Materials Required
- Red cabbage
- Hot water
- Large bowl
- Coffee filter (or sieve or colander)
- Measuring cup
- Small containers for testing
- Liquids to test (lemon juice, orange juice, tomato juice, vinegar, baking soda solution, hand soap, toothpaste, water, etc.)
Instructions
- Chop the red cabbage into small pieces and place them in a large bowl.
- Cover the cabbage with hot water from your sink.
- Allow the cabbage to steep for 30 minutes until the water has turned purple.
- Carefully pour the cabbage juice through a coffee filter, sieve, or colander into a measuring cup.
- Notice how the color of the liquid is purple - this will be your base color for testing.
- Pour a small amount of cabbage liquid into a small container.
- Add another liquid (like lemon juice or soap) to see what happens to the color!
- Test different liquids and observe the color changes.
What's Happening?
Substances can be acids or bases. Lemon juice and vinegar are both examples of acids. On the other end of the spectrum are bases like baking soda (used in cookies and cakes) and many soaps. Some substances are neutral, meaning they are neither an acid nor a base, like water.
Cabbage juice contains a molecule called anthocyanin, which is a natural dye. Anthocyanins react to the pH of the environment:
- Acidic environment (more H+ molecules): The solution turns pink or red
- Basic environment (more OH- molecules): The solution turns green or blue
- Neutral: The solution stays purple
The anthocyanin molecules change their structure depending on the pH, which causes the color change!
What Colors to Expect
- Red/Pink: Strong acids (lemon juice, vinegar)
- Purple: Neutral (water)
- Green/Blue: Bases (soap, baking soda solution)
- Yellow/Green: Strong bases (bleach - don't test this!)