It’s a scream!

As a child I could put away sweets without a second’s thought for their energy content. As an abstract concept, energy is really difficult to visualise. You can look at the numbers on the back of a sweet packet and calculate that with just one jelly baby you’d have enough energy to run for a few minutes or bring a cup of water to the boil. However, a simple calculation doesn’t necessarily bring the idea home because we take the heating power of a kettle for granted and metabolic processes take their time. To get a real sense of what approximately 100 kilojoules look like in action, you need a speedy reaction that packs a visceral punch. Having a sympathetic figure to destroy in the process can’t hurt the storytelling either, and this is where the ‘screaming’ or ‘howling’ jelly baby demonstration can really help.

Kit

• 7 g potassium chlorate(V) – reagent grade (may cause fire or explosion, strong oxidiser, harmful by inhalation/ingestion, toxic to aquatic life)
• UK-style jelly baby sweet
• Clean borosilicate boiling tube
• Bunsen burner
• Clamp stand and clamp (without rubber grips)
• Tongs
• Safety screen(s)
• Face shield
• Heat-resistant gloves
• Heat-resistant mats

Preparation

Set up this practical in the middle of a well-ventilated room, well away from the walls and with the windows wide open. Ensure there are no smoke detectors. You will need at least 2–3 metres of bench space, perpendicular to the audience. Protect the bench with heat-resistant mats. Set up safety screens to protect the audience and the demonstrator, and to ensure no ejected particles can come into contact with combustibles.

Weigh out 7 g of reagent grade potassium chlorate(V) and add it to the bottom of a thoroughly clean borosilicate boiling tube clamped at an angle steeper than 45° (so that the sweet will drop straight to the bottom of the tube) over a Bunsen burner. Line up the tube to point along the bench, away from the audience and demonstrator as well as from walls, displays and windows.

In front of the class

Students should remain 3–4 metres away wearing eye protection. The demonstrator should wear a face shield and heat-resistant gloves. Heat the base of the tube with the Bunsen burner until the potassium chlorate(V) just melts. Then do not wait: immediately remove the flame and drop in the sweet using a pair of tongs in your gloved hand.

Teaching goal

This demonstration is all about the drama, but a discussion of the energy on display is always helpful to anchor students’ understanding of this concept. According to the packet, the UK jelly baby is approximately 75% sugar by mass, with each sweet delivering approximately 100 kilojoules of energy.

Assuming no heat loss, this would be sufficient to raise just over 300 cm³ of water from 20°C to 100°C or, based on figures from Harvard Health Publishing, for a 70 kg person to:

• sleep for about 30 minutes
• walk for about 4 minutes
• cycle for about 2 minutes
• or sit in your class for another 10 minutes!