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

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."