Question

Washing soda gives pink colour with phenolpthalein

Answer 1

Its unionized molecules are colourless whilst on ionization give colourless H+ and pink coloured phenolphthalein ions. In the presence of acid due to increase in the concentration of common ions (H+), the dissociation of phenolphthalein is suppressed and thus the solution becomes colourless

Answer 2

Rubbing some washing soda on the paper reveals the writing by turning it pink. This happens because phenolphthalein turns pink in the presence of a base like washing soda. Phenolphthalein is commonly used as an indicator for titration, i.e. for determining when a solution turns basic from acidic or vice versa.

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