Red cabbage pH indicator lab
01 // Question
How does the pH of a household liquid change the color it turns red cabbage indicator?
A testable question names one thing you change (the liquid tested) and one thing you measure (the resulting color).
02 // Variables
Independent
Liquid tested- Vinegar
- Lemon juice
- Baking soda solution
- Ammonia-based cleaner
Dependent
Color the indicator turns- Resulting color
- Estimated pH from a color reference chart
03 // Hypothesis
IF a more acidic liquid is added to red cabbage indicator, THEN the indicator will turn a shade of red or pink instead of blue or green, BECAUSE the pigment in red cabbage changes its molecular structure differently depending on the concentration of hydrogen ions in the liquid it's mixed with.
04 // Procedure
1. Make the indicator
Boil chopped red cabbage in water for 10 minutes, then cool and strain the purple liquid.
2. Portion the indicator
Pour an equal amount of cabbage indicator into each clear cup or test tube.
3. Add the test liquid
Add a small, equal amount of each household liquid to its own cup of indicator.
4. Observe the color
Record the resulting color immediately, since some colors shift slightly over a few minutes.
5. Rank by color
Line the cups up from reddest to bluest and compare against a pH color reference chart.
05 // Data table
| Liquid tested | Resulting color | Estimated pH | Acid, base, or neutral? |
|---|---|---|---|
| Vinegar | ____ | ____ | ____ |
| Lemon juice | ____ | ____ | ____ |
| Baking soda solution | ____ | ____ | ____ |
| Ammonia-based cleaner | ____ | ____ | ____ |
06 // What to expect
Acidic liquids like vinegar and lemon juice turn the purple indicator shades of red or pink, while basic liquids like baking soda solution and ammonia cleaner turn it blue, green, or even yellow at the strongest end.
Red cabbage contains a pigment called anthocyanin, which changes its structure — and the wavelengths of light it reflects — depending on how many hydrogen ions are in the surrounding liquid, which is exactly what a pH indicator is designed to reveal.
07 // Lab notes
- Let the boiled cabbage water cool before handling — it comes off the stove very hot.
- Use clear or white cups so the color change is easy to see and compare.
- Safety. Ammonia-based cleaners can irritate eyes and airways — test them in a ventilated area and avoid inhaling fumes directly.
08 // Remember
Purple cabbage juice is not just purple. It is a pH scale you can see with your own eyes.
09 // Questions people ask
Why does red cabbage juice change color in acids and bases?
Red cabbage contains a pigment called anthocyanin, the same family of pigment found in many red and purple fruits and vegetables. Its molecular structure shifts depending on the concentration of hydrogen ions in a liquid, which changes which wavelengths of light it absorbs and reflects — seen as a color change.
What colors does red cabbage indicator turn for acids versus bases?
Strong acids typically turn it red or pink, weak acids a purplish-red, neutral liquids stay closer to purple, weak bases shift toward blue-green, and strong bases turn it yellow-green or yellow.
Is red cabbage indicator as accurate as a lab pH meter?
No — it gives an estimate, not a precise number. It's read against a color chart rather than a numeric sensor, so it's best used to sort liquids into acidic, neutral, or basic categories rather than to pin down an exact pH value.