In case of qualitative analysis of anions, sodium carbonate extract is prepared for insoluble salts.
It is done by heating (for 10 minutes) an aqueous solution of a mixture of the solid salt and sodium carbonate in 1:4 ratio (by mass). As far as I understand, the principle behind preparing sodium carbonate extract is to prepare a clear solution containing the anion by precipitating the cation as insoluble oxide or carbonate.
But is this process applicable for salts that do not ionize at all in water, or covalent salts like $\ce{SnCl2}$, $\ce{SbCl3}$ etc.? I have never tried this myself.