Yeast is a fungus. A teaspoon of dry yeast is made up of millions of single celled organisms. In the right conditions these organisms will feed on sugar and convert it into carbon dioxide and alcohol. The cells will multiply. This process, fermentation, will only stop when the food runs out, or the conditions change.
Take a half litre drink bottle and add three teaspoons of sugar and three teaspoons of dried yeast. Fill the bottle a quarter full with warm water and swirl the mixture around to mix it.
Put a party balloon over the opening of the bottle. Over time the balloon will inflate as the yeast ferments, producing carbon dioxide gas. Once all the sugar has been consumed, the yeast will stop fermenting, and the balloon will stop inflating.
Ask your students how they could adapt this experiment as a basis of exploring the best conditions for yeast to ferment. How will they ensure the tests are fair?
Take three bottles. Add three level teaspoons of dried yeast and three level teaspoons of sugar to each. Chill 150ml of water to almost freezing point and add this to the first bottle. Add 150ml of warm water to the second, and 150ml of nearly boiling water to the third. Swirl the bottles and place balloons over the top. Observe the bottles every 10 minutes for the next hour. You should find that the cold water inhibited the fermentation, there may be bubbles in the liquid, but the balloon will not have inflated. If you warm this bottle, the yeast should ferment.
The hot water kills the yeast - there are no bubbles and the balloon has not inflated. Even if you cool this bottle down, the yeast will not ferment. The warm water should work really well, and the balloon may be fully inflated.
Try the experiment again, but vary the other conditions. For example try shutting out the light, or adding different amounts of sugar. Use the size of the balloon as a measure of how much fermentation is going on.
Sophie Duncan is project manager for science at the BBC www.bbc.co.uksn