Question

A white shirt has a yellow stain of curry. When soap is rubbed on this shirt during washing, the yellow stain turns reddish-brown. On rinsing the shirt with plenty of water, the reddish-brown stain turns yellow again.

1. Name the natural indicator present in curry stain.
2. Explain the changes in colour of this indicator which take place during washing and rinsing the shirt.
3. What is the nature of soap (acidic/basic) as shown by the indicator present in curry stain?

Answer 1

1. Turmeric
2. The colour changes occur because turmeric responds to changes in pH. Soaps and detergents are alkaline/basic in nature. When the basic soap is rubbed on the yellow turmeric stain, the indicator changes colour from yellow to reddish-brown.  Rinsing the shirt with plenty of water removes the soap. As the basic soap is washed away, the stain’s neutral or original pH is restored, causing the indicator to return to its original yellow colour.
3. The indicator shows that soap is basic (or alkaline) in nature.