Risk Assessment for a Sucrose Dehydration Demonstration
A teacher plans a demonstration for 28 ninth graders: adding 30 mL of 18 M sulfuric acid to 20 g of sucrose in a 250 mL beaker on a hot plate to show the dehydration of sugar. The classroom has no fume hood, one window that opens, and a fire extinguisher 15 m down the hall. The teacher has splash goggles and nitrile gloves but no face shield or acid apron. The school's chemical hygiene plan prohibits using more than 10 mL of concentrated acid outside a fume hood and requires a written risk assessment for demonstrations that produce corrosive mists or toxic gases. The teacher wants students to see 'real chemistry' and asks whether to proceed as planned, modify the demonstration, or substitute another activity.
1. Identify the principal hazards and the missing controls, then make a safety decision.
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The main hazards are concentrated sulfuric acid (corrosive, dehydrating, and exothermic on mixing), spattering from the hot plate, and acid mist or sulfur oxides from the reaction; the reaction also produces steam and may eject carbonaceous material. Missing controls include a fume hood, face shield, acid apron, secondary containment, a nearby extinguisher, and a written risk assessment. The planned 30 mL exceeds the school's 10 mL limit outside a hood. Decision: do not proceed as planned. The risk cannot be made acceptable by goggles and gloves alone.
2. Propose a safer modification or substitute that preserves the learning objective. Justify your choice with the hierarchy of controls.
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Apply the hierarchy of controls. First eliminate or substitute: use a teacher-made video or a microscale caramelization of 1–2 g sucrose in a test tube on a hot plate, which still shows dehydration to carbon and steam without concentrated acid. If the acid reaction is retained, reduce to a few grams and a few milliliters, perform it in a fume hood behind a shield, use face shield and apron, and keep students at a distance. Engineering and substitution controls come before PPE. The modified version must still fit the chemical hygiene plan.
3. How would you assess whether students met the intended learning objective from the substitute?
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Assess the objective, not the spectacle. Ask students to write the dehydration equation for sucrose, predict the products, explain why the acid removes water, and list the controls that made the substitute safer. A short pre-lab prediction and a post-activity explanation can show whether they understand dehydration and risk assessment. If the video is used, pause it and have students justify the observed carbon and steam rather than simply recall the demonstration.