Soap solution is basic in nature.
Explanation
Turmeric is a natural acid-base indicator derived from the Curcuma longa plant. It contains a yellow pigment called curcumin, which changes colour depending on the pH of the solution it interacts with.
Detailed Analysis
- Nature of Turmeric: Turmeric remains yellow in acidic and neutral solutions. However, in the presence of a base (alkaline medium), it turns reddish-brown.
- Nature of Soap: Soap is chemically a salt of a strong base (such as Sodium Hydroxide) and a weak acid (fatty acids). Consequently, an aqueous solution of soap is basic (alkaline) in nature.
- The Reaction: When soap is applied to a turmeric stain on a white shirt, the basic nature of the soap interacts with the turmeric indicator, causing the yellow pigment to turn red.
- Reversal: If the cloth is washed with plenty of water or treated with an acid (like lemon juice), the stain reverts to yellow as the basic soap is removed or neutralized.
Therefore, the colour change is strictly due to the basic nature of the soap solution acting on the natural indicator.
Key Takeaway: Turmeric is a natural pH indicator that remains yellow in acidic or neutral media but turns reddish-brown in basic (alkaline) media.