What it shows
Living cells make an enzyme, catalase, that breaks hydrogen peroxide into water and oxygen. Raw potato foams strongly when added to dilute peroxide, boiled potato hardly foams, and crushed potato foams fastest.
The science
Cells make hydrogen peroxide as a harmful by-product of metabolism, and the enzyme catalase breaks it down: 2 H2O2 gives 2 H2O and O2. The oxygen gas, trapped by a drop of soap, makes foam, so the foam height shows how fast the enzyme works. Heat changes the shape of the enzyme (denaturation), so boiled potato barely reacts. Crushing exposes more cells and more enzyme to the peroxide, so the reaction is faster.
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.
- One raw potato
- A pan of boiling water (for the adult)
- 3 percent hydrogen peroxide solution from a pharmacy (about 200 mL for three rounds)
- 4 tall narrow glasses or measuring cylinders
- A few drops of dish soap
- A grater and a knife
- A ruler and a timer
- Goggles and gloves
Safety
For this project: Three percent hydrogen peroxide can irritate skin and eyes. An adult must handle and measure it; wear goggles and gloves, never taste it, wash splashes with plenty of water and pour leftovers down the sink. An adult must cut the potato and boil the 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.
Step 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.
Cut four equal pieces of potato of about 5 g each, and keep one raw as a whole piece.
Ask an adult to boil one piece for five minutes and let it cool, grate a raw piece to a fine pulp, and keep a fourth cube in a dish for a water control.
Pour 20 mL of peroxide into each of three glasses, add one drop of dish soap and swirl gently, and use a fourth glass with plain water and soap as a control.
Drop the raw whole piece into glass 1, the boiled piece into glass 2, the grated piece into glass 3 and a raw piece into the water control, one after another.
Start the timer as each piece goes in and measure the foam height with the ruler after 1 and 2 minutes.
Write the readings in a table and note which glass foamed fastest.
Repeat the test twice more with new pieces and calculate the average height.
Plot a bar graph of the foam height for each treatment and write the explanation using the enzyme idea.
What you should see
The raw whole piece makes a good foam, the grated pulp makes more foam and faster, the boiled piece gives very little and the water control shows none. The bar graph shows the order grated, raw, boiled.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
01What does catalase do?
It breaks down hydrogen peroxide into water and oxygen.
02Why does boiled potato not foam?
Heat changed the enzyme’s shape (denatured it), so it no longer works.
03Why does grated potato foam faster?
It has more surface area, so more enzyme meets the peroxide.
04Why add soap?
It traps the oxygen as bubbles so the reaction is easy to see.
Take it further
Test the foam from potato pieces kept in ice water, at room temperature and in warm water at 40 degrees Celsius.
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.
Free ebooks for this project
Science projects
Biology Models Guide
Make the heart, lungs, kidney, eye, cell and more, and explain them well
Science projects
Your Science Exhibition Guide
Choose a project, build it, present it and answer the judges with confidence
Find out more about biology
Free, well-known places to explore the subject further. Ask a grown-up before you sign up for anything or type your name anywhere.