Why is it important to use de-ionized water when making an aqueous solution?
Because tap water (or any water that is not de-ionized) is filled with ions that should not be in the solution. For example, if you want to prepare a 1.0 M solution of HCl, the bottle will only say 1.0 M HCl and people who use it will assume it only has H+ and Cl- ions in it. If you use tap water, it will have other junk in it that could react with/interfere with the experiments.
