Making Invisible Ink
Challenge: Can you write a secret message in invisible ink?
- Juice from ½ a lemon
- Cotton buds or thin paint brush
- Source of heat, e.g. oven, iron, hair dryer
- Dip the cotton bud tip or paint brush into the lemon juice.
- Write or draw a map, picture or secret message on your paper.
- Wait for the lemon juice to dry.
- Heat the paper using a heat source. You can place the paper in an oven, on a high setting for 3 or 4 minutes. You can place a cloth over your paper and iron it. You can use a hair dryer on a high heat setting.
The lemon juice contains citric acid. Where you have touched the citric acid to the paper, the acid thins the fibres under it. When the lemon juiced dries, it is clear. Although the fibres where the lemon juice was are now thinner than the other areas of the paper, but not so thin that you can see the difference with your eyes.
When the paper is exposed to heat, the parts that have been thinned by the acid burn first, exposing your message, picture or map!
For more exciting experiments why not grab our Chemistry-in-Action kit?