A student spills some soap solution on a yellow curry stain on a white cloth. The stain turns reddish-brown. When the cloth is later rinsed thoroughly with plenty of water, the stain turns yellow again. What do these two colour changes tell you about the chemical nature of soap and water, and what property of the indicators involved explains why the colour reverses on rinsing?
Generated by claude-sonnet-4-6 · 2026-06-26 01:03 · grounding rag
Model Answer
Soap is basic in nature. When soap solution (a base) is applied to the curry stain, turmeric (a natural indicator present in curry) changes from yellow to reddish-brown, indicating a basic medium.
Water is neutral (or very slightly acidic due to dissolved CO₂). When the cloth is rinsed with plenty of water, the base is washed away and the medium returns to neutral/acidic, so turmeric changes back to yellow.
Turmeric is a natural acid-base indicator — it is yellow in acidic or neutral conditions and reddish-brown in basic conditions. Since this colour change is reversible, the indicator returns to its original colour once the base is removed by rinsing.
Source: Chapter 2, Introduction (Acids, Bases and Salts)
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Explanation
- Key point 1: Soap is basic → turmeric turns reddish-brown (base indicator response).
- Key point 2: Water is neutral → rinsing removes the base, turmeric reverts to yellow.
- Key point 3: Examiners expect you to name turmeric as a natural indicator and state that its colour change is reversible — this directly answers "why the colour reverses."
- Avoid confusing turmeric with litmus; turmeric is yellow in neutral/acidic and reddish-brown in basic medium (opposite pattern from litmus in one direction).
- Do not write about micelles or soap chemistry — that is not what this question tests.