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 safety equipment such as goggles, gloves, and aprons. Adult supervision required.)

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:

  1. 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!
  2. 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.
  3. 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?