Question

why does turmeric stain on a cloth turns red when it is washed with soap

Answer 1

This is because turmeric is a natural indicator while soap is a base (sodium hydroxide). The reaction between the basic soap and the turmeric changes the color of the turmeric to red. This is because turmeric turns red in base and dark yellow in acids.

Answer 2

Tumeric is a natural indicator. It turns red in bases and dark yellow in acids. Now what nature does a soap have ? It's basic. So, it will turn red :)