There are many different ways to create inivisible ink. I didn’t want to use heat with a group of first graders, so we experimented at home with baking soda, vinegar, grape juice and red cabbage water (from wikipedia).
It turns out after boiling red cabbage, the water turns a deep purple. This interacts with both vinegar (turning it bright pink) and baking soda (turning it blue-green). The color changes are dramatic when the liquids are mixed together – I thought that was even more fun than the invisible ink, which took most of the day to dry.
Continuing with our theme that science is more fun when its a surprise, we called this science class “mixing liquids” and didn’t let on to where these experiments would lead.
I started out the class by introducing the concept of how a clear liquid will change color when you mix it with a colored liquid. I held up two cups in front of the class, one with dark green liquid (food coloring and water) and one with plain water. I asked them to tell me what they saw. They observed that one cup had something that was green and we came up with a few words to describe the other: clear, invisible, transparent. Then I asked them what might happen if I mixed them. One child said that if I mixed the clear water with the green, it would turn lighter. Knowing that its actually hard to see the difference if you dilute colored water with just a little clear water. I mixed in the other direction.
I took a teaspoon of the green water and dropped it into the clear water. I didn’t expect the great number of “ooohs” and “aahs,” but it did look rather cool as the liquids mixed. We could have probably done a whole section on mixing colors before this one. I was glad that I had prepared this basic introduction before moving on to the others.
I told the class that I had created the green liquid with food coloring and water (and we talked a bit about food coloring and its uses), but for their experiment I had made my own colored liquid the old-fashioned way. I passed around a red cabbage, so they could see where it came from, then we passed out little cups of cabbage water and plain water along with plastic spoons for mixing.
They tried the different liquids and recorded their results on a worksheet. When they were done, I introduced the words “acid” and “base” and asked them what we might do with what we learned from this experiment. None of them guessed that we could create invisible ink. We let them all write a message or draw a picture, to be revealed the following week…
The next week every student revealed their own pictures. Many were hard or impossible to see, but two came out very nicely. If I were to do this again, I would have each student create several invisible pictures to provide more of a chance for success. In any case, they all seemed to enjoy the experiment.