We ended up only using one lemon. Cut lemon in half and squeeze juice onto saucer. We diluted with a teaspoon of water which I don't think is really necessary.
A quick lick of lemon juice before the real science began.
Dip the cotton swab in the lemon juice and write a message on plain white paper. We also think a blunted toothpick would work well, because you'd get a thinner line of writing. Allow the invisible ink to dry thoroughly.
To read the secret message, hold the paper ink side down over a lightbulb until the message shows up. THIS DID NOT WORK!
With the blowdryer on HIGH, heat the paper until the message appears. THIS DID NOT WORK!
Once the kids are thoroughly discouraged by this chemistry experiment, think hard about other heating sources. Rule out the lit candle (fire hazard), microwave and oven. Then in a last ditch effort to prove to the kids you really were once a science teacher in a real school, pull out the iron, turn on high and start ironing the paper. After about 30 seconds the message will magically appear in a dark yellow, tannish color. Once the paper cooled the message wasn't as noticeable so the picture didn't turn out so well. But our secret messages were as follows:
Mom Loves Dad
Lee - cool bike
Grace horses
pictures of hearts and flowers
Science behind the project - the acid in the lemon juice contains carbon compounds. When those compounds are heated they turn brown (or in our case dark yellow-maybe because we diluted it a little). We also talked about ascorbic acid and how it prevents oxidation when squeezed over fruit, preventing it from turning brown. Of course, the scientific method was discussed once our original plan had failed.
