Why does the green precipitate obtained from an iron (II) salt slowly turn brown when in contact with air?
Due to oxidation.
When iron (ll) is exposed to air, oxidation will occur and it will form iron (lll) salt which gives the brown colour.
Oxidation refers to the loss of electrons, gain in oxygen, loss of hydrogen or increase in the oxidation state.
In this case, the iron in the salt increases from +2 to +3.