What better way to get kids excited about science than with an experiment straight out of a spy novel? Write a secret message using colorless ink, wait for it to dry, then use a special secret solution to reveal the message. This activity was inspired by the Berlin blue experiment in this chemistry kit. However, while that experiment uses strange-sounding chemicals and warns against putting the substances down the drain, I thought it would be fun to make something similar at home using common ingredients. I experimented a bit and came up with something that works well using only water, baking soda, turmeric, and rubbing alcohol.
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My kids and I have also done the lemon juice invisible ink experiment in the past. It works pretty well, but the one downside is that you really need to apply a lot of heat (like with an iron or hot plate) to get it to work. In my experience, holding the paper over a lightbulb just doesn’t cut it. That means I end up being the one to reveal the message while my kids stand by and watch. The nice thing about this color-changing invisible ink experiment is that no heat is required. Instead, it uses a homemade acid/base indicator to reveal the secret message.
I knew that I wanted to use a natural indicator (like red cabbage juice or turmeric) along with a basic solution. As you can see, I tried several different combinations before I found one that I was really happy with. It turns out a solution containing only baking soda and water really has a remarkable change in color when used with the turmeric/rubbing alcohol indicator. (See below for detailed instructions.)
The kids had such a great time making and reading secret messages. The best part…even the baby could reveal the message on his own since no irons or hotplates were involved. (Though I will warn you that if you’re worried about stains, maybe have them use paintbrushes instead of paper towels!)
Recommended Age Range: Preschool, Kindergarten, Elementary, Middle School, High School
Time Required: ~30 minutes (including about 15 minutes to let paper dry in the middle)
Difficulty: Easy
Cost: Under $10 (much, much less in used supplies). We had everything on hand, but turmeric can be a little expensive if you have to buy more than you will use. Maybe borrow a teaspoon from a friend!
Materials:
- water (1/2 cup)
- baking soda (1 Tablespoon)
- paper
- q-tips (or a paintbrush) to apply invisible ink
- rubbing alcohol (1/2 cup)
- turmeric (1 teaspoon)
- paper towels (or a paintbrush) to apply color-changing solution
Supplies:
- measuring glass
- measuring spoons
- spoon for stirring
Instructions:
1. First, add 1 Tablespoon of baking soda to 1/2 cup of water.
2. Stir the baking soda into the water to make the “invisible ink”.
3. Use q-tips (or paintbrushes) to draw a picture with the invisible ink on paper.
4. While you are waiting for your paper to dry, mix up your color changing-solution. First, add 1 teaspoon of turmeric to 1/2 cup of rubbing alcohol.
5. Stir the turmeric into the rubbing alcohol. (Note: The solution can stain a little bit. I used a white plastic bowl, which was not the smartest, because it was a little difficult to clean afterwards. You might want to use a glass bowl.)
6. After the paper is dry, use a paper towel (or paintbrush) to apply the color-changing solution to the paper to reveal the secret message! (Note: We used paper towels, but our hands were all a little orange for a few hours afterward, so you might want to use a paintbrush instead!)
Enjoy reveling in the mysteries of science!
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Shevella
Is it possible to change the turmeric into something else that have a same kind reaction?
Michelle
The only other household natural indicator I’m familiar with is red cabbage juice. You can just blend some chopped up red cabbage with a little water and strain it. However, I tried that as well and found the color change to not be anywhere near as dramatic (at least using the baking soda solution to write the message). You could try the red cabbage juice with other basic or acidic solutions though (like maybe lemon juice) to see if it works better. Good luck!
Regina
I used curry powder and it worked okay. I haven’t tried the turmeric so, it may not have been as dramatic. However, my 4 and 6 year olds were still happy with the results.
Michelle
That’s great to know in case people have curry powder but not turmeric on hand. Thanks for sharing, Regina! 😄
Amaley
Is substituting the rubbing alcohol for something else a possibility?
Michelle
It might not work as well, but you could try it with water. I would let it sit for several minutes and stir or shake it really well to try and get as much of the pigment dissolved in the water as possible.
Ishita
Yes you can substitute rubbing alcohol for hand sanitizer with more than 60% alcohol. I tried it , it worked .
Michelle
That’s great to know!
zoe
does anyone know why this works?
Michelle
pH indicators change color based on the acidity of the substances they are exposed to. They basically undergo a chemical reaction that causes a change in color. Turmeric is a natural indicator, so it changes color when the turmeric and rubbing alcohol solution is mixed with baking soda which is a weak base.
Shawn
Does anybody know how to do this method, or really a very close version of this method only you would utilize an Ultra Violet light or even Black Light? Thank You
Michelle
Hi, Shawn. Amazon sells really inexpensive invisible ink pens that come with UV lights built into the cap. I have purchased those for my kids and they had a blast using them to write secret messages. The only downside is that the ink seems to dry up after a few months. It would be great to try and figure out how to make homemade replacement ink to use with those pen lights. Maybe tonic water? I’m not sure. I haven’t tried!
Kate
Liquid Tide laundry detergent (I haven’t tried it with other brands) will glow under a blacklight. It lasts for quite some time too!
Michelle
What a great idea, Kate! We will have to try this out!
Janelle
Does anyone know if I could write the secret message with the baking soda solution and the message can still be discovered with the tumeric solution a couple days after writing the message? Planning a escape room party for my son and have to deliver all activities the night before😊
Michelle
Hi Janelle! I haven’t tried that, but yes, it should definitely work! :)
Jenna Avery
Hi there, I was wondering if this worked for you. I wanted to try to do this and send an actual letter in the mail. Hoping it would still work after a few days! Thanks!
Michelle
Hi Jenna! Sorry it took so long to get back to you! I don’t see why the color changing ink wouldn’t work a few days later. I think it should! You would just have to explain to the recipient how to reveal it! If you tried it and had success or not, please let me know!
Kristen
I wrote all my students names ion the first day and when they discovered the secret message they were so excited! Thanks for a wonderful activity!
Michelle
Yay! I’m so glad your students had fun with this activity, Kristen!
Michelle
Could you do this activity using a small canvas instead of on paper?
Michelle
Hi Michelle! I don’t see why not. I think it should work. If you have any issues, let me know!
Frances Sinclair
We are going to try putting the solution in a small spray bottle to see how that goes for revealing the messages a bit easier. Who knows, maybe we could make artworks by putting both the “ink” and solution in spritz bottles. We will see.
Michelle
I hope it went well Frances! I would make sure you use the squirt bottle outside as the turmeric solution can stain! The spray bottle might keep it a little farther away from clothes though!