Question
What are natural indicators? Explain the colour changes shown by turmeric, china rose (Hibiscus), and red cabbage in acidic and basic solutions.
Solution — Step by Step
Natural indicators are substances extracted from plants that show different colours in acids and bases. They contain pigments (chemicals that absorb different wavelengths of light depending on pH) that change colour as the medium becomes acidic or basic.
Natural indicators were the original way chemists identified acids and bases before synthetic indicators (like litmus, phenolphthalein) were developed.
Source: Rhizome of the turmeric plant (Curcuma longa). Active pigment: curcumin (yellow).
| Medium | Colour |
|---|---|
| Neutral | Yellow |
| Acid | Yellow (no change) |
| Base/Alkali | Red/Reddish-brown |
Practical use: The turmeric stain on clothes turns reddish when washed with soap (soap is basic). It returns to yellow when washed with acidic water (dilute acid). This is a real-life demonstration you can show examiners.
Limitation: Turmeric doesn’t change colour in acids — it only shows a colour change in bases. So it cannot distinguish between neutral and acidic solutions.
Source: Petals of the Hibiscus rosa-sinensis plant. Active pigments: anthocyanins (magenta/red in the flower).
| Medium | Colour |
|---|---|
| Acid | Dark pink / Magenta |
| Neutral | Original pink |
| Base | Green / Yellow-green |
Method: Crush the petals in water to extract the pink juice. Use this as indicator solution.
China rose is a good two-way indicator — it shows clearly different colours in both acids and bases.
Source: Leaves of red cabbage (Brassica oleracea). Active pigment: anthocyanins (same class as china rose, but different specific compounds).
| pH / Medium | Colour |
|---|---|
| pH 2–3 (strongly acidic) | Red |
| pH 4–6 (weakly acidic) | Purple-pink |
| pH 7 (neutral) | Purple |
| pH 8–10 (weakly basic) | Blue-green |
| pH 11–14 (strongly basic) | Yellow-green |
Red cabbage is exceptional — it changes through multiple colours across the entire pH range, making it a rough universal indicator.
Method: Boil red cabbage in water, filter the liquid. Use the blue-purple liquid as indicator.
Why This Works
The colour changes in natural indicators occur because the pigment molecules (primarily anthocyanins) are weak acids themselves. Their molecular structure changes when is added (acidic conditions) or is present (basic conditions).
The protonated form (in acid, with extra ) and deprotonated form (in base, removed) absorb different wavelengths of light — hence different colours. This is a general principle: acid-base indicators are all weak acids or bases whose protonated and deprotonated forms have different colours.
Alternative Method — Quick Summary Table
| Indicator | Acidic | Neutral | Basic |
|---|---|---|---|
| Turmeric | Yellow | Yellow | Red |
| China Rose | Dark pink | Pink | Green |
| Red Cabbage | Red | Purple | Blue/Green |
| Litmus (for comparison) | Red | Purple | Blue |
Common Mistake
Saying turmeric turns yellow in acid and red in base — and stopping there. The common mistake is not mentioning that turmeric gives NO colour change in acids (stays yellow). This is actually the key limitation. If a question asks “why is turmeric not a good indicator for acids?” — the answer is: it doesn’t change colour in acidic solutions, so it cannot distinguish acid from neutral.