Question

The ph of coldrink is 5.what will be its action on red and blue litmus paper

Answer 1

red litmus - no action
blue litmus - turns red

since cold drinks contain carbonic acid

Answer 2

pH = 5  of cold drink.  it means that it is a mild acid.

obviously it will turn blue litmus into red color.... and does not have an affect on the red litmus paper.