One of these test -tubes contain distilled water and the other two contain an acidic and a basic solution respectively. If you are given only blue litmus paper, how will you identify the contents of each test-tube?

The color of blue litmus paper will not change when distilled water is added to the test tube. When the same litmus paper is dipped in an acidic solution, it turns blue to red, whereas the third test tube has no effect because it is a basic solution.