What Happens When You Dip Turmeric Paper in Soap Solution?
The Question
What colour change will you observe when turmeric paper is dipped in soap solution? Give a reason.
Step 1 — What Is Turmeric Paper?
Turmeric paper is made by dipping paper strips in turmeric (haldi) solution and letting them dry.
Turmeric is a natural indicator — it changes colour when it comes in contact with certain substances.
| In contact with… | Turmeric colour |
|---|---|
| Acid | Yellow (no change) |
| Neutral substance | Yellow (no change) |
| Base | Red-brown (changes!) |
Turmeric only reacts to bases. It cannot distinguish between acids and neutral substances (both remain yellow).
Step 2 — Is Soap Acidic, Basic, or Neutral?
Soap is a base.
You can remember this because:
- Soap feels slippery — a characteristic property of bases.
- Soap turns red litmus paper blue (another indicator reaction for bases).
- Soap has a pH greater than 7.
Common soaps have a pH of around 9-10 — clearly basic.
Step 3 — What Happens?
When turmeric paper is dipped in soap solution:
The basic soap solution reacts with the turmeric dye in the paper.
The turmeric paper changes from yellow to red-brown.
Answer: The turmeric paper turns red-brown (reddish) when dipped in soap solution, because soap is basic in nature.
Why Does This Happen?
The pigment in turmeric is a compound called curcumin. Curcumin is yellow in neutral or acidic conditions, but it reacts with the OH⁻ ions present in basic solutions and transforms into a red-brown compound.
This is a chemical reaction — the colour change is not just mixing, it’s the molecule actually changing form.
(Don’t worry about curcumin or OH⁻ for Class 7 — just know the observation and the reason: soap is basic → turmeric turns red-brown.)
You can try this at home safely! Dip a small piece of white cloth or paper in diluted turmeric water, let it dry, then dab soap water on it. You’ll see the yellow turn red-brown. When you rinse with plain water (neutral), it doesn’t reverse. That’s chemistry in action in your kitchen!
Common mistake: Saying turmeric turns blue in a base.
Turmeric turns red-brown in a base — NOT blue. Blue is what happens to RED litmus in a base. Turmeric and litmus have completely different colour changes. Don’t mix them up!
Try These Similar Problems
Problem 1: Turmeric paper is dipped in lemon juice. What colour will it be?
Lemon juice is acidic. Turmeric remains yellow in acids (no colour change). The turmeric paper stays yellow.
Problem 2: Turmeric paper is dipped in pure water. What happens?
Pure water is neutral. Turmeric remains yellow in neutral substances (no colour change). The turmeric paper stays yellow.
This shows a limitation of turmeric as an indicator — it cannot tell acids apart from neutral substances.
Problem 3: Name the substance present in turmeric that acts as an indicator.
The substance is curcumin. It is a yellow pigment found in turmeric that turns red-brown in basic conditions.
Exam tip: Questions about turmeric as an indicator often ask: “What is the limitation of turmeric as an indicator?” The answer is: turmeric cannot distinguish between an acid and a neutral substance — both remain yellow. Only bases cause a colour change (yellow to red-brown). This limitation makes litmus a more versatile indicator.