In this experiment, we'll learn how to extract colorings from a red (purple) cabbage and make them change color.
This experiment requires adult supervision!

Equipment

  • Red cabbage
  • A large bowl or jar
  • 4 small transparent jars, glasses or beakers
  • Boiling water
  • Baking soda
  • Trisodium Citrate
  • Vinegar or lemon juice
  • Laundry detergent

Instructions
The instructions can be viewed in the following video:

Explanation
Cabbage contains a compound called anthocyanin, which protects the plant from radiation, like sunscreen. Anthocyanin is found not only in cabbage but in flowers (anemones, cyclamens, etc.) and certain types of fruits (grapes, apples, etc.). In autumn, many leaves turn red mainly because of their anthocyanin content.

Besides providing protection from the sun, anthocyanin can be used as a pH indicator that changes color when mixed with acids or bases. Each of the reagents we used has its own typical effect on the pH in water: Lemon juice and vinegar, for example, are acidic. By contrast, baking soda and laundry detergent are basic. Anthocyanin reacts with acids and bases so that the products have different colors. This is how all indicators work.

For example:

Purple anthocyanin + acid --> Red product

Purple anthocyanin + base --> Green product

The acidity or basicity of a material is measured by the pH scale. Values lower than seven on this scale indicate an acidic solution, while values higher than seven indicate basic solutions. A solution at pH 7 is considered neutral in terms of acidity/basicity.

The beautiful thing about anthocyanins is that for every 2-3 pH units their chemical structure changes, and so do their colors. So a cabbage extract solution lets us know not only if a material is acidic or basic, but also the strength of its acidity or basicity.

Since this is not only due to one material in the cabbage extract changing color but rather a family of related materials that each change their color under slightly different pH values, we can get many different combinations of colors. Hence, by playing with materials of different acidity or alkalinity, bright yellow and lilac-purple colors can be obtained as well.

Dr. Avi Saig
Department of Neurobiology and Davidson Institute of Science Education
Weizmann Institute of Science

Article translated from Hebrew by Aviv J. Sharon, M.Sc. student at the Weizmann Institute of Science.

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