A solution with a pH value of 7 is a neutral solution.
(i) To increase the pH value of a neutral solution, we need to add a base to it because bases have a pH value greater than 7.
(ii) To decrease the pH value of a neutral solution, we need to add an acid to it because acids have a pH value less than 7.
If a solution changes the colour of litmus from red to blue, then its pH value is greater than 7. Also, it is basic in nature.