Question

Why do group 1 cations form precipitates when mixed with hcl?

Answer 1

Both of you are overlooking a pretty big component of the question...the Group I cation isn't being dissociated into water. We're testing the solubility of the cation when mixed with HCl. And this IS a legitimate question, seeing as our lab manual is the one asking.

By the way, the answer you're looking for is "Because Group I cations have insoluble chlorides".

"In order...to distinguish cation Group I, one adds HCl to a sample. If a Group I cation is present in the sample, a precipitate will form."

Answer 2

They form insoluble chlorides

Step-by-step explanation:

The cations of group 1 are Ag+,Pb2+,Hg22+. These cations are known to form insoluble chlorides.

The solubility product of the chloride salts by these cations is too low hence the chlorides do not dissolve in water thus forming precipitates which help in identifying he group 1 cations during qualitative analysis.