Invisible Ink & vitamin C

In this experiment, kids learn about indicators by writing a secret message. An indicator is a chemical that is used to test for the presence of another chemical. Iodine is an indicator for vitamin C. In the absence of vitamin C, iodine forms a purple complex with starch. Vitamin C blocks the purple color formation. So if starch and iodine mixed together don't turn purple, you know that vitamin C is present.

Ages 6 and up; Download Printable PDF

Iodine forms a purple complex with the starch in paper.
Vitamin C blocks this reaction, so parts of the paper coated
in lemon juice remain white.

You will need:

measuring cup

teaspoon measure

Q-tip or small paint brush

large paint brush
lemon juice

printer paper

iodine solution (available at drug stores)

newspaper to absorb spills

Preparation
Stir 2 teaspoons of 10 % iodine solution into 1 cup of water.

Instructions

Write your message Using a small paint brush or Q-tip as pen and lemon juice as ink, write a message on paper. Let dry.

Reveal your secret Using a large paintbrush, coat the paper with a thin layer of iodine solution. Your message will be revealed!

What’s Happening?
The sort of simple explanation Paper has starch in it. Iodine reacts with starch to form a purple color. Vitamin C in lemon juice blocks this reaction, so areas of the paper coated in lemon juice remain white. Scientists use iodine as an indicator to tell whether or not vitamin C is present. If they measure carefully, they can even find out exactly how much vitamin C is present.

The hard-core explanation A form of iodine called triiodide [I3-] reacts with starch to form a purple complex:

[I3-]  +  starch  ->  [I3-• starch complex]

In the presence of vitamin C (also known as ascorbic acid), triiodide [I3-] is converted to iodide ion [I-], which doesn’t react with starch:

Since there is no triiodide to react with the starch, no purple color is formed.

More to try

Use your iodine vitamin C indicator to find out if other juices contain vitamin C. Paint a patch of liquid onto paper as you did the lemon juice, and coat the paper with iodine solution to see if the patch remains white. You might also want to try milk, white vinegar, and dissolved solid foods. 

Did you know?
People who don’t get enough vitamin C can develop a disease called scurvy. Scurvy used to be common among sailors who had no access to fresh fruit. Today scurvy is rare, but it’s still important to get enough vitamin C. So make sure to eat plenty of fruit!