a. A solution has

(a) The pH of seven indicates that the given solution is neutral in nature.

(i) In order to increase the pH of the solution that is to make the solution more alkaline, one needs to add base to the solution.

(ii) In the similar manner to decrease the pH which means the solution should become more acidic, one needs to add an acid to the given

(b) The pH of the solution which can liberate CO₂ from sodium carbonate has to be less than seven because sodium carbonate is a basic salt and an acidic solution can only liberate the CO₂ from it. The reaction is as follows:

Na₂CO₃ + 2HCl → 2NaCl + H₂O + CO₂