An unknown inorganic compound (X) loses it water of crystallisation on heating and its aqueous solution gives the following reaction.
(i) It gives a white turbidity with dilute hydrochloric acid solution.
(ii) It decolourises a solution of iodine in potassium iodide.
(iii) It gives a white precipitate with silver nitrate solution which turns black on standing. Identify the compound (X) and give chemical equations for the reaction at step (i) to (iii).

Explanation

Since the compound (X) decolourises a solution of iodine in potassium iodide it should contain thiosulphate ion. `S_(2)O_(3)^(2-)` which also coincides with two other given facts i.e. (i) and (iii) hence the compound (X) is sodium thiosulphate, `Na_(2)S_(2)O_(3).5H_(2)O`. Which explains the given reaction as below.
`Na_(2)S_(2)O_(3).5H_(2)O overset("Heat")rarr Na_(2)S_(2)O_(3)+5H_(2)O uarr`
(i) `Na_(2)S_(2)O_(3)+2HCl rarr 2NaCl+H_(2)O+SO_(2)+S" (White turbidity)"`
(ii) `2Na_(2)S_(2)O_(3)+l_(2)rarr Na_(2)S_(4)O_(6)+2Nal`
(iii) `Na_(2)S_(2)O_(3)+2AgNO_(3) rarr underset("(white ppt)")underset("Silver thiosulphate")(Ag_(2)S_(2)O_(3)darr)+2NaNO_(3)`
`Ag_(2)S_(2)O_(3)+H_(2)O rarr underset("(black ppt)")(Ag_(2)Sdarr)+H_(2)SO_(4)`