Question

The colour change is due to ionisation of the acid base indicators​

Answer 1

The change in pH converts one tautomeric form into other and thus, the colour change occurs. Phenolphthalein has benziod form in acidic medium and thus, it is colourless while it has quinonoid form in alkaline medium which has pink colour.

Answer 2

Colour is changed due to ionisation of the acid base indicators​ :

Explanation:

• Due to ionization of the acid-base indicator it changes colour. The unionized form has several colour than the ionized form.
• The ionization of the indicator is much affected in acids and bases as it is either a weak acid or a weak base.
• If 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.