[DOracle]

Why is Al2O3 soluble in sodium hydroxide? What´s the minimal required concentration for?

I have no idea. THx.

[Arkcon]

Al₂O₃ reacts with sodium hydroxide, to produce aluminum hydroxide, which is also insoluble.  But, in an excess of hydroxide, aluminum from a soluble complex.  "Minimum required concentration for" is a little hard to answer, any concentration (more than a trace, likely) so long as there is a "excess" which is a little hard to pin down.