Rust in test-tubes of HCl FeO or Fe3O4?

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rust in test-tubes of HCl FeO or Fe3O4??

I am doing experiments on steel nails in HCl solutions with a pH of about 1.5 - 2.
The rust is black in colour, and is not attached to the surface of the nails. It stays at the bottom of the test-tube. Is this FeO or Fe3O4. Why is it that the rust I am getting in solutions of just salt is not the same colour?. And why is it red and attaching to the nail?

 

[GCT]

I am doing experiments on steel nails in HCl solutions with a pH of about 1.5 - 2.
The rust is black in colour, and is not attached to the surface of the nails. It stays at the bottom of the test-tube. Is this FeO or Fe3O4. Why is it that the rust I am getting in solutions of just salt is not the same colour?. And why is it red and attaching to the nail?

It has a lot to do with coordination inorganic chemistry which depends on quantum mechanical explanations which relate the iron to the " field " of ligands around it to explain the color , you may want to start off with Crystal Field Theory.

 

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I am doing experiments on steel nails in HCl solutions with a pH of about 1.5 - 2.
The rust is black in colour, and is not attached to the surface of the nails. It stays at the bottom of the test-tube. Is this FeO or Fe3O4. Why is it that the rust I am getting in solutions of just salt is not the same colour?. And why is it red and attaching to the nail?

Maybe with HCl you get more Fe²⁺ ions, that don't have enough time to be oxidized by air oxygen into Fe³⁺. In the first case you would get FeO (black), in the second Fe₂O₃ (red).