|III||Base insoluble Sulfides|
In Qualitative analysis, the grouping of cations is based on how the first steps of separation can proceed. Each Group has the general property described. This property can be used to separate that particular group of metal ions from other groups of metal ions.
For example, when separating group I from the other groups, the first step requires adding HCl (Hydrochloric Acid). The chlorine atoms from HCl dissociate from the Chlorine atoms and then precipitate with the group I cations. This general feature of group I ions is what separates these metals from all of the other groups.
It is, in essence, a parsing mechanism. Because the metals could be isolated from each other in any of probably one hundred possible methods, the parsing method allows for the fastest and most convenient way. It's fastest because you don't have to do re-work when the metals from multiple groups are active with a particular reagent that would otherwise separate a metal within its own group and it's the most convenient because our brains memorize steps better when they are broken down into large generalized "chunks" of data.