Explain the following :
(i). `FeCl_3` and `FeBr_3` are well known, but `FeI_3` has doubtful existence.
(ii). Anhydrous `FeCl_3` cannot be obtained by heating hydrated ferric chloride.
(iii). Cast iron is hard but pure iron is soft in nature.
(iv) . A ferrous salt decolourises `KMnO_4` solution.
(v). A ferrous salt turns brouwn air.

Explanation

(i). `I^(ɵ)` ion being reducing agent reduces `Fe^(3+)` iions into `Fe^(2+)` as: `Fe^(3+)+I^(ɵ)toFe^(2+)+(1)/(2)I_2`
(ii). On heating hydrated ferric chloride, anhydrous ferric chloride is not formed as water of crystallision reacts to form `Fe2O_3` and HCl.
(iii). Cast iron contains carbon `2.5%` to `5%` which is responsible for its hardness.
(iv). Ferrous salt acts as a reducing agent. It reduces acidified `KMnO_4` into `K_2SO_4` and `MnSO_4` which form colourless solution i.e., declourisation of `KMnO_4` solution takes place.
`2KMnO_4+10FeSO_4+8H_2SO_4toK_2SO_4+2MnSO_4+5Fe_2(SO_2)_3+8H_2O`
(v). A ferrous salt turns brows in air due to oxidation to ferric salt.