I have known about using red cabbage as a color-changing acid base indicator for years. However, this experiment always seemed like too much work. Who has time to boil cabbage? It turns out ME during quarantine, that’s who. I cannot believe how easy this red cabbage acid/base experiment was to put together. The kids had so much fun playing with science and it really is as simple as boiling some cabbage.
In the past, I’ve done acid/base experiments with my kids using a turmeric solution as the color-changing indicator. (This invisible ink experiment using turmeric is particularly fun!) The problem with turmeric is that the color change for bases is much more impressive than for acids. Red cabbage juice is really cool, because the color change is dramatic in both directions. The purple solution changes to bright pink or red for bases and green or blue for acids.
If you are looking for a good reference book to teach elementary-aged kids about acids and bases, I recommend Matter Matters! by Tom Adams and Thomas Flintham. It is full of lots of fun chemistry pop-ups and interactive flaps. Though not a long book, it covers topic such as atoms and molecules, states of matter, chemical reactions, and radioactivity in a way that is fun and engaging for young kids. It has a double page spread just on acids and bases.
If you are feeling like boiling cabbage is too much work for you (and I hear you), you can always just buy a jar of red cabbage. It will just cost a little more money and you will have less indicator to start with.
I also explain in this post the very simple process of making indicator strips from the red cabbage indicator solution. This just allows the child to dip the color-changing paper into the test solution to test for acidity. It is an easy extension to this experiment, but honestly, I think most kids will have more fun stirring the test substances into the indicator solution. If the indicator paper seems too intimidating or time-consuming, just skip those steps!
Recommended Age Range: Preschool, Kindergarten, Elementary
Time Required: About 10 minutes of prep, with 2 hours of letting the solution cool to prepare indicator solution. If making the indicator paper, that will take an addition couple hours of drying time.
Difficulty: Easy
Cost: Less than $2 in used supplies
Materials:
- head of red cabbage
- 4-6 cups distilled water
- 2 to 10 coffee filters (optional, only if making indicator strips)
Supplies:
- knife
- cutting board
- a large pot
- measuring glass
- spoon for stirring (not pictured)
- jar with lid for storing leftover solution
- funnel for pouring in jar
- strainer (not pictured, because my pot lid contains strainer holes)
- scissors (optional, only if making indicator strips)
- cookie sheet or drying rack (optional, only if making indicator strips)
Test Substances:
In addition to the materials and supplies needed to make the acid base indicator and test strips, you will also want to look around your house to find substances you can check for acidity. These can be anything! Grape juice, coca cola, olive oil, sand…let your kids be creative. Below are the substances we tested. Whatever you decide, make sure you test baking soda and vinegar! If you don’t have those, soap and lemon juice are great too.
- soap
- baking powder
- borax
- lemon
- orange
- baking soda
- water
- vinegar
Instructions:
- Measure 4 to 6 cups of distilled water. We used 6 cups so we would be sure to have some solution left over for a different day.
- Pour the water in a pot and bring it to a boil.
- Meanwhile, chop up your head of cabbage into chunks. You want your pieces to be bigger than whatever you will use to strain the solution.
- When the water is boiling, place the cabbage in the pot and turn off the heat.
- Stir.
- Wait for the solution to cool, stirring occasionally. For us, this took a couple hours.
- While the solution is cooling, get your coffee filter paper ready. If needed, cut it into pieces that will lay flat and are smaller than your pot. Each filter will makes several indicator strips. We used about 10 filters, but we have lots left over to save.
- When the solution is cool, place the filter paper in the red cabbage juice solution and make sure it is saturated.
- Lay the filter paper on a cookie sheet or other suitable place to dry.
- Strain the red cabbage indicator solution into either test containers (like cups) or into a jar to save for later, or both.
- Label the leftover indicator solution so you remember what it is and store it in your fridge.
- After the filter paper dries, cut it into strips. This may take a couple hours. I assisted the process with a blow dryer.
Now you are ready to let your kids experiment! How you want to conduct this experiment is entirely up to you. Since I have 4 kids, I gave them each 2 cups with cabbage juice indicator and as many test strips as they wanted. I let them each pick 2 test substances (one for each of their cups). They took turns guessing what color the solution would change before the poured their substances in. Red/pink means that the substance was an acid and blue or green means it was a base.
By the end, my kids were pretty good at guessing which substances were acids (like vinegar and citrus fruits), which were bases (like baking soda and soap), and which were neutral (like water). In the beginning, my 10 year old son guessed that soap was an acid, since it burns if it gets in your eyes. However, he questioned his guess, since soap does not sting in cuts like lemon juice.
After we used the indicator solution I put some more test substances (lemon juice, vinegar, baking soda stirred into warm water, etc.) in separate containers. Then they shared the substances and dipped their strips in whichever container they wanted. My daughter discovered it was fun to use a q-tip to draw on indicator paper. She was especially excited to discover she could change the indicator paper from blue to pink back to blue.
However your kids decide to play with the indicator solution and indicator paper, I hope they have fun. It’s experiments like these that make my kids say, “I love science.” At this age, that is the absolute ultimate goal when I do activities like this with my kids.
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