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