What it shows
Hydrogen peroxide breaks down very slowly on its own but very fast when yeast is present. Yeast contains the enzyme catalase, a catalyst, and the oxygen released makes a tall foam with a little dish soap.
The science
Hydrogen peroxide decomposes: 2H2O2 gives 2H2O + O2. A catalyst speeds up the reaction without being used up. The enzyme catalase in yeast lowers the energy needed, so the oxygen comes out in seconds, and the soap traps it as foam. More yeast gives a faster reaction, up to a limit, and the reaction gives out some heat, so the bottle becomes warm.
Materials
Cost: under Rs 200. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- 3 percent hydrogen peroxide from a pharmacy, 200 ml
- Dry yeast, 2 teaspoons
- Warm water, 50 ml
- Liquid dish soap
- A tall narrow bottle or measuring cylinder of 250 ml
- A tray to catch the foam
- A ruler
- Safety glasses and gloves
Safety
For this project: Wear safety glasses and gloves. Use only 3 percent peroxide and never a stronger hair-bleach grade. An adult must supervise. Do not close the bottle, because oxygen is released. Wash the foam down the sink with plenty of water.
Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.
All the safety rulesStep by step
8 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.
Mix one teaspoon of yeast in 50 ml of warm water and let it stand for 5 minutes.
Stand the bottle on the tray, and pour in 50 ml of 3 percent hydrogen peroxide and a squirt of dish soap.
Pour the yeast mixture in quickly and step back. Note the time and the maximum foam height with the ruler.
Touch the bottle to feel whether it is warm and write down what you feel.
Repeat with a half spoon of yeast and then with two spoons, using a clean bottle each time.
Repeat once without any yeast as a control and note that very little happens.
Plot the amount of yeast against foam height and write a conclusion.
Try a pinch of raw liver or a slice of raw potato in place of the yeast and see which catalyst works well.
What you should see
The yeast reaction produces a tall foam within 20 to 30 seconds, taller with more yeast, the bottle feels warm, and the control shows nearly none.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1What is a catalyst?
A substance that speeds up a reaction but is not used up.
2What gas forms?
Oxygen.
3Why is the bottle left open?
The gas must escape or pressure could build up.
4Why is the bottle warm?
The reaction gives out heat, so it is exothermic.
Take it further
Test whether boiled yeast works and explain what happens to the enzyme when it is heated.
Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.
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