(i)    An indicator is weak acid or weak base.

(ii)    Ionization of the indicator causes change in its colour. In other words, the unionized molecules of the indicator possess different colour than the ions.

(iii)    Being weak electrolytes, indicators are feebly ionized in solution. However, the addition of a strong acid or strong base increases their ionization considerably which causes change in the number of coloured ions.

(iv)    An acidic indicator such as phenolphthalein must possess a coloured anion while a basic indicator such as methyl orange must possess a coloured cation.

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