I found invisible ink to be one of the more fun projects to play with. If you are not familiar with it, invisible ink revolves around writing with a liquid which does not leave a visible mark, then treating the paper later to reveal the writing. For some background reading, you may want to look at this page on the use of
invisible ink in spy letters during the American revolution
Below you will find information I have gleaned from various sources, including experimenting with it myself. Sometimes it takes a bit of trial and error to get it just right, but kid-craft-central will get you off to a good start.
The simplest method is to use lemon or onion juice to write with and let it dry. To reveal the writing, heat the paper with a iron, or other heat source.
A more sophisticated approach uses an acid or base to write with and a pH indicator to reveal the writing. (pH is a measure of how acidic or alkaline a substance is - a pH of 1 is extremely acidic, 7 is neutral, and 14 is extremely alkaline). pH indicators change colour when mixed with acidic or alkaline substances.
To write with: White vinegar (which contains acetic acid) is probably the most convenient source of an acidic 'ink'. To make an alkaline 'ink' try a mixture of baking soda and water (around 50/50). With a small paint brush, write on a piece of paper using your chosen 'ink' and allow it to dry.
To reveal the writing:There is a natural pH indicator (a pigment called flavin) found in apple skin, grapes and red cabbage (amongst other things). Boil half a red cabbage in 5 cups of water. Filter out the cabbage, and spray or paint the juice over your invisible writing. I tried doing this with apple skin (I imagine this is only red apples), which also worked.
If you just want to see the pH indicator working, put a little of your cabbage/apple juice in the bottom of a glass and note the colour. Now add some white vinegar, noticing the colour change. Next add some baking soda. You will see the colour change back again.
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