Question

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

Answer 1

Acids turn blue litmus solution to red while bases turn red litmus solution to blue. When student added blue litmus solution to a colourless aqueous solution, the colour of litmus solution changed from blue to red, it means it was an acid. To reverse this process, a base should be added in excess in the solution. Bases change red litmus solution to blue and out of given options, caustic soda(chemically known as NaOH) is a strong base which can convert red litmus solution to blue.

Hence, the correct answer is option 2.