Cabbage Chemistry

cabbage-chemistry

This red cabbage science project is a bit smelly (eww) but LOTS of fun! Use this ordinary vegetable to make a pH indicator that tests the alkalinity and acidity of various liquids.

Pour some cabbage juice and watch acids (like vinegar and lemon juice) turn green, while bases (like baking soda and laundry detergent) turn a bluish-green hue. Plug your nose and bring some friends; you won’t regret it…

Materials:

Instructions:

  1. Peel about four red cabbage leaves and place them in your blender filled halfway with water. Blend on the highest setting until you have purple cabbage juice.
  2. Pour the cabbage juice through the strainer to filter the chunks. Save the liquid for later.
  3. Set up three glasses or plastic cups next to each other against a white sheet of paper. The paper will be the background. Fill each glass halfway with cabbage juice.
  4. Add a small amount of vinegar to the first glass and stir with a spoon until the mixture turns red. This means vinegar is an acid.
  5. Add a teaspoon of laundry detergent or washing soda to the second glass. Watch how the liquid turns green, indicating a chemical base. Keep these glasses for reference along with the third glass of cabbage juice.
  6. Fill more glasses with cabbage juice. Try adding the other chemicals (lemon juice, baking soda, washing soda, laundry detergent, Alka-Seltzer, and clear soda) to a small amount of cabbage juice and take note of the color change. Determine if each chemical is an acid or a base.

Tips and Tricks:

  • Use the purple cabbage indicator to test other substances. Try orange juice, lemonade, milk, salt, ammonia, or soap. Are they acids or bases?
  • Make pH indicator strips. Soak some coffee filter paper in cabbage juice. Then, remove the filter and hang it with a clothespin to dry completely. Cut the paper into thin strips and dip these into different liquids to test their acidity or alkalinity. The redder the strip, the more acidic the liquid is. The greener the strip, the more basic it is.

Explanation:

Many substances are either acids or bases. Acids have a low pH, while bases have a high pH. On the other hand, water is neutral with a pH of 7 on a scale of 0-14. An indicator tells us if a substance is an acid or a base. It is usually a chemical that changes color when it comes in contact with an acid or base.

As you know, purple cabbage juice turns red when it mixes with something acidic and turns green when it mixes with something basic. Therefore, cabbage juice is an indicator.

So, why does this happen?

Well, red cabbage contains a pigment called anthocyanin. This water-soluble chemical changes color when mixed with an acid or base. When the pH is less than 7, the pigment turns red. When the pH is greater than 7, the pigment turns a bluish-green hue.

When there is no color change, the substance is most likely neutral, like water.

 

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Image attribute:
By Ian Alexander (Own work) [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons

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