boiling point difference (ethanol 78 °C, water 100 °C — large enough for simple distillation). Sulfate concentration: Gravimetric analysis • Forms insoluble BaSO4 precipitate; quantitative measurement by weighing.
Q1.2 — Why filtration fails for dissolved Cl− (2 marks)
Dissolved Cl− ions are too small to be retained by filter paper — they pass through with the filtrate [1]. The appropriate technique is gravimetric analysis: add excess AgNO3 solution to precipitate Cl− as AgCl, then filter, wash, dry and weigh the precipitate [1].
Q2.1 — Trend and constant mass (2 marks)
The mass of the precipitate decreases steadily from 0.512 g at time zero as water evaporates during drying [1]. The mass levels off and remains constant at 0.466 g from approximately 100 minutes onwards, indicating that constant mass has been reached at 100 minutes [1].
Q2.2 — Mass of water removed and effect of early stopping (2 marks)
Mass of water removed = 0.512 − 0.466 = 0.046 g [1]. If the student used the mass at 40 minutes (approximately 0.485 g), the measured mass would be higher than the true dry precipitate mass (0.466 g). This would lead to an overestimate of the sulfate concentration in the original sample, because the residual moisture adds to the apparent mass of BaSO4 [1].
Q2.3 — Evaluate the drying procedure (3 marks)
The procedure is valid in principle but only if the student used sufficient drying intervals between weighings [1]. Two consecutive weighings with no change confirms there is no more moisture to drive off — this is the definition of constant mass, and it is the correct criterion to use [1]. However, if the interval between weighings was very short (e.g. only 2 minutes), the oven may not have had enough time to dry the sample between measurements, giving a false “constant” reading. To be rigorous, the student should use intervals of at least 15–20 minutes per drying cycle and use a desiccator to cool the sample before each weighing (to prevent reabsorption of atmospheric moisture) [1].
Q3 — Compare and contrast table
Type of result: Gravimetric: Quantitative (exact mass / concentration). Chromatography: Qualitative (identity / number of components; can be semi-quantitative with instrumentation).
Property exploited: Gravimetric: Formation of an insoluble precipitate + mass measurement. Chromatography: Differential affinity for stationary and mobile phases.
Mixture it suits: Gravimetric: A solution containing a specific ion that forms an insoluble precipitate with a known reagent. Chromatography: A mixture of dissolved compounds that differ in polarity or structure (e.g. amino acids, pigments, drugs).
What is measured: Gravimetric: Mass of the dried precipitate on an analytical balance. Chromatography: Distance travelled by each spot relative to the solvent front (Rf value); or peak area in instrumental methods.
Australian context: Gravimetric: Sydney Water testing tap water for sulfate (BaSO4 precipitation method). Chromatography: WADA anti-doping testing of athletes at the Australian Open or Commonwealth Games; analysis of plant pigments from eucalyptus leaves.
Q4.1 — Remove fine rock particles (2 marks)
Technique: filtration [1]. Justification: The rock particles are insoluble solids suspended in the solution. Filtration exploits the difference in particle size — the solid rock particles are retained by filter paper while the dissolved gold ions, cyanide, and water pass through in the filtrate [1].
Q4.2 — Gravimetric procedure for gold (3 marks)
Step 1 — Filter the precipitate: after the gold compound precipitates, filter the solution through ashless filter paper to collect all the gold precipitate; necessary to separate the solid from the solution [1]. Step 2 — Wash with distilled water: rinse the precipitate with distilled water to remove any co-precipitated impurities or cyanide ions adsorbed to the surface; necessary to ensure only pure gold compound is weighed [1]. Step 3 — Dry to constant mass and weigh: dry the precipitate in an oven, cooling in a desiccator, and weigh repeatedly until the mass stops changing; then use stoichiometry (balanced equation, molar masses of gold compound and Au) to calculate the mass of gold ions in the original 1 L sample [1].