(i) First take three strips of blue litmus paper and dip one in each test tube.
(ii) 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).
(iii) 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.
(iv) Now put the blue litmus paper which is turned red by the acid of test tube A in the remaining two test tubes.
(v) If the liquid of test tube B turns that red litmus paper to blue again, it shows that it is a base.
(vi) The liquid in test tube C does not turn the colour of either blue litmus or red litmus thus is distilled water.