Learning by Accident
Learning by Accident is an ongoing Crucible feature, in which real-life lab accidents or incidents are recounted and explained. The goal is to highlight the consequence of ignoring safety rules so that science educators will be further encouraged to become knowledgeable, and to take appropriate action, in areas of safety that affect their daily activities in the science classroom. Submissions are encouraged. Anonymity will be guaranteed. Please send written descriptions to Ian Mackellar, STAO Safety Committee Past-Chair, Box 191, MAITLAND, ON K0E 1P0

««« Submitted by a STAO member, Region 4

Glowing Test TubeExplosion

A chemistry teacher was demonstrating a reaction between sulfur and zinc. The powdered sulfur and zinc were mixed in a test tube and heated in a fume hood. Usually this produced a minor explosion out of the top of the test tube. However, this time the explosion occurred at the bottom of the test tube and caused the tube to explode. Despite the fact that the experiment was done in a fume hood some of the glass escaped and traveled some distance from the hood. What happened was that the teacher stopped heating halfway through to correct student behavior and a plug formed at the top of the tube. In addition, this was a demonstration that the teacher had done many times and due to the usually predictable nature of the explosion a safety shield was not used. The teacher and all students were wearing protective eyewear, however. This incident created a lot of concern but fortunately no damage other than to the test tube.

Comments from the STAO Safety Committee
The STAO Safety Committee does not endorse the use of uncontrolled explosions for demonstration purposes. However controlled reactions can produce much student interest in the material if done following a proper procedure.

When demonstrating the reaction of zinc with sulfur, always use a safety shield and wear eye protection. Use powder, not zinc dust. Students should
move to the back of the room. Protect the bench with a large piece of hardboard. The demonstration is best done at the end of a lesson. The room should be well-ventilated.

When the zinc/sulfur mixture is heated, the sulfur melts and reacts with the zinc exothermically to form zinc sulfide. However, as a side-reaction, a small amount of the sulfur burns in air to form sulfur dioxide, a TOXIC and CORROSIVE gas. Accordingly, it is best to use a fume hood if available, to protect students susceptible to asthma attacks.
The reaction of zinc with sulfur may be carried out as a teacher demonstration by heating a small heap of a freshly prepared mixture (5 g of zinc, 2.5 g of powdered sulfur) on a tin lid. Once the heating starts, move away from the area. NEVER carry out this reaction with the mixture confined in any way e.g., in a test-tube!

Perhaps the main lesson to be taken from this is that you should not take safety precautions for granted just because a demonstration has worked predictably in the past.