Please note that the groups of cations mentioned/discussed here do not refer to the groups of the periodic table! The various cations have been arranged into several groups for the purpose of qualitative analysis and these are the groups that we are talking about.
-->Insert a proper explanation of groups. Also accompany with a table.
In the first group of cations, we include Silver(I), Mercury(I)and Lead(II) cations. Before we begin, it is good to note that mercury(I) ions - or mercurous ions - are NOT Hg+ but Hg22+.
The group reagent is dilute Hydrochloric acid.
When we pass dil. Hcl through the solution, we expect the precipitation of the chlorides of the aforementioned cations. ANY Precipitation is nothing but a chloride and the cation of the ppt. can not be any other than the three.
If a ppt is obtained, group I cations are confirmed. If not, we wll test the solution for the presence of group II cations.
Suppose we obtain a ppt on adding dil. HCl to the solution. It is inferred that the cation in the solution is either Ag+, Hg22+ or Pb2+.
Our goal now would be to distinguish between these three cations.
Before we pass onto this section, please realize that dil. HCl was added to only a part of the solution, taken in another testtube. The rest of the solution would be preserved for use in other tsts should it be needed.
Adapted from the Inorganic Wikibook.