In the pictures you can se two photos of the same unknown substance! What cations or what compounds would you predict in this substance according to what you see. I already have Fe 3+ and Al 3* Hint: When I give acid to the substance there are a white powdery AND a black residue. So this residue is NOT soluble in diluted acid. But when I give K2cr04 to this two residues the colour of the solution is black!!! What could those grey particles be