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Cabbage Juice Indicator

Test the pH of various substances in your kitchen, and be wowed by the amazing changing colours of cabbage juice! 

“Chemical” is really just a fancy name for matter or stuff. So chemicals are everywhere, and make up everything from the air around us, to you yourself! A useful way to make sense of all of the chemicals around us is to find out whether they are acids, bases, or neutral.

Acids and bases are opposite like hot and cold; neutral means neither or in-between.

Acids create hydronium ions when in contact with water and have a sour taste. Lemon juice and vinegar are both acids. Bases release (or create) hydroxide ions in water and taste bitter. Many soaps and cleaning products are bases. When hydroxide ions and hydronum ions combine, they create water again which is neutral.

Cabbage juice contains a special molecule called anthocyanin (flavins), which gives red cabbage its colour. Anthocyanin is also found in blueberries, grapes and lots of other plants. When anthocyanin comes in contact with the hydronium ions in an acid it turns pink, and when it comes in contact with the hydroxide ions in a base it turns blue or green.

We refer to cabbage juice as a pH indicator because it can tell us if a substance is acidic or basic by changing colour. Other pH indicators are litmus paper and phenolphthalein. 

Vocabulary:

  • Acid: Acids create H3O+ ions when in contact with water (for simplicity sometimes called H+). The term comes from the Latin word acidus that means “sharp” or “sour”. Some properties of acids are that they taste sour, react with metal, change the colour of litmus paper from blue to red, and have a low pH (0 – 7). Examples of acids are vinegar and lemon juice.
  • Base: A compound that produces OH- ions when in contact with water. Another word for base is alkali. Some basic properties of bases are that they can be used as household cleaners, turn red litmus paper blue, and have a high pH (7 – 14). Some examples of bases are ammonia and baking soda.
  • Indicator: A substance that indicates the degree of acidity or alkalinity of a solution through characteristic colour changes.

Objectives

  • Explain the importance of observation when doing science.

  • Explain the chemistry behind a simple acid-base reaction.

Materials

  • Per Student:
    Safety goggles
    Red cabbage solution
    Small clear plastic or glass cups or beakers

  • Per Class:

    Red Cabbage Solution:
    Heat proof bowl or cup
    Boiling water
    Sharp knife
    Strainer

    A variety of household substances, such as:
    Lemon juice (citric acid)
    Vinegar (acetic acid)
    Baking soda (sodium bicarbonate, a base)
    Washing soda (sodium carbonate, a base)
    Cream of tartar (potassium bitartrate, an acid)
    Antacids (various bases)
    Soda water (carbonic acid)
    Ammonia (a strong base)
    Ivory soap (not detergent, a base)

Key Questions

  • Which solutions are acids?
  • Which solutions are bases?
  • How can you tell and acid from a base?

What To Do

Preparation: 

To make the cabbage juice:

  1. Coarsely chop about 1 cup of red cabbage into small pieces, and add to the heat proof container.
  2. Pour boiling water over the cabbage until well submerged. Let steep for one hour.
  3. Allow to cool, and then strain to remove cabbage pieces, reserving liquid.
  4. Refrigerate cabbage juice until you’re ready to use it.

Instructions: 

  1. In a clear container (plastic cup or glass), add about 50mL of one of the substances you wish to test. (For baking soda or other dry materials place about 1 tsp and top up with water). Label the container!
  2. Add about 1 tbsp of cabbage juice to the test substance and observe.
  3. Try other substances, making sure to always start with a clean cup.

Students can do steps 1 and 2 themselves for most of the household chemicals listed above.

Adults should demonstrate the results with ammonia, which is strong enough to be dangerous (Wear safety goggles).

Extensions

  • Do the cabbage juice test on other safe household fluids (milk, orange juice, coffee).
  • Can you predict what colour will result (and whether they are acids or bases) based on the ingredients listed?