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Narrative
Petri dishes contain aqueous solutions of crystal violet. One dish also contains the cationic detergent cetyl trimethylammonium bromide. Sodium hydroxide is added to each dish. The mixture containing detergent reacts much faster.

Discussion
The addition of hydroxide ion destroys the conjugation between the aromatic rings and results in the formation of a colorless compound. The cationic detergent forms positively charged micelles which accelerate the rate of the reaction.