You have been provided with three test tubes. One of them contains distilled water and the other two contain an acidic solution and a basic solution, respectively. If you are given only red litmus paper, how will you identify the contents of each test tube?
Answer:
We know that,
 In acidic medium, red litmus doesn't change colour
 In basic medium, red litmus turns blue.
 In neutral medium, red litmus doesn't change colour

Start by dipping the red litmus paper into all three solutions.

The solution which turns the red litmus paper into blue will be the basic solution
.

The solution which turns the red litmus paper into blue will be the basic solution
.

Since the red litmus paper has now turned blue , it can be used as a blue litmus paper.

Dip the newly formed blue litmus paper into the remaining two solutions.

We know blue litmus turns red in an acidic solution.

The solution which turns the blue litmus paper into red is the acidic solution.

The solution which turns the blue litmus paper into red is the acidic solution.
 The only remaining solution is the distilled water.