Two solutions are prepared where a few drops of 0.2 M calcium nitrate is added to distilled water. In the second solution, a zeolite is added that will exchange Na+ ions for Ca2+ ions reducing the water hardness. Upon addition of carbonate ions, the precipitate formed in the solution with zeolites is noticeably less and eventually clears up completely.
Hard water occurs when magnesium or calcium is dissolved in water and can form deposits with carbonate or with soap. Reducing the amount of calcium or magnesium ions is the goal for water softening.