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<<Forensic Science Products
Invisible inks are also called sympathetic inks, and can be made with many
different substances. Sometimes they appear when you heat them up; other times
another chemical can reveal them. Get creative and see how many kinds of
invisible ink you can find.
(NOTE: Some of the chemicals used in this project can be hazardous
if misused. Use caution when working with chemicals! Read the information on the
chemical label before you start, and always wear protective
such as goggles, gloves, and aprons. Adult supervision required.)
- Ink type 1 - lemon juice, grapefruit juice, vinegar, milk, onion
- Ink type 2 -
- Ink type 3 -
- Q-tips or paintbrush
What to do:
- Choose a liquid like lemon juice to use as ink. Write a message on a
piece of white paper using a paintbrush or q-tip, dipping in the ink
frequently. Let the message dry. Turn on the toaster and carefully hold your paper
over it. The heat will make your secret writing appear!
- Some sympathetic inks appear and disappear again based on humidity. To
try one of these, make a solution of 1/8 teaspoon cobalt chloride and 1/2
cup water. After writing your message and letting it dry, heat the paper to
see your message appear in blue. Now hold the paper face down over a pan of
steaming water. The steam will cause the writing to disappear. If you reheat
it (evaporating all the water) the writing will appear again.
- Other sympathetic inks work because of acid-base reactions. Use
phenolphthalein solution to write a message on a piece of paper and let it dry.
After it is dry, spray it with some Windex. Phenolphthalein is a pH indicator that is pink in the presence of bases. Since Windex is a base, it causes your phenolphthalein writing to appear.
What other sympathetic inks can you find? Which kind shows up best? Which
kind lasts longest?