Red cabbage juice is a fun and colorful way to test whether everyday liquids are acids, bases, or neutral. The juice contains natural pigments that change color when the amount of acid or base changes. This makes it a safe classroom pH indicator for comparing lemon juice, vinegar, water, milk, baking soda solution, and soap.
Watching the colors change helps students connect chemistry to materials they see at home.
Key Facts
- Acids usually have pH values less than 7.
- Bases usually have pH values greater than 7.
- Neutral substances have pH = 7, like pure water.
- Red cabbage juice contains anthocyanins, pigments that change color with pH.
- Cabbage indicator often turns red or pink in acids, purple near neutral, blue or green in bases.
- A pH scale usually runs from 0 to 14, with lower numbers more acidic and higher numbers more basic.
Vocabulary
- pH
- pH is a number that shows how acidic or basic a substance is.
- Acid
- An acid is a substance with a pH less than 7 that often tastes sour, but unknown substances should never be tasted.
- Base
- A base is a substance with a pH greater than 7 that can feel slippery, but unknown substances should never be touched without permission.
- Indicator
- An indicator is a substance that changes color to show a chemical property such as pH.
- Anthocyanin
- Anthocyanin is a natural pigment found in red cabbage that changes shape and color in acids and bases.
Common Mistakes to Avoid
- Using colored or cloudy cups, which makes the color change hard to see. Use clear cups or tubes and place them on white paper for better comparison.
- Adding different amounts of cabbage juice to each sample, which can make one color look stronger than another. Use the same number of drops or the same measured volume for every test.
- Assuming every red liquid is a strong acid, which is not always true. The cabbage color gives an estimate, so compare it with a pH scale instead of guessing from color alone.
- Mixing test substances together, which can change the result and make the test unfair. Use a clean spoon, dropper, or cup for each substance.
Practice Questions
- 1 A student tests 6 liquids with cabbage juice. Lemon juice turns pink, vinegar turns red, water turns purple, milk turns light purple, baking soda solution turns blue, and soap turns green. List the liquids from most acidic to most basic using these color clues.
- 2 You make 120 mL of cabbage indicator and want to divide it equally into 6 clear cups. How many milliliters of indicator should go into each cup?
- 3 A mystery liquid turns cabbage juice green. Explain whether it is likely an acid, base, or neutral substance, and describe one safety rule the student should follow before testing more of it.