You are provided with three test tubes A, B and C which contain distilled water, acidic solution and basic solution respectively. If you are given blue litmus paper only, how will you identify the contents of each test tube ?

Explanation

First take three strips of blue litmus paper and dip one in each test tube.
The liquid in test tube A turns blue litmus to red. It means test tube A contains acid because (acid turns blue litmus to red).
If other two test tubes do not change the colour of blue litmus paper, it shows that one of them contains a base and the other contains distilled water.
Now put the blue litmus paper which is turned red by the acide of test tube A in the remaining two test tubes.
If the liquid of test tube B turns that red litmus paper to blue again, it shows that it is a base.
The liquid in test tube C does not turn the colour of either blue litmus or red litmus thus is distilled water.