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• 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 acid 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.