Presumably you want to do a Chem lesson. 1 minutes isn't very long so you need to choose 1 learning objective, what about a 'How Science Works' question. Give each member of the 'class' a wooden splint. Have one yourself. Weigh your splint accuratly, the dept does have some accurate balances (I did my Chem PGCE at Brum). Light your splint then light the splint of the nearest student, get them to pass the flame from splint to splint. Give them some H&S warnings about not letting it burn too close to their fingers, you may also want to demonstrate the correct way of holding the splint (45 degrees sloping downwards from your fingers) and also hold over the bench rather than over their legs, have a couple of elastic bands handy to give to anyone with long hair (If Roger Locke, the Bio lecturer is in the audience he will need one)! LOL While the splint burns they need to consider what is happening to the wood. Carefully burn your splint making sure you collect any ash that falls. The place your stub and ash back on the balance and weigh again. Why does it weigh less than before? Ask them for their ideas. Explain that there was a very old idea that materials that burn such as wood contain a magical substance called phlogisten and what you see when something burns is this stuff escaping into the air.(An idea by a block called Becker) . Wood is a mixture of 'true wood' (the ash) and phlogiston (according to Becker). Then place a burning candle under an upturned coffee jar or beaker until it goes out. Ask them why they think it went out. Explain that scientists such as Priestley, Lavoisier and Scheele postulated that the air contained a gas (Lavoisier named it oxygen) that combined with the chemicals in the wood to create new materials called oxides. In wood the oxides are themselves gases so leave the reaction and go into the air. Thus the Phlogiston theory was proved to be false. That should fill 15 minutes nicely. Be prepared for the 'class' asking some searching questions to test your knowledge. Your lucky, when I went there there wasn't a Chem lecturer and I was quizzed on my knowledge of Physics! Thank heavens I remembered Flemings left hand rule!