Five Groups, Five Anomalies, Ten Marks
Your Class 9 practical on separating a sand–salt–water mixture runs in five groups of six. The expected mass of recovered dry salt is 19.0 g. Results: Group 1 recovered 18.2 g; Group 2's filtrate is still cloudy; Group 3's 'dry' salt is damp and weighs 21.4 g; Group 4's thermometer read 96 °C when distillate began to collect, and they report that water 'boils at 96 °C'; Group 5 finished in 12 minutes and one student tasted the distillate to 'check it is pure'. The lab has one balance accurate to 0.1 g, an unreliable electrical supply, and no fume hood. The textbook states that mass is conserved. A student asks: 'If mass is conserved, where did Group 1's 0.8 g go - did the salt disappear?' The practical is worth 10 marks, and the inherited rubric gives 8 marks for a neat record and 2 for the result.
1. Diagnose each group's anomaly as technique error, measurement/calibration error, or conceptual confusion, and state the single action you would take for each. Then answer the student's conservation question.
Read the worked answer
| Group | Result | Diagnosis | Action |
|---|---|---|---|
| 1 | 18.2 g vs 19.0 g | Transfer loss (technique), not vanished matter | Rinse the dish and filter paper into the account; teach a closed-system mass balance over every vessel |
| 2 | Cloudy filtrate | Technique: torn paper, pouring above the rim, or no settling | Identify the cause before re-filtering; ask them to name it |
| 3 | 21.4 g, damp | Incomplete drying - mass too high | Dry to constant mass; explain why a damp mass is not the salt mass |
| 4 | 96 °C 'boiling' | Calibration or ambient pressure, not a broken law | Compare with a second thermometer; explain that boiling point depends on pressure |
| 5 | Tasting | Safety violation, but a sound instinct about testing purity | Stop, explain the risk, redirect to a safe test |
The conservation question: the answer is not 'the salt disappeared' but 'matter is conserved in the system; the 0.8 g stayed on the glassware, the filter paper or the bench'. Make the class account for every vessel - that single exercise fixes the concept and the technique together.
The 96 °C reading is a chance to separate 'what the thermometer says' from 'what the definition says': pure water boils at 100 °C only at 1 atm, and a thermometer can also be out of calibration. The concept is intact; the measurement is not.
2. How will you handle the tasting incident so that the safety rule holds without shutting down the student's curiosity?
Read the worked answer
- Stop the group immediately, take the sample away, and check for symptoms. Even a 'pure' distillate has passed through glassware that may carry residues.
- Do not shame the student. The instinct - test whether it is pure - is scientific; the method is not. Name the risk precisely: unknown contamination, and the habit generalises dangerously to acids and unknowns.
- Replace the test with a safe one: evaporate a drop on a watch glass and look for residue. Discuss what that evidence does and does not prove about purity.
- Log the incident as the school's laboratory policy requires, and use three minutes of the next class for a student-led rule-setting discussion ('what may never be tasted, and why'), so the rule is owned rather than imposed.
- Check first-aid and eyewash access while the near-miss is fresh; this is the cheapest moment to fix the routine.
The teaching aim is a class that still asks 'is it pure?' but reaches for a watch glass, not a tongue.
3. Redesign the 10-mark rubric so it rewards process rather than neatness, and adjust the practical for the unreliable electrical supply.
Read the worked answer
Rebalanced 10-mark rubric
- Planning and prediction (2): written prediction of the mass and the reason for it.
- Technique (3): filtration, evaporation, drying to constant mass - observed by the teacher at the bench.
- Data and calculation (2): mass account for every vessel, percentage recovery calculated.
- Explanation (2): one anomaly explained in the record.
- Safety and lab discipline (1).
Neatness gets no separate marks; if legibility matters, fold it into one criterion. Otherwise the rubric rewards handwriting over chemistry, which is exactly what the inherited version did (8 marks for a neat record).
Guard against group-level masking: add a 2-mark individual viva - 'why is your salt mass higher than 19.0 g?' - and mark the record individually, since a group can produce one correct answer and five non-understanders.
Electrical constraint: run distillation as a single teacher-led demonstration for the whole class, and have the other four groups do evaporation on the same day. No group is idle, no group depends on a stable supply, and the demonstration still gives everyone the distillate observation.