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I think you may not understand what a precipitate is. If a powder is added to a liquid and it doesn't completely dissolve, then the powder is partially soluble, or insoluble if it doesn't dissolve at all. However, that has nothing to do with precipitates. If two liquids are mixed (i.e. the aqueous solution of the powder and one of the chemicals) and a solid forms (it should be fairly obvious that it is not a normal solution), then that is a precipitate. A powder added to a liquid could never form a precipitate, by definition. Also, if a liquid remains above the precipitate, that is called a supernate (just more information that might be helpful). As far as the chemistry behind it, this might help- http://wiki.answers.com/Q/Why_do_precipitates_form