Question

What happens when litmus paper is dipped in aqueous solution of MgO

Answer 1

Assuming that the MgO has some solubility associated with it. The O2- ion will accept a proton from a water molecule to produce OH-. Therefore, now having some basicity associated with the solution, providing red litmus paper is added to the solution it will change to a blue colour. (Note:- An initial piece of blue litmus would not change its colour.)