A student added blue litmus solution to a colourless aqueous solution. The solution turned red. Which one of the following chemicals should be added in excess so that the change in colour is reversed?
(1) ethanol solution
(2) caustic soda solution
(3) hydrochloric acid solution
(4) vinegar solution

Correct answer: (2) caustic soda solution

Explanation: Acids turn blue litmus to red while bases turn red litmus to blue. When the student added blue litmus solution to a colourless aqueous solution, the colour changes from blue to red, it means it was acid. In order to reverse the process, a base should be added in excess to the solution as caustic soda (NaOH) is a strong base, hence it should be added.

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Updated on: 10-Oct-2022

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