Red cabbage pH indicator lab
01 // Question
How does the sample change the color of red-cabbage indicator under the same test conditions?
Compare one prepared sample at a time with a purple reference; record the observed color before classifying it approximately.
02 // Variables
Independent
Liquid tested- Lemon juice
- Vinegar
- Plain water
- Baking-soda solution
Dependent
Color the indicator turns- Observed color
- Approximate acidic / near-neutral / basic category
03 // Hypothesis
IF lemon juice or vinegar is added to red-cabbage indicator, THEN the color will shift differently from the plain-water reference, BECAUSE anthocyanin pigments change their light absorption across acidity conditions, though the exact shade depends on the extract and setup.
04 // Procedure
1. Prepare the indicator
An adult steeps chopped red cabbage in hot water, then cools and strains one purple batch before class.
2. Set up a reference
Place 10 mL of extract in a purple reference cup and 10 mL in each labeled test cup; keep them on white paper under the same light.
3. Test separately
Add 2 mL of lemon juice, vinegar, plain water or baking-soda solution to its own cup. Use a clean dropper for each sample.
4. Record what you see
Describe each color promptly and compare it with the purple reference; do not guess an exact pH from hue.
5. Classify cautiously
Sort the observed pattern as acidic, near neutral or basic. Use calibrated pH paper or a meter when a more precise reading matters.
Materials // Prepare the bench
- Red cabbage
- hot water
- clear sample cups
- strainer
- vinegar
- baking-soda solution
- lemon juice
- droppers
- goggles
- heat-safe container
05 // Data table
| Sample | Observed color | Shift from purple reference | Approximate group |
|---|---|---|---|
| Lemon juice | ____ | ____ | ____ |
| Vinegar | ____ | ____ | ____ |
| Plain water | ____ | ____ | ____ |
| Baking-soda solution | ____ | ____ | ____ |
06 // What to expect
In a common classroom setup, lemon juice and vinegar shift the purple extract toward pink or red, plain water remains closer to purple, and baking-soda solution often shifts toward blue or green. These are examples, not guaranteed shades: cabbage variety, extract strength, dilution, sample color and lighting can change what a class observes.
Red cabbage contains anthocyanin pigments whose molecular forms and light absorption vary across acidity conditions. The extract is a broad visual indicator, not a calibrated pH scale; it cannot identify an unknown mixture or provide an exact number by color alone.
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.
08 // Remember
Cabbage extract shows a color pattern. Compare the pattern; do not read an exact pH from hue.
09 // Use the cabbage extract as an indicator
Red cabbage contains pigments called anthocyanins that change color across different acidity conditions. The extract is a natural indicator, not a calibrated pH meter. Make one batch of extract and distribute equal volumes into labeled samples. Add each listed food-grade solution in measured amounts and use a clean dropper for each one so the liquids do not contaminate one another.
Compare the four listed food-grade samples—lemon juice, vinegar, plain water, and baking-soda solution—with a separate purple starting reference. Record each color on the same white background and under the same light. Classify the pattern broadly as acidic, near neutral, or basic; do not assign a precise pH number from hue alone. Household cleaners and unknown products are outside this student procedure.
10 // Control concentration and color observations
The amount of cabbage, extraction water, heating time, and extract age can change the color strength. Keep the indicator batch constant within a comparison. If investigating sample concentration, prepare measured dilutions and add equal volumes to each cup. If investigating indicator preparation, hold the test solutions constant and change only one extraction factor, such as heating time.
Color perception differs by observer and by lighting. Use a white background, photograph all cups in one frame, and write a color description before discussing what it means. A weak color change may indicate a dilute sample, an old extract, or a small volume difference. Do not infer that no color change proves a liquid is neutral; it may simply be too weak to distinguish with this indicator.
11 // Heat and chemical safety
The cabbage extraction uses hot water. A teacher should manage the heating step, use a heat-safe container, and let the extract cool before students handle it. Wear goggles and do not taste the samples. Keep household cleaners and unknown products out of this student activity; never combine samples.
