Preliminary tests on insoluble substances

5 PRELIMINARY TESTS ON INSOLUBLE SUBSTANCES A substance which cannot be dissolved by concentrated acids (hydrochloric or nitric) or by aqua regia is described as insoluble . Special methods for solution must therefore be employed, the actual process chosen depending largely upon the nature of the insoluble substance. 

The most common insoluble substances encountered in qualitative analysis are  

and various silicates SnS2 (mosaic gold) C and S metallic silicides carborundum. 

The insoluble substance should be subjected to the tests enumerated below in the order given until it is brought into solution. The substance should be in the form of a fine powder use an agate mortar, if necessary. 

The remaining substances listed above are white or nearly so, but may be slightly coloured by traces of impurities the effect of the latter is less marked when the substance is in the form of a fine powder. 

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