Colourless salt `(A)` decolourise `I_(2)` solution and gives white precipitate (change to black) with `AgNO_(3)` solution. `(A)` also produces pink colour with `FeCl_(3)` solution. Identigy `(A)` and explain reactions.

Explanation

`2S_(2)O_(3)^(2-)+I_(2) to S_(4)O_(6)^(2-) +2I^(-)`
`S_(2)O_(3)^(2-) + 2Ag^(+) to Ag_(2)S_(2)O_(3) darr`(white)
`Ag_(2)S_(2)O_(3) + H_(2)O to Ag_(2)S darr` (Black)`+H_(2)SO_(4)`
`Fe^(3+) +2S_(2)O_(3)^(2-) to [Fe(S_(2)O_(3))_(2)]^(-)`(Pink or violet)