Dispose of the small classroom samples according to school instructions and wash the cups and droppers. A useful extension is to compare indicator color with pH paper for the same samples, then explain why a broad color indicator and a numerical strip provide different kinds of evidence. The conclusion should state that cabbage extract suggests relative acidity/basicity under this procedure; it does not identify every chemical in a mixture.
12 // Interpret color as a range, not an exact pH
Red-cabbage pigments called anthocyanins change their light absorption as acidity changes, producing a range of colors. The shade depends on pigment concentration, lighting, sample volume, and the substance being tested. A color chart can help compare samples, but an indicator color does not provide an exact pH unless it is calibrated against known standards under the same conditions.
Use equal indicator volumes and small, measured additions of each listed sample. Photograph the cups against a white background under consistent lighting. A numeric color chart is appropriate only when the indicator has been calibrated with known standards under the same conditions; this classroom comparison is not calibrated, so report broad categories rather than exact pH values. Use only the food-grade samples in the procedure and dispose of them according to school instructions.
13 // 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 might red-cabbage indicator show?
Acidic samples often shift it toward pink or red, a plain-water comparison may stay closer to purple, and basic samples may shift toward blue or green. The shade varies with the cabbage, extract, dilution, sample and light, so record what your class actually sees.
Can cabbage indicator give an exact pH number?
No. It can support a broad comparison under consistent conditions, but it is not calibrated and color alone does not identify a solution or give an exact number. Use pH paper or a calibrated meter for a more precise reading.
14 // Keep going
Can you explain the investigation?
Red cabbage pH indicator lab · Student Data Sheet
Question: How does the sample change the color of red-cabbage indicator under the same test conditions?
Hypothesis: If lemon juice or vinegar is added to red-cabbage indicator, then the color will shift differently from the plain-water reference, because anthocyanin pigments change their light absorption across acidity conditions, though the exact shade depends on the extract and setup.
Materials
- Red cabbage
- hot water
- clear sample cups
- strainer
- vinegar
- baking-soda solution
- lemon juice
- droppers
- goggles
- heat-safe container
Safety before starting
- An adult should prepare the cabbage extract with hot water and let it cool before students handle it. Wear goggles and do not taste any test sample.
- Use only the listed small food-grade samples: lemon juice, vinegar, plain water and baking-soda solution. Keep samples in separate labeled cups, never taste them, and do not bring household cleaners into this activity.
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.
Variables
Independent: Liquid tested · Dependent: Color the indicator turns
Keep constant: same amount of cabbage indicator per sample · same amount of test liquid · same clear cup or tube for each test
Procedure
- Prepare the indicator: An adult steeps chopped red cabbage in hot water, then cools and strains one purple batch before class.
- Set up a reference: Place 10 mL of extract in a purple reference cup and 10 mL in each labeled test cup; keep them on white paper under the same light.
- Test separately: Add 2 mL of lemon juice, vinegar, plain water or baking-soda solution to its own cup. Use a clean dropper for each sample.
- Record what you see: Describe each color promptly and compare it with the purple reference; do not guess an exact pH from hue.
- Classify cautiously: Sort the observed pattern as acidic, near neutral or basic. Use calibrated pH paper or a meter when a more precise reading matters.
Record your observations
| Sample | Observed color | Shift from purple reference | Approximate group |
|---|---|---|---|
Answer key and expected pattern
In a common classroom setup, lemon juice and vinegar shift the purple extract toward pink or red, plain water remains closer to purple, and baking-soda solution often shifts toward blue or green. These are examples, not guaranteed shades: cabbage variety, extract strength, dilution, sample color and lighting can change what a class observes.
Red cabbage contains anthocyanin pigments whose molecular forms and light absorption vary across acidity conditions. The extract is a broad visual indicator, not a calibrated pH scale; it cannot identify an unknown mixture or provide an exact number by color alone.
Remember: Cabbage extract shows a color pattern. Compare the pattern; do not read an exact pH from hue.