RED - CABBAGE INDICATOR

A sour taste is one indication of a type of solution called an acid. The word acid comes from a Latin word meaning sharp or biting to the taste. Acids also conduct electricity. A light bulb connected by wires to two electrodes will light up when the electrodes are submerged in an acid. Acids are not the only solutions that conduct electricity. Bases, also called alkalies, do the job as well.

Acids are present in many of the foods we eat. Lemon juice and vinegar are good examples. We also eat certain bases, although they are not as common as acids. Baking soda, for example, is a base when it is dissolved in water.

There are, of course, many stronger acids and bases that we don't eat cecause they are poisonous or extremely damaging to living tissues. Tasting is not a test used by chemists to determine whether or not a substance is an acid or a base. Instead, they use a dye called an indicator that changes color depending on what the solution is. Litmus paper turns blue when it is dipped in a base and pink when it is dipped in acid.

The pigment in red cabbage can be used as your own personal "litmus." Here's how to make some red-cabbage indicator.

Materials & Equipment
  • 1 whole red cabbage
  • water
  • a knife
  • a grater
  • 2 large bowls
  • measuring cups and spoons
  • a slotted spoon
  • a strainer
  • a very clean glass jar
  • a small white dish

Procedure

Cabbage Grater

1 Cut the cabbage into quarters. Grate it section by section into a large bowl. Add between 1 and 2 cups water, enough to cover the cabbage. Let the cabbage stand in the water, stirring occasionally so that all the cabbage is moistened

2 When the water is a strong red, remove as much of the grated cabbage as you can with the slotted spoon and save it in your second bowl. Pour the water solution through the strainer into the glass jar. Add the strained cabbage to the rest of the cabbage you have saved.

3 Put about a tablespoon of your cabbage-juice indicator into a small white dish. Test for an acid by adding a substance you know to be an acid to the cabbage juice. Notice the color it becomes in acid. Now use a fresh sample of cabbage juice and add a substance you know to be a base (baking soda). Again notice the color change. Pretty gorgeous! Now add acid to the base-and-indicator mixture to reverse the color change. What happens when you add base to the indicator showing the acid color? An acid and a base will react with each other to neutralize each other. The original color of the red cabbage is pretty close to neutral. Now you can test some foods to see which category they fall into. Here is a list to get you started:

  • Cooking water from boiled vegetables, including beans, peas, onions, carrots, turnips, celery, asparagus, etc.
  • Liquids from canned vegetables and fruits
  • Cream of tartar
  • Soda pop
  • Egg whites
  • Fruit juices
  • Tomatoes
  • Cottage cheese
You can use the grated red cabbage raw in a salad or cole slaw, or you can experiment with it further.
Materials & Equipment
  • shredded cabbage from above experiment
  • 1 tart apple
  • water
  • a knife
  • a wooden spoon
  • 2 aluminum pots (The pot metal may be written on its underside. If you're not sure, ask the cook in your house.)
Procedure

1 Divide the cabbage evenly between the two aluminum pots. Cut the apple into quarters and remove the core. Cut the apple into chunks and add it to one of the pots. Add just enough water to cover the bottom of each pot (1/4 - 1/2 cup).

2 Then cook the cabbage over low heat about 20 minutes, stirring occasionally.

Observations

Which cooked cabbage is redder? Which one contained acid? How do your finding support the idea that small amounts of aluminum combine with water to form aluminum hydroxide, a base? What did the apple do to this base?

You can also prepare your indicator from cooked cabbage but, of course, you can't cook it in an aluminum pot. Put raw grated cabbage in a stainless steel, porcelain, or glass pot and cover with water. Cook over low heat until it boils for 3 or 5 minutes. Then drain off your liquid. It will be a darker red than the uncooked indicator. Compare indicators to see which one you prefer using.

Mixed with salt, pepper, and butter, the cooked cabbage goes well with pot roast.

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