Ages 7 and up; Download Printable PDF
You will need:
red cabbage
knife
large pot
stove
clear bowl
measuring cup
clear plastic cups
spoons
straw
chemicals to test: water, baking soda, vinegar, lemon juice, ammonia, soda pop, etc.; clear liquids work best. Caution! ammonia is caustic and should only be handled by an adult.
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| from left to right: vinegar, lemon juice, Sprite, water, baking soda, ammonia |
Preparation
Place half the cabbage in a large pot and cover with water. Boil until the water turns a purplish green. After the juice has cooled, pour some into a clear bowl and dilute as necessary so you can see to the other side of the container. Congratulations! You have just made a pH indicator, a substance that changes color with pH.
Instructions
Set out a clear plastic cup for each chemical you plan to test, and pour a quarter cup of your pH indicator into each. Pour or spoon a little of each substance into the appropriate cup, and stir. Make sure to test some acids (like soda pop, vinegar, and lemon juice); some bases (like baking soda and ammonia); and water.
If you add water the pH stays neutral and the indicator has a blue color. Add a mild acid like fruit juice and the indicator turns purple. Add a stronger acid like vinegar and it turns pink. Add a base like baking soda or detergent and it turns green.
Watch carbon dioxide acidify water
Pour a quarter cup of pH indicator into a cup. Using a straw, blow bubbles into one of the cups for about a minute. Compare the color of the liquid in the two cups.
What’s Happening?
When you exhale, you breathe out carbon dioxide. The carbon dioxide reacts with the water to form carbonic acid, which acidifies the water and turns the pH indicator more blue.
Did you know?
Chemicals called anthocyanins are responsible for red or purple pigmentation in many plants, including red cabbage, berries, grapes, and red fall foliage. Anthocyanins change color with pH, and anthocyanins extracted from almost any red pigmented plant can be used to make a pH indicator.
Place half the cabbage in a large pot and cover with water. Boil until the water turns a purplish green. After the juice has cooled, pour some into a clear bowl and dilute as necessary so you can see to the other side of the container. Congratulations! You have just made a pH indicator, a substance that changes color with pH.
Instructions
Set out a clear plastic cup for each chemical you plan to test, and pour a quarter cup of your pH indicator into each. Pour or spoon a little of each substance into the appropriate cup, and stir. Make sure to test some acids (like soda pop, vinegar, and lemon juice); some bases (like baking soda and ammonia); and water.
If you add water the pH stays neutral and the indicator has a blue color. Add a mild acid like fruit juice and the indicator turns purple. Add a stronger acid like vinegar and it turns pink. Add a base like baking soda or detergent and it turns green.
Watch carbon dioxide acidify water
Pour a quarter cup of pH indicator into a cup. Using a straw, blow bubbles into one of the cups for about a minute. Compare the color of the liquid in the two cups.
What’s Happening?
When you exhale, you breathe out carbon dioxide. The carbon dioxide reacts with the water to form carbonic acid, which acidifies the water and turns the pH indicator more blue.
Chemicals called anthocyanins are responsible for red or purple pigmentation in many plants, including red cabbage, berries, grapes, and red fall foliage. Anthocyanins change color with pH, and anthocyanins extracted from almost any red pigmented plant can be used to make a pH indicator.
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| pH indicator color before (left) and after (right) blowing carbon dioxide bubbles into the liquid |
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