How could dissolved minerals interfere with an experiment ysis?

Regular tap water contains ions such as Na+, K+, Ca+2, Mg+2, Cl-, CO3-2, SO4-2, etc. if you are testing for an ion in an unknown sample and that ion is already present as a contaminant in the water used for the experiment, it will make it appear as if there was more of that ion present than was actually the case. Also, the presence of some of these ions may interfere with tests for other ions and make it appear as if there were less of those ions than is actually the case.