The Science of Halloween: Invisible Inks
Making invisible ink is one of those classic, old-time science experiments that practically everybody has done at least once in their lives. Invisible Inks, also known as sympathetic ink lets you write secret messages that can only be seen through the use of heat or the use of another chemical. Most invisible ink forumulas I have encountered use lemon juice but apparently all kinds of other ingredients can be used to produce an invisible ink including spit and sweat! That's pretty gross which of course means that this fits in nicely with our Halloween theme.
One non-lemony way of making invisible ink uses baking soda! You mix about 1/4 cup (60 ml) of baking soda and 1/4 cup (60 ml) of water. Then, using a Q-tip or a toothpick or a brush, you use the baking soda solution to write your magic message on a piece of paper. Let your message dry completely. To read the secret message, you paint grape juice concentrate across the paper with a paint brush or a sponge. Viola, the message appears! Please keep in mind that if you use this method, grape juice stains. Badly.
This method works because grape juice contains an acid that reacts with the baking soda. A different color appears wherever the secret message is written.How cool is that? Pretty darn cool I think.
Here's a new one, making invisible ink using milk! We all know (excepting the lactose intolerant) that milk is good stuff, but I did not know there was also magic in there along with the Vitamin D and calcium.
Put a touch of milk in a small bowl. Write with the milk on a piece of paper with a Q-tip or a brush. Let your message dry completely. To read the message just heat the paper. Use an iron or 100-watt light bulb or stove element. Don't rest the paper on the bulb because paper burns (that's another example of science but save that for another time). Children should not do this without an adult to help in case a fire. You should also not use a halogen light as those things get very hot and the possibility of a fire is that much greater. Yes science can be dangerous.
Magic Milk Invisible Ink works because milk is organic which means it comes from a living thing. When it's heated, it burns at a slower rate than the paper. Your invisible message shows up brown.
And now, because you can't talk about making invisible inks without mentioning the use of lemon, here is that classic formula for those who may need it.
Dip a Q-tip or brush into a bowl of lemon juice and write your message on ordinary paper. Try not to use too much. Let the lemon dry completely. To see the message, heat the paper using one of the same methods as you used for the milk solution.
A really cool variation is to put salt on the drying ink. Give it a minute and then wipe the salt off. Use a wax crayon to color over the message. What happens?
Lemon juice, like milk, are mildly acidic and acid weakens paper. The acid remains in the paper after the juice or milk has dried. When the paper is held near heat the acidic parts of the paper burn or turn brown before the rest of the paper does.
Now here comes the real science. Find out what other ingredients you can use to make additional invisible ink solutions. If grape juice can be used to reveal the baking soda message, what about apple juice or tomato juice? Test them out. Do they work? Or not? If not, why do you think not? You should probably lay in a supply of pH paper before you do this experiment so you can find a possible answer.
If milk works what kind of milk did you use? If you used whole milk, will 2% or 1% or Skim milk work as well? How about condensed milk? How about coconut milk? How about that lactose-free milk stuff?
If lemon juice works so well what about lime juice?
This is what science is really about all - asking questions and finding answers. Enjoy!