Following are some of the certain facts of Ostwald's theory of acid-base indicators.
I : Ionised and unionised forms have different colours.
II : Colour change is indicated at the end point when unionised form changes to ionised form due to change in pH.
III : Benzenoid form changes to quinonoid form and vice-versa due to change in PH.
Select the correct facts

detailed solution

Correct option is C

Ostwald's theory: According to this theory:(a) The colour change is due to ionisation of the acid-base indicator. The unionised form has different colour than the ionised form.(b) The ionisation of the indicator is largely affected in acids and bases as it is either a weak acid or a weak base. In case, the indicator is a weak acid, its ionisation is very much low in acids due to common H+ ions while it is fairly ionised in alkalies. Similarly if the indicator is a weak base, its ionisation is large in acids and low in alkalies due tocommon OH- ions.