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ExposeExciteIgniteMay2012

3. Replace the butane

3. Replace the butane with hair spray or deodorant spray. You will now have a mixture of butane and propane plus some impurities you did not have before. 4. Over-fueling will create a more visible flame. This is pure edutainment! It is important (and educational) to emphasize the result of an over or under supply of reactants. Refer students to the balanced equation. When using an oversupply of butane, the gas mixture usually does not ignite due to insufficient oxygen. But when the flame emerges from the can, it finds new oxygen in the outside air and burns brightly. Try it: Inject double the volume you used before and you will find it will be hard to ignite. But, when the lighter is held outside the opening, the gas ignites and produces a rocketlike fire plume (Figure 2.13). Figure 2.13 Combustion reactions perform at their peak when combustion ratios are stoichiometrically ideal as per the balanced equations. But it is important to realise that fuel gases can react over much wider ranges than that predicted by the balanced equations. Not withstanding this, this is still a great way to make students realise the importance of balancing equations to find the ideal stoichiometric ratios! Table 2.1 lists the explosive or flammable range of gas/air mixtures. Below the indicated value, the mixture is too lean to burn and above the upper explosive limit the mixture is too rich to burn. The limits indicated are for gas and air at 20! and atmospheric pressure 2 . Let’s take butane as an example: We injected 15.7 / 500 x 100 = 3.13% fuel into the can which is fine as butane can ignite between 1.8 to 8.4% 2 The Engineering Toolbox, www.engineeringtoolbox.com An invaluable resource. 19

Fuel Gas Flammable Range (% by volume) Hydrogen 4 - 75 Methane 5.0 - 15.0 Butane 1.8 - 8.4 Propane 2.1 - 10.1 Table 2.1 B. The Tube Cannon! [] You may want to extend the Bunny Can demonstration by launching the lid or a plastic cup from a Pringles ® tube. This is loud but fortunately the projectiles are safe. What you will need • all items listed for the Bunny Can project except the cable tie and fishing line • a large Pringles ® tube • plastic cup ! 100 mL (3.4 oz) that will fit the tube - or almost • scissors Here’s how 1. To make the hole for the gas lighter, hammer a nail through the centre of the tube’s base or use a poking tool. 2. Enlarge this hole to the diameter of the gas lighter shaft with a hand drill or pointy pliers. The lighter shaft should fit tightly through the hole. 3. There is no need here to tie the plastic lid down due to its small mass. 4. Measure the volume of the Pringles ® tube. It should be close to 950 mL (32 oz). Divide this by 32 if you use butane gas: 950/32 = 30 mL (1 oz) Operating the cannon 1. Place the plastic lid onto the Pringles ® tube. 20