Assertion:Fluorescein is an adsorption indicator.
Reasoning : The indicator fluorescein is a dye

Options

A. If both assertion and reason are true and the reason is the correct explanation of the assertion
B. If both assertion and reason are true but reason is not the correct explanation of the assertion.
C. If assertion is true but reason is false
D. If the assertion and reason both are false

Answer

B

Explanation

An adsorption indicator whose colour when adsorbed to the precipitate is different from that when is in solution, is used on titrating `Cl^( -)` with `Ag^(+)` . The anionic dye dichloro–fluorescien is used as an indicator. Before the end point, the precipitate of AgCl has a negative surface charge due to excess` Cl^( -)` ion. The anionic indicator is repelled by the precipitate and remains in solution where it has a greenish yellow colour. At the end point the precipitate has a positive surface charge due to the adsorption of excess `Ag^(+)` . The anionic indicator now adsorbs to the precipitate’s surface where its colour is pink. This change in colour signals the end point.
Hence both assertion and reason are correct statements but reason is not the correct explanation of the assertion.