Question

What happens to red litmus paper when it touches sour milk

Answer 1

as sour milk is acidic in nature so its don't changes its colour

Answer 2

Sour milk is acidic. Red litmus paper should not change. The sour part of sour milk is

lactic acid

. As litmus paper turns red when exposed to acidic compounds, litmus paper that is already red should not react when exposed to lactic acid.