To a solution of sodium hydroxide in a test tube two drops of phenolphthalein is added (i) State the colour change observed (ii) If dil HCL is added drop wise to the solution, what will be the colour change (iii) on adding few drops of NaOH solution to the above mixture the colour of the solution reappears. Why ?

Phenolphthalein is basic indicator which remain colorless in acidic and neutral solution and turns pink or magenta in basic solution. 

(i) when few drops of phenolphthalein is added to sodium hydroxide, the solution turns deep pink as NaOH is a strong base. 

(ii) If dilute HCl is added to the solution then the color of the solution get fades and if we keep on adding HCl, the solution becomes colurless. As HCl being a strong acid will neutralize NaOH. 

(iii) If few drops of NaOH is added again to the same solution, the solution become basic and the pink color of the phenolphthalein reappears.