Make a pH indicator!

Key Concepts

Materials: 
1    blender
5    large red cabbage leaves
6     cups (clear or white if possible)
6    different substances from around the house (We recommend water, baking soda, vinegar, lemon juice, bleach, and soda.) ​

Activity Directions:
  1. Put 8 cups of water and 5 large red cabbage leaves into the blender. Blend until smooth.
  2. Fill each cup halfway with cabbage juice. 
  3. Add each substance to a different cup of cabbage juice. Each cup should be about ¾ full.
  4. Observe the color changes!
Discussion Questions & Science Explanations:
  1. What is pH and the pH scale?
    1. pH is a measurement of how strongly acidic or basic a substance is. The pH scale starts at 7, and substances with a pH of 7 are neutral, like water. Substances with a pH less than 7 are acids, while substances with a pH greater than 7 are bases. In fact, the further away a substance’s pH is away from 7, the stronger it is.
    2. For instance, the pH of milk is about 6.8. On the other hand, the pH of stomach acid is 2.5. Both of these substances would be acids; however, because 2.5 is much further away from 7 than 6.8, stomach acid is a much stronger acid.
  2. Why did the cabbage juice change color when some of the other substances were added? 
    1. The cabbage juice changes color when it’s mixed with an acidic substance or a basic substance. Therefore, cabbage juice is a pH indicator, because it changes color to indicate a substance’s pH.
  3. Why is cabbage a pH indicator? 
    1. Anthocyanin, the pigment that makes red cabbage red, changes color when it’s mixed with an acid or a base. Therefore, we can use cabbage juice, which contains anthocyanin, to determine the pH of different substances.
  4. What does each color mean? 
    1. When you put an acidic substance (like vinegar and lemon juice) into the cabbage juice, it turns red or red-purple. When you put a basic (like baking soda and bleach) substance into the cabbage juice, it turns green or blue-green. If you add a neutral substance (like water), the color of the cabbage juice will stay the same. 
  5. Why did the color of the cabbage juice change more when bleach was added than when baking soda was added?
    1. The color change is more dramatic when the substance has an extreme pH. Bleach’s pH is 12.6, while baking soda’s pH is only 8.3. Therefore, bleach has an extreme pH and an extreme color change, while baking soda has a slightly basic pH and a less intense color change.
Additional Experimentation:
  1. Try adding different substances to the cabbage juice to test out their pH.
  2. Blend another vegetable in water, then mix in some acidic and basic substances. See if it’s a pH indicator that changes color like the cabbage juice.
Suggested video: https://www.youtube.com/watch?v=I18K2upEHLc&feature=youtu.be

​Published 6.12.2020

Author: Anna Hudson