Make your own acid/base indicator with red cabbage!
Here’s a science activity that will have your kids saying, “Wow!” Did you know that red cabbage can be used as a pH indicator? Yep, you can tell if something is an acid or a base just by mixing it with red cabbage juice! It’s pretty cool!
How does it work?
Red cabbage contains a chemical called anthocyanin, which changes color in the presence of an acid or a base. Water is neutral, with a pH of 7. Acids have a pH lower then 7, and bases have a pH higher than 7.
To make the cabbage juice indicator, cut a red cabbage into chunks. I cut mine into 8 pieces. Then cover it with water in a large pot and bring it to a boil. Let the cabbage boil a couple of minutes and then turn off the heat. Let it cool for several minutes and then strain out the cabbage from the juice. I ended up with about 4 cups of purple colored juice.
Once the cabbage juice had cooled completely, I set up our experiment! I used plastic condiment cups, and they worked well.
We tested lemon juice, vinegar, baking soda, dish soap, toothpaste, citric acid (I had some left over from making dinosaur egg bath bombs), alka seltzer, tomato juice, and washing soda (purchased in the laundry aisle, used for making homemade laundry soap).
First, we tested the substances that they boys already knew were an acid and a base – baking soda and vinegar. You can see the results below.
I assumed that the washing soda (a base) would turn the cabbage juice blue just like the baking soda did. But it actually turned it GREEN!
I did a quick search and found this article that shows the colors that red cabbage juice will turn according to the pH. Green means that it is a stronger base than blue – a 12 on the pH scale!
(Washing soda is not good for kids to handle. My teenage son did the testing on this one.)
After we had tested everything, we had a nice rainbow of colors!
I wasn’t sure if the tomato juice would be a strong enough acid to test, but it was! All I did was to squish a tomato wedge into the little condiment cup. It turned a purplish pink – more pink than the control (plain cabbage juice) but not as red as the vinegar and citric acid.
Overall, our bases were less impressive. The baking soda and washing soda had the most noticeable color change. The dish soap might have turned the indicator blue, but it was hard to tell. We have some of the cabbage juice left, so I’m going to try cream of tartar and an antacid tablet crushed and mixed with water before we dump the rest.
Jonathan and Janie were napping when we did the experiment, so we did a preschool version when they got up.
So then Aidan wanted to take the blue cup (with baking soda in it) and add vinegar to make a reaction.
And it reacted!
Notice also that the liquid changed back to purple! It wasn’t sure if it would change back once it had turned blue, but it did. Aidan added more vinegar after the reaction settled down, and the liquid turned to pink. Pretty cool that the same liquid will show a pH change more than once!
Make your Own Litmus Paper
One more thing that we tried was making our own litmus paper. I soaked some coffee filters in the cabbage juice and then let them dry. Owen had a great time dripping vinegar and baking soda in water onto the coffee filter with some plastic pipettes. It was so cool to see the paper turn pink and blue by dripping clear liquids on it!
Have fun exploring with chemistry!