Rosie Research just had a major breakfast debacle. The color is off, the taste is off, and now her day is off. Let’s help her find out what is going on!
Let’s help Rosie find out.
Making your own purple smoothie is easy! Just chop and boil a purple cabbage for 35-40 minutes. The cabbage should be lightly colored while the water will be a very deep dark purple. Congrats! You now have a cabbage pH indicator!
You will need our fun kids science activity download. Our Cabbage pH Indicator Lab is a great activity for all ages. Along the way, you will get to read Rosie’s story, figure out what she really added to her drink and solve some acid/base puzzles.
Kids in Action
Purple cabbage pH indicator – how does it work?
Red cabbage juice is purple because of anthocyanin, a pigment often found in flowers, fruits, and leaves. In fact, it is the cause of many of the reds, blues purples, and oranges we see around us everywhere.
Colors of the world around us are determined by how molecules like anthocyanin absorb light. They can absorb light into their outer shell electrons (called valence electrons). The energy absorbed into the bonds through light is light that will never reach our eyes, so if red light is absorbed, a substance would look blue.
When we add in acid we change the structure of anthocyanin. That is because adding acid is adding in a bunch of hydrogen ions (H+), and oxygen can bind with that hydrogen. Can you see the difference between the acidic molecule below and the one above?
Adding base is like adding a bunch of hydroxide ions (OH-). This also changes the molecule. Can you spot the difference?
All of these changes to the molecule changes the bonds. With different bonds, we absorb different energies. That means absorbing different wavelengths, or colors, of light! Think about it this way. When you are tuning a guitar tighter you hear a higher note. When you tune it lower you hear a lower note. Those different frequencies correspond to different wavelengths. In light, the wavelength determines the color. Tuning the bonds in the molecules changes the color absorbed!
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Flavinoids, Flavinoids, Everywhere.
The molecule anthocyanin, a flavonoid, is found in the coloring of fruits, vegetables and even leaves all around us. Blackberries, blueberries, cherries, plums, red apples, wines and even red beets all are chalk full of flavonoids. Experiment with other plants that could be used as a pH indicator like cabbage. Which one works the best?
Not so secret anymore.
Red cabbage juice will only change color where a strong acid or strong base is present. Using full-strength lemon juice write a secret message on a piece of paper. After it dries use red cabbage juice to paint over the paper. What do you see?
Homemade pH paper.
You can make your own pH paper, also known as litmus paper, using red cabbage juice and coffee filters. Just soak the filters, dry, and cut into strips. Now you can take your pH testing out of the lab and into the wild!