Chemicals Used In Precipitation Softening When the PH of an aqueous solution is increased enough, metallic ions begin to precipitate out either as hydroxides or carbonates. The following chemicals are used in this chemical process known as precipitation softening: CaO: Quick lime Ca(OH)2: hydrated or slaked lime Na2CO3: soda ash or sodium carbonate CaSO4: calcium sulfate or gypsum CaCl2: Calcium Chloride NaOH: Caustic Soda Alum (Al2[SO4]3) and Sodium Aluminate Na2Al2O4 Sulfides can be used to precipitate some heavy metals too   Compiled by Rami E. Kremesti