Moldy Blue Mandarin Turns Deliciously Yellow!? The Secret Behind the “Magic Paper” That Amazed a Kindergarten
I’m Ken Kuwako, a science trainer. Every day is an experiment.
“Teacher, is this magic paper!?”
Those amazed voices filled the kindergarten classroom. This year, we launched a Science Show Development Seminar at our school, bringing together 20 students who absolutely love science. This time, I visited Sonohara Kindergarten with these students, where we put on a science show for two classes of senior kindergarteners. Today, I’d like to share the star experiment that stole the show.
The secret is “color-changing paper”! Simple to prepare, but full of science
To reveal the secret, this magic paper is simply Japanese calligraphy paper soaked in BTB solution, an indicator that tells us whether a substance is acidic or alkaline. We added a little baking soda to the BTB solution to make it alkaline, soaked the calligraphy paper in it, and let it dry. The result was bright blue paper. For this event, we prepared about 30 sheets of this “magic paper.”
We also prepared two liquids: water containing citric acid, and water containing baking soda and sodium carbonate. These two liquids were the real secret behind the paper’s magical color changes.
Showtime! The Doctor and the Great Adventure of the Orange
The show began with a little skit. A “doctor” from the junior high school appeared and spoke to the children.
Doctor: “I have something I want to show you today! I finally finished it yesterday! It’s magic paper—the ‘Magic Paper’!”
First, we asked the children to draw an orange on the paper with crayons. But because the paper was blue, their finished oranges looked almost like moldy oranges. The children looked worried and said things like, “Hmm… it kind of looks moldy.”
That was when the first liquid appeared: citric acid solution. According to the teacher playing the doctor, “It contains the same ingredient that you find in lemons!” We dipped a brush into the liquid and painted it onto the blue orange.
And then—amazing! The paper gradually changed to a vivid yellow. The moment the blue, moldy-looking orange suddenly transformed into a juicy, fresh-looking orange, cheers erupted throughout the room. The children’s eyes lit up as they eagerly enjoyed painting their pictures.
The mysterious return to blue—and the world of acids and bases beyond it
But the excitement didn’t stop there. Next came the second liquid: baking soda solution. As the doctor explained, “It’s a liquid that’s part of the same family as soap,” we painted the yellow oranges with the baking soda solution. The color changed back to blue once again. The delicious-looking oranges had suddenly turned back into “moldy oranges”!
The science behind this magical transformation is the basic property of acids and bases. BTB solution turns yellow when it comes into contact with an acidic liquid and blue when it comes into contact with an alkaline liquid. The citric acid solution is acidic, while the baking soda solution is alkaline, so the color kept switching back and forth every time we painted it.
Another fun experiment involved mixing the two liquids in a plastic bottle. When we added citric acid solution to the green liquid, it turned yellow. Then, when we added baking soda solution, it turned blue again. The children shouted, “It’s yellow!” and “It turned blue!” The dramatic, visible color changes grabbed their curiosity in an instant. That, to me, is one of the very best things about science.
The surprising world of acids and bases was right there in a familiar snack
This experiment is actually connected to a popular Japanese candy called “Nerunerunerune,” a familiar favorite among children. When you mix its powders with water, the color and texture change—and, surprisingly, the process also makes use of the action of an acid, similar to the acids found in lemons.
The children were fascinated when the doctor explained, “Sour things belong to the ‘acidic’ family, while soap belongs to the ‘alkaline’ family.” The detailed science is something they will learn later in elementary school, but planting the seeds of curiosity early on—making children think, “That’s strange!” and “I want to know more!”—can have a powerful impact on their future learning.
Finally, instead of revealing the secret behind the experiment, we gave each group a set containing BTB paper, citric acid solution, baking soda solution, and brushes. The children were then free to create their own “magic pictures.” Watching them change the colors with sparkling eyes and complete concentration was, I think, an unforgettable experience not only for the children but also for the students who led the activity.
For the second experiment, we made paper-cup gyro rockets.
Let’s Make a Paper Cup Gyro Rocket!
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Let’s bring the wonder and fun of science a little closer to everyday life! On this site, you’ll find fun science experiments you can try at home, along with easy-to-understand tips and explanations. Take a look around and see what you can discover!
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