What it shows
Vitamin C is destroyed by heat and dissolves in cooking water. A starch and iodine indicator that loses its blue colour when vitamin C is present lets you compare how much each sample contains.
The science
Vitamin C reacts with iodine and turns it to colourless iodide. When starch is present the iodine gives an intense blue-black colour only if free iodine remains. So the more drops of iodine you can add before the blue colour stays, the more vitamin C the sample contains.
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.
- Chopped pale vegetables such as white cabbage, green capsicum or cauliflower (coloured juices like tomato hide the colour change)
- A diluted iodine solution (tincture diluted with water)
- A teaspoon of starch cooked in a cup of water
- Droppers or a clean straw
- Test tubes or small glasses
- A pan and a small stove
- A pestle and a mortar or a grater
Safety
For this project: An adult must handle the stove and the iodine tincture, which is harmful if swallowed, stains skin and is flammable because it contains alcohol, so keep it well away from the flame. Wear gloves and eye protection and never taste anything used in a test. This is a classroom comparison, not a nutritional measurement.
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.
Grate 20 g of raw vegetable, add 20 ml of water, and strain the juice.
Boil another 20 g in 20 ml of water for 10 minutes and cool it. Pour the cooking water into a glass, then grate the boiled vegetable, add 20 ml of fresh water and strain to get the boiled-vegetable juice.
Put 5 ml of each liquid (raw juice, boiled-vegetable juice and cooking water) in separate clean glasses and add 5 drops of cooled starch solution.
Add the diluted iodine drop by drop, swirling after each drop, until a permanent blue colour appears.
Count the drops needed for each sample and repeat three times.
Calculate the average and compare the raw, boiled vegetable and cooking water samples.
Make a bar graph of the average number of drops.
Write what the data suggest about cooking time and water, and discuss the limits of the test.
What you should see
More drops of iodine are needed for the raw juice than for the boiled vegetable juice, and the cooking water holds some of the lost vitamin C.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
01Why does the colour appear only after vitamin C is used up?
Vitamin C removes the iodine until none is left.
02Why does boiling reduce vitamin C?
Heat breaks it down, and it also dissolves in the water.
03Why repeat the test?
To check that the drop counts are consistent.
04What is the limit of this method?
It is a comparison; other substances can react with iodine.
Take it further
Compare steaming, pressure cooking and microwaving using the same vegetable.
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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