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Friday, December 10, 2010

pH & Indicators

Lemon juice is an acid. It tastes sour. Soap and baking soda are both bases. They taste bitter. Pure water, which is neither acidic nor basic, is considered neutral. Acidity and basicity is measured using the pH scale. Neutral substances such as water have a pH of 7. Acids have a pH less than seven. The lower the pH, the more acidic the substance. Bases have a pH greater than 7. The higher the pH, the more basic the substance. In this experiment you will make a pH indicator, a substance that changes color with pH, and use it to measure the pH of common substances.    

Ages 7 and up; Download Printable PDF

You will need:

red cabbage
large pot
clear bowl
measuring cup
clear plastic cups
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.
from left to right: vinegar, lemon juice, Sprite, water, baking soda, ammonia

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.


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.
pH indicator color before (left) and after (right) blowing carbon dioxide bubbles into the liquid

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