Wikijunior:Big Book of Fun Science Experiments/Red cabbage indicator

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An acid and base indicator is a chemical that shows whether different chemicals are acids (like lemon juice) or bases (like baking powder). This is called a chemical's pH. You can make an indicator using red cabbage juice. The cabbage juice changes color depending on how acidic or basic (alkaline) the chemical is. The pH level is indicated by the amount of hydrogen (the "H" in "pH") ions in the tested chemical. Hydrogen ions are protons that had their electrons stripped off them (since hydrogen atoms consist of one proton and one electron).

We often use a test like this to make sure swimming pools are kept at the right pH. If the water in a swimming pool is too acidic or too basic (alkaline), it can damage pool equipment, bathing suits, or stop the chlorine that keeps pools clean from working properly.

pH is measured on a scale of 1 to 14. Neutral chemicals have a pH of 7. Acidic chemicals have a pH of 1-6. Basic (alkaline) chemicals have a pH of 8-14.

The Experiment[edit | edit source]

Safety[edit | edit source]

As with all experiments, you should always get help from an adult. Some acids and bases can hurt your skin, so you should wear:

  • Rubber gloves
  • An apron
  • Safety glasses (regular glasses, sunglasses or swimming goggles are okay if you don't have safety glasses).

If you get chemicals on your hands or gloves, do not touch your eyes! Wash your hands after completing the experiment.

What You Need[edit | edit source]

  • A red cabbage
  • A knife or grater
  • A medium sized bowl
  • At least 4 small cups
  • Chemicals to test such as lemon juice and baking powder or soda
  • coffee filter

Step 1[edit | edit source]

Red Cabbage Cups.jpg
Spotting tile.png
  • Have a parent, guardian, or teacher help you cut or grate between a quarter or a half of the cabbage into small pieces.
  • Put the cabbage into a medium sized bowl.
  • Pour water over the cabbage. You can use cold water, warm or boiling water. Boiling water will work the fastest, but only boil the water if you have an adult to help you.
  • Let the cabbage sit in the water for a while (30-45 minutes), until the water turns a deep purple colour. You can stir the mixture to speed up the process.

Step 2[edit | edit source]

  • Using a coffee filter or a fine sieve, strain the juice into a cup. It should be deep bluish purple in colour.
  • Pour equal amounts of the strained liquid into small plastic cups. These are your 'test tubes' in which you will test different chemicals.

Step 3[edit | edit source]

  • Set one of your cups ('test tubes') aside. This is your control. It already contains water, which usually has a pH of 7. This means that water is neither an acid nor a base.
  • Assemble some testing chemicals (some good ones to try are below).
  • Label each cup with the name of the test chemical each cup will contain, so you will remember what you put in each cup.
  • Which chemical do you think is the most acidic?

Step 4[edit | edit source]

  • Pour, drip or spoon a small quantity of each test chemical into the cup labeled with its name.
  • Observe the change in colour of the juice. If the juice turns reddish, the chemical you are testing is an acid. If the water turns bluish, the chemical you are testing is a base. If it stays the same, the chemical is neutral, like water.

Some Chemicals to Test[edit | edit source]

  • Lemon juice
  • Baking powder or soda
  • Milk
  • Yogurt
  • Vinegar
  • Soap

Observations[edit | edit source]

  • Arrange your cups from red, to pink, to violet, to purple, to blue, and to green. You might not have all of these colors, but that's okay.
  • The cup that is most red is the most acidic. The cup that is the most blue or green is the most basic (alkaline). Which chemical is the most acidic? Which is the most basic (alkaline)?