Running a diffusion practical with six beakers, 44 students and 35 minutes
Your Class VIII science section has 44 students and one 35-minute period. The prescribed activity is to show that diffusion is faster in hot water: potassium permanganate crystals in water at room temperature and in water at about 60 degrees Celsius. Your stock contains 6 glass beakers of 250 ml, 2 spirit thermometers, 1 electronic balance (200 g capacity, 0.01 g resolution), a supply of potassium permanganate crystals, hot water from the kitchen, and no safety goggles for students. Desks are fixed in rows of two. One student is on crutches after a leg injury; another has been photographed earlier that week playing with crystals from the store and has been told the store is out of bounds. The head of department expects a written practical record from every student and a one-line conclusion, and the lab assistant is available for the first ten minutes only.
1. Redesign the activity so that all 44 students do something hands-on in 35 minutes with six beakers. Show grouping, timing and role rotation.
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The binding constraint is 6 beakers, not 44 students, so the unit of work must be the group and the unit of learning must be the observation, not the handling of glassware.
Structure: 8 groups of 5–6. Each group receives one beaker of room-temperature water prepared in advance, and one group at a time collects hot water. Six groups work simultaneously; two groups start on a written prediction task and rotate in.
Timing: 0–4 min, whole class: the question and the prediction sentence, written by every student. 4–8 min: group leaders collect water; the two thermometers circulate, one measuring room temperature and one the hot beaker. 8–24 min: each group adds a single crystal at a marked point, starts timing, and records the time for the colour to reach a drawn line on the beaker; two groups repeat to give a check. 24–30 min: results table on the board, group by group. 30–35 min: every student writes the one-line conclusion.
Roles: materials collector, timer, recorder, reporter - rotated from the previous practical, and posted on the board so no group argues about it. The student on crutches is the timer and recorder so participation does not depend on mobility.
2. List the safety and inclusion risks in this plan and the control you will put against each, including the conduct issue with the student who took crystals.
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Enumerate the hazards against the actual plan rather than in general terms.
- Hot water at 60 degrees: scald risk with 44 students moving. Control: you pour the hot water yourself at a fixed pouring station at the front; students never carry a hot beaker across the room; no student walks between the station and the desks while pouring is happening.
- Potassium permanganate: strong staining and harmful if swallowed; it is also the substance this student has already taken. Control: pre-weigh six portions of about 0.01 g on the balance yourself, dispense one portion per group in a small dish, count them out and count them back in, and keep the stock bottle in your hand throughout. Record the count in writing at the end. Do not confront the student publicly; give that student a clearly defined role at the front where he is occupied, and speak to him privately after the period with the class teacher informed.
- No goggles: control is reduction, not reassurance - smallest possible quantity, no heating by students, no vigorous stirring, and teacher-only handling of the crystals. Record the absence of goggles in your practical note as a resource gap for the head of department.
- Broken glass: one dustpan and brush placed at the front, breakage reported not cleared by students.
- Inclusion: fixed rows make group movement awkward; designate a reporter from each group to walk to the board instead of groups gathering around beakers, so the student on crutches is not asked to move repeatedly.
3. How will you assess practical skill validly under this arrangement, and what will you deliberately not assess?
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With 8 groups and one teacher, a per-student apparatus-handling assessment is not possible in this period, so assess only what is genuinely observable and be explicit about the rest.
Assess (observation checklist, 4 items, ticked during the 24-minute window): whether the group's crystals were added at the same instant the timer started; whether the observation was recorded at the marked line rather than "it spread fast"; whether the results table has the manipulated variable (temperature) and the measured variable (time) in separate columns; whether the conclusion refers to the comparison, not just to one beaker. Score individually from each student's own record sheet, not the group's, and add one oral question per group to a named student so you sample understanding.
Deliberately do not assess: accuracy of weighing (you did it), correct use of a spirit thermometer to read 60 degrees (the hot water is prepared, so the reading is a demonstration), and glassware technique, since only one student per group handles a beaker. Say so in the record.
Close the gap next period: a six-station rotation with the same beakers over two periods, where each student personally adds a crystal and times it. That is where handling skill is legitimately assessed, and it protects the record from claiming evidence you did not collect.