What happens when ?
(A) Aqueous solution of `CrCl_(3)` is added to ammonia solution.
(B) Ammonium carbonates reacts with `MgCl_(2)` (i) in absence of ammonium salts and (ii) in presence of ammonium salts:

Explanation

(A)`Cr^(3+)+3NH_(3)+3H_(2)OtoCr(OH)_(3)darr` (green)+`3NH_(4)^(+)`
`Cr(OH)_(3)` precipitate formed becomes slightly soluble in excess of precipitant in cold forming a violet or pink solution containing `[Cr(NH_(3))_(6)]^(3+)` complex ions.
`Cr(OH)_(3)darr+6 NH_(3)to[Cr(NH_(3))_(6)]^(3+)+3OH^(-)`
(B)(i)`5Mg^(2+)+6CO_(3)^(2-)+7H_(2)Oto4MgCO_(3).Mg(OH)_(2).5H_(2)Odarr+2HCO_(3)^(-)`
White precipitate of basic magnesium carbonate is formed.
(ii)In presence of ammonium salts no precipitation occurs, because the equilibrium
`NH_(4)^(+)+CO_(3)^(2-)toNH_(3)+HCO_(3)^(-)` is shifted towards the formation of `HCO_(3)^(-)` ions.