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The substances, which give different colours with acid and base, are called indicators, more precisely acid – base indicators.

Example : Methyl orange, phenolphthalein and litmus are acid - base indicators.


Methyl orange gives yellow colour with a solution of base (for example, NaOH) and orange colour or pink colour in acidic solution (for example, HCl). When acid reacts exactly with the given amount of base, the colour at the end of this reaction is orange red.


Phenolphthalein is colourless in acidic solution and pink in alkali solutions. Litmus solution gives red colour with acids and blue colour with bases.

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