Did you know that Red Cabbage Juice can be used as an acid-base indicator? Well I guess now you do! I use this easy acid base lab to get my high school chemistry students to begin working with acid-base indicators. As cool as universal indicator is, it’s a pretty yucky chemical, not to mention, highly flammable.
I started out my teaching career in a retired art classroom. We had no lab, no equipment and basically no chemicals. Because of the no-chemicals chemistry situation, I was forced to resort to some green chemistry and kitchen chemistry options for my classroom. Hence the stinky cabbage that I could simply grab at the grocery store. Unfortunately, we didn’t have very many outlets, so I had to extract the cabbage juice at home in my kitchen.
One of the few things we had was spot plates, which was pretty surprising. This acid base lab activity could totally be done with paper or plastic cups as well. Because most of what we dealt with was household items, I wasn’t too worried about all the regular safety protocol.
When talking real life acid base, I also love to bring up blueberry muffins because the same thing happens. The blueberries act as an acid-base indicator. The top of blueberry muffin can sometimes look green all the way to purple. It’s not a coincidence! I mean hydrangeas also are acid base indicators… I’m going a little off the cabbage path here. Sorry! I just get excited about this kind of stuff.
Setting Up The Kitchen Chemistry Acid Base Lab
I’ve done this both ways – one is less stinky than the other. Either boil the cabbage (stinky!) until the water is a nice deep purple, or you can set the cabbage in water overnight and let the juice ease out on its own, almost like a cold brew coffee.
The cabbage juice will act as the acid base indicator. Then I give the kids a few different samples of known pH. I’ve always just worked with whatever was in my chem lab at the time. When I was doing this with no chemicals, I used vinegar and ammonia. I would use pH paper or universal indicator to test for the pH values myself and dilute with water as needed.
With known pH values, the kids are able to set up the color changes the cabbage juice undergoes. They drop the solutions into a spot plate and add a few drops of the cabbage juice. The color changes are quite incredible! Most of the pHs turn out to be some shade of purpley magenta but there is a nice denim blue and green that show up as well.
Student Lab Report
I have my students describe the color it makes in words (which is always fun – the girls seem to be especially good at this). They also need to color in a box with colored pencils. Now that I am in a 1:1 school, I will have the students take pictures of their color charts and upload them into their lab reports.
Then I give the kids a few substances of unknown (to them) pHs. Now they need to use their color charts to determine the pH of the unknown solutions. I love this lab for a lot of reasons, but the standard solutions really do it for me. It gets me excited to see them build real chemistry skills! I make sure that kids actually take the time to write thorough lab reports for this activity. I especially love when the kids extract the cabbage juice themselves. It allows them to take ownership of the entire process.
Teaching Chemistry Through this Lab Activity
This lab activity is a great tool to introduce standard solutions. The solutions with known pH values are known as standards, and the kids will effectively “calibrate” their color chart in accordance. Then they will test unknown solutions against the standard. While this is a super easy acid base lab, you can also dig into the analytical chemistry in a high school setting without getting too technical. Knowing that quantitative analysis was my favorite chemistry class, this lab has a special place in my heart.
I have been interested in using the cabbage juice in a titration in place of phenolphthalein, or some other indicator, but I’ve never done it. I didn’t have burets in my first school setting so I never added this to my lab book. I’ll have to get back to you on that one.
Red Cabbage Lab Makeup Sessions
This lab is a little tougher to schedule make ups for unless you have a freezer in your lab area. You can pour some of the extra cabbage juice in an ice cube tray and save the juice. Usually I just take a sample of juice from some kids in the class that have leftovers when their lab is done. I know this can be a pain, but I’ve found that freezing the juice helps to keep me sane, since cabbage isn’t a huge part of my diet. It saves me from running to the store over and over for a few leafs of cabbage.
