Question

When Phenolphthalein was added to a base, its color changed to pink. Despite adding some acid to the mixture, its colour remained pink. What could be the reason?

Answer 1

When you add phenolphthalein into a base its colour changes to pink. After that addition of acid to mixture does not change the pink colour because the amount of acid added is less than base, so total neutralisation of base do not occur or even if the amount of acid was same as base then may be the acid was very dilute in comparison of base in that case also neutralisation of base by acid do not occur. Due to these reasons amount of base will left in mixture which will show pink colour.