When the iron (II) ammonium sulfate was mixed with the [Fe2+(CN-)6]4- ion, the precipitate was initially white but turned blue upon exposure to air. What happened to the iron (II) ion when the precipitate turned blue?

Iron(II) was oxidized to iron(III).

When the precipitate turned blue upon exposure to air, it indicates that the iron (II) ion in the iron (II) ammonium sulfate has been oxidized to iron (III) ion. The oxidation state of iron changed from +2 (Fe2+) to +3 (Fe3+).

The blue color is due to the formation of a complex compound known as Prussian blue or Berlin blue. This complex is formed when iron (III) ion reacts with the [Fe2+(CN-)6]4- ion present in the solution. The reaction leads to the formation of Fe4[Fe(CN)6]3, which is a deep blue solid precipitate.

So, the iron (II) ion got oxidized to iron (III) ion, resulting in the formation of the blue precipitate.

When the iron (II) ammonium sulfate ([Fe(NH4)2(SO4)2]) was mixed with the [Fe2+(CN-)6]4- (hexacyanoferrate(II) or ferrous hexacyanide) ion, a reaction occurred resulting in the formation of a precipitate. Initially, the precipitate appeared white, but upon exposure to air, it turned blue. This color change indicates a chemical reaction known as oxidation.

To understand what happened to the iron (II) ion when the precipitate turned blue, we need to consider the oxidation states of the iron atom involved. Iron can exist in two common oxidation states - +2 (ferrous) and +3 (ferric). In this case, the iron (II) ion [Fe2+] was oxidized to form the iron (III) ion [Fe3+].

This oxidation process occurred due to the presence of atmospheric oxygen (O2) in the air. Oxygen readily reacts with the iron (II) ion, causing it to lose an electron and increase its oxidation state. This results in the formation of the iron (III) ion, which imparts a blue color to the precipitate.

To summarize, the iron (II) ion present in the precipitate was oxidized to iron (III) in the presence of oxygen from the air. This oxidation reaction led to the color change from white to blue.