Question

When a student added red litmus to an aqueous solution, the red litmus turned blue. Which one of the following should be added in excess so that the change in colour is reversed?
(1) baking soda solution
(2) lime water
(3) vinegar solution
(4) ammonia solution

Answer 1

Acids turn blue litmus to red and bases turn red litmus to blue. When the student added red litmus to an aqueous solution, it turned blue which means, the solution is basic. To reverse the change in colour, now he has to add an acid to the solution in excess. So, from the given options only vinegar solution is acidic in nature. It is made up of ethanoic acid. When it will be added in excess in the solution, the solution will become acidic and the colour of the blue litmus then it can be changed back to red.

Hence, the correct answer is option 3.