Why does the colour changes when the phenolpthaline is added dropwise to washing soda solution?

Why does the colour changes when the phenolpthaline is added dropwise

1.	Why does the colour changes when the phenolpthaline is added dropwise to washing soda solution?
Answer» This is so because the last drop of added alkali brings the pH of the solution in the range in which phenolphthalein shows sharp colour change. In the titration of strong acid and a weak base, methyl orange is chosen as indicator.