A solution with pH less than 7 is acidic while that with a pH greater than 7 is alkaline. So, in order to test the nature of solution( whether acidic or basic) we would use a litmus paper. Litmus paper contains a pH sensitive dye which is red colored in acidic form and blue colored in basic form. Acidic solution turns blue litmus paper red. Alkaline solutions turn red litmus paper blue. Hence, the nature of solution can be detected by using litmus paper test.