pH Rainbow  as seen on Let's Talk Live
more experiments

Materials:
purple cabbage
vinegar
lemon juice
water (preferably distilled)
baking soda & water solution
egg whites
liquid Drano
clear glasses or test tubes

Directions: Coarsely chop at least a cup of purple cabbage and cover with water.  Bring to a boil, then simmer for about 10 minutes.  Turn off the heat and allow to soak another 20 minutes or so.  Strain out the cabbage and allow the liquid to cool.  This is your cabbage juice pH indicator.

Pour at least 1/8 cup of each of the fluids (vinegar, lemon juice, water, baking soda & water, egg whites, liquid Drano) into a separate glasses, cups or test tubes.  For fun, mix them up so it is more difficult to tell which is which.  Now add about a teaspoon of the cabbage juice indicator to each.  When you are finished, you can arrange your cups into an acid and base rainbow!

Extensions:  Test lots of fluids in your house using the cabbage juice.  Try orange juice, tomato juice, soapy water, dish detergent and various household cleaners.

Science:  Most of us know about acids and bases in the context of making pretend volcanoes.  If you combine vinegar (an acid) with baking soda (a base) you will get a great bubbling, fizzing chemical reaction.  (CAUTION:  sometimes household chemicals, when combined, make poisonous gases, so when combining your acids and bases, stick to vinegar and baking soda.)  The reaction happens as the two substances give or receive ions and neutralize each other. 

Acids typically taste sour (think of lemon juice -- though not all acids are edible!) and bases typically feel slippery (think of soapy water).  Kids probably won't understand the specifics of the molecular structure that makes up an acid or a base -- it's enough to know that sometimes things are acids and sometimes things are bases, just like sometimes things are insects and sometimes they are birds, and you can tell them apart with a pH indicator like cabbage juice.  It is also worth noting that not all things are an acid or a base.  Many things are neither. 

Red cabbage works as an indicator because it contains a pigment called anthocyanin that changes color when it is mixed with an acid or a base.