Starring
You and Your Students!
Script By
Vicki Cobb, Education World Science Editor
Synopsis
Change the color of a mixture by adding different substances.
Genre
Chemistry
Props Required
- red cabbage juice (see instructions for creating it)
- large jar
- teaspoon
- white vinegar
- chlorine bleach, ammonia, and other cleaning supplies
- soap flakes (not detergent)
- cream of tartar
- baking soda
- sink (or an alternative basin) in case mixtures overflow from jar
Setting the Scene (Background)
I've known about the color-changing properties of red cabbage juice for so long that I sometimes forget how exciting and effective it is for a fresh audience. It's like the old magician's "water changing into wine" trick. Most importantly, it gives kids ideas about trying things just for fun to see what happens.
If you remember the "litmus" test from your own school days you have a general idea of what this demonstration will illustrate. Cabbage juice can be used to distinguish between two important kinds of chemicals -- acids and bases
Stage Direction
Do this as a demonstration; but explain to students how they can make the cabbage juice in case they want to try it when they get home.
Plot
Act I
To prepare the red cabbage juice:
- Cut up a small red cabbage and shred it in a food processor (or grate into a bowl).
- Add enough water to cover, then stir. The water will turn the reddish-purple color of the cabbage.
- Collect the water by straining the cabbage mixture over a bowl.
Act II
Perform the following demonstration for students:
- Put about a cup of the red cabbage juice into a large jar. Fill the jar to between 1/3 and 1/2 full.
- Perform the next steps over a sink or another overflow basin -- in case the container overflows: Stir in, one ingredient at a time, about a teaspoon of white vinegar, then soap flakes, then cream of tartar.
What color does the mixture become with each added ingredient? (Some things will turn the red juice pink, while other things will turn it green…)
- When the container gets nearly full, pour out the liquid and start again. Be sure to wash out the jar between experiments.
Act III
Ask students to take notes -- to answer these questions as you proceed:
CAUTION:
Be sure students do not mix bleach and ammonia. The combination of bleach and ammonia can be very harmful.
- Which items turn the juice green? (soap, baking soda, ammonia)
- Which items turn the juice pink? (cream of tartar, vinegar, lemon juice)
- Which items produce foam? (baking soda and an acid - vinegar; they are the two chemicals needed to release carbon dioxide) Check your hunches by mixing the ingredients for foam in water instead of cabbage juice.
- Chlorine is a bleach -- it reacts with the pigment in the cabbage juice to make it colorless.
Behind the Scenes
The cabbage juice contains a pigment that changes color depending on
the chemicals around it.
- Acids are solutions that have a sour taste. They turn the cabbage juice pink.
- Bases, or alkalis, have a slippery feel. They turn the cabbage juice green.
- Acids and bases react to neutralize each other. When the juice returns to its original purple color, you're working once again with a neutral solution.
The End
See if other colored pigments from fruits and vegetables react the way
the cabbage juice reacted. Try making extracts of beets, cherries, or
blueberries. You might try violets or irises by chopping them up and mashing
them in water.
Article By Vicki Cobb
Education World®
Copyright © 2005 Education World
Originally published 03/04/2005
Last updated 03/28/2008
|
