Inside: This cranberry juice and baking soda invisible ink recipe is so fun to use for secret messages, cards, and gifts. It’s also great for learning about acid + base chemical reactions.
We can try to imagine what life was like for spies in the old days – their level codes and secret strategies – but it’s much more fun to try our hand at it! We discovered that writing with invisible ink can be challenging and entertaining for a number of reasons.
First, you don’t always remember what you wrote last, and it’s not easy to retrace your steps to refresh your memory (unless you want to risk making your invisible message visible).
Second, since there is no “back” button and retyping is not possible, you have the challenge of staying focused and going over the text mentally before committing it to paper.
Finally, as my son pointed out, writing a longer message, like one detailing the outline of a proposed battle or secret plot, would have been a major time commitment and strenuous finger exercise.
Overall, we decided we are very happy to live in the age of computers, instant messaging, Google docs, and password-protected pages. Still, writing with invisible ink is an engaging and fun experience that gets kids excited and reflective.
The History of Invisible Ink
Invisible ink has been around for over two thousand years. During ancient times, common invisible inks included lemon and onion juice, wine and vinegar, milk, and even bodily fluids (remember those blood-written messages on the walls of Bastille in the old French novels?). And there were two ways to make invisible inks appear – acid and heat. That’s because acid and heat are major catalysts for chemical reactions that change substances and can make the invisible visible. But more on that later…
In today’s activity, and in the spirit of the season, we’re using the acidity of cranberry juice to make our secret message reveal itself.
If you are looking for more cranberry juice activities, try our Cranberry juice play dough. And if you have any cranberries left after experimenting, toss them into any dish you’re making for dinner. I threw a handful of cranberries into the pot of chili I was cooking the other day, and we all thought it added an intriguing touch to the overall bouquet of flavor. A kitchen is a wonderful place for all sorts of experiments!
Magic Invisible Ink for Spies Recipe
What you need
Cranberries (about 2 cups)
Baking Soda (about 2 teaspoons)
Q-tips, paintbrushes, cake-pop sticks, feathers, or toothpicks
What to do
1. Cook Cranberries
We used just under 2 cups of berries. If you are in the USA, it’s about half a bag of Ocean Splash cranberries. We boiled them covered with about 3 cups of water for 20 minutes, then blended them with a hand-held mixer and pushed them through a sieve with a spoon to form a nice smooth, just lightly thick, juice.
2. Prepare Invisible Ink
Thoroughly stir 2 teaspoons of baking soda with ⅓ cup of water. It’s much easier to do if the water is warm or hot.
3. Write a secret message
We cut regular printing A4 paper in half and in quarters (because, let’s be real, filling up a whole page of white space with secret messages might be intimidating). My younger kids drew hearts and other simple drawings, while my older kids (with my encouragement 🙂 wrote things like:
You are sweet
I love you
You are one of a kind
To find your best writing tool, try different implements like Q-tips, thin paint brushes, cake-pop sticks, feathers, and toothpicks. My kids preferred Q-tips, while I liked the small paint brushes.
>>> Please, let your message dry before the next step! It doesn’t take long…
4. Reveal the secret message
Pour cranberry juice into a cake pan, casserole dish, or Pyrex glass container. The liquid should be like tomato juice. If it’s thicker than that, add a bit of water to dilute it.
Submerge the piece of paper with the secret message in the cranberry juice. Did your message come out?
The Science Behind the Activity
Cranberries are acidic. They make you pucker your cheeks with their sourness and add a bright pop to any recipe you add them to.
But what does acidity mean? You might remember that atoms have electrons, which have a negative charge, and protons, which have a positive charge. If the atom has the same number of protons and electrons, it’s balanced and happy and what scientists call “stable.” But if the numbers are uneven, the atom is called an ion, and it will easily get itself into chemical reactions in order to even out its electrons. Acids have a lot of Hydrogen ions, and when those unhappy, unbalanced electrons bounce around everywhere trying to even out – it feels like a sour punch!
Bases are substances with lots of Hydroxide ions. Things like ammonia, soap, and bleach are bases, and, unlike acid, they feel sort of slimy. The really amazing thing is that when you put an acid and a base together, they actually even each other out! Together, they can produce a substance that is neutral, with all the electrons peaceful and content. But to do that, the acid and the base have to go through a chemical reaction that produces water and carbon dioxide, which makes them bubble and steam. So cool!