[Nemesisof99]

When FeO is present as an impurity an acidic flux like SiO2 is added which forms slag.
Why is FeO said to be basic?

[Borek]

FeO + SO₃ -> FeSO₄

Can you tell which oxide is basic and which is acidic?

[Nemesisof99]

Hmm..no. The Lewis concept?
Basic compounds are electron pair donors but I don't really know how to find out which one's basic among the two.
Pls explain..

[Borek]

Much simpler concept here.

Obvious acid/base reaction:

Ca(OH)₂ + H₂SO₄ -> CaSO₄ + 2H₂O

Less obvious one, but analogy is strong:

CaO + SO₃ -> CaSO₄

CaO is so called basic oxide, SO₃ acidic oxide. Generally that means that when they react with water they give either acid or base. In the case of FeO and SiO₂ there is no such reactions taking place, but the concept can be easily extended and adapted.

[Nemesisof99]

So the idea is that all metallic oxides are basic and all non metallic oxides are acidic. Is this always true? Any exceptions.

[Borek]

This is chemistry, "always" is of no use

Tungsten trioxide is definitely acidic, think about chromates, manganates...

[Nemesisof99]

Hmm Potassium chromate K₂CrO₄. Can this compound be labelled acidic/basic? Few tips on the criteria in doing so?

[Borek]

Chromate can be thouht of as a product of K₂O + CrO₃ reaction - these are two oxides, and it should be obvious which is considered basic and which one acidic.

Don't treat it too seriously, IMHO that's more of a rule of thumb then real science.