Invisible inks are also called sympathetic
What You Need:
Ink type 1 - lemon juice, grapefruit juice, vinegar, milk, onion juice
Ink type 2 - cobalt chloride
Ink type 3 - phenolphthalein, Windex
Q-tips or paintbrush
What You 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. Since Windex is a base, it will turn the phenolphthalein bright pink.
What other sympathetic inks can you find? Which kind shows up best? Which kind lasts longest? That discovery, is up to you.
© Rounak Chakraborty, 2019. All rights reserved.