Science Saturday: Chemistry and the Cabbage
We have been working on a project for the past week where we are dying t-shirts in sunbath. This is something you could just as easily do on a stovetop, but sometimes I like to do things the hard way because I want the kids to see how things use to be done way, way, way back in the old days. The process stinks- and I mean STINKS, but it is still interesting to see what color the vegetables create. I will be posting more on this activity next week. We haven’t pulled our shirts from the jars yet, and I wanted a finished project to show you.
In the meantime, a curious thing occurred. After making what was essentially a red cabbage tea it was time to add a mordant to the mixture in order to “fix” the dye. At this point the color of the dye was reddish purple but after adding alum to it it turned a very deep shade of purple almost blue. I found this rather perplexing and wanted to find out why.
Red Cabbage is a natural acid-base indicator. So in adding alum to my solution the cabbage mixture immediately turned blue indicating a base had been added.
Here is a PH scale
Want to try this experiment?
This is what you need:
Fresh red cabbage
A sharp knife
A cutting board
Hot tap water
You will need a cup for each household substance you test
A spoon for each corresponding cup
A large plastic bottle or glass jar
A range of the household substances which may include: (acids) toilet cleaner, vinegar, lemon juice, (neutrals) pure water or baby shampoo, (base) baking soda, liquid dish soap or milk of magnesia.
This is what you do:
Cut your red cabbage into small chunks and place in a plastic bottle or glass jar. Now fill the container to the top with hot tap water, and allow the container to set for approximately 20 minutes or until the mixture has cooled. Once the mixture has cooled, strain the cabbage from the mixture but retain the liquid because it will serve as your base-acid indicator. Take the liquid and pour it evenly in the glasses you will be using to test your household substances. Add one household substance per cup and stir. What color is the liquid now?
Below you can see some of our photos. I apologize because the lighting isn’t the best. We used baking soda, vinegar and dish detergent. The baking soda changed our solution to a blue, vinegar to a hot pink and our dish detergent appears to be more on the neutral side because it really had very little affect when it came to changing the color.

Filed under: Science on June 30th, 2007 | 2 Comments »









