1.que:-what is softning? 1. ans :- Water softening is the reduction of the concentration of calcium, magnesium, and certain other metal cations in hard water. These "hardness ions" can cause a variety of undesired effects including interfering with the action of soaps, the build up of limescale, which can foul plumbing, and galvanic corrosion.[1] Conventional water-softening appliances intended for household use depend on an ion-exchange resin in which hardness ions are exchanged for sodium ions. Water softening may be desirable where the source of water is hard.[2] However, hard water also conveys some benefits to health by reducing the solubility of potentially toxic metal ions such as lead and copper. Methods for water softeningWater softening methods mainly rely on the removal of Ca2+ and Mg2+ from a solution or the sequestration of these ions, i.e. binding them to a molecule that removes their ability to form scale or interfere with soaps. Removal is achieved by ion exchange and by precipitation methods. Sequestration entails the addition of chemical compounds called sequestration (or chelating) agents. Since Ca2+ and Mg2+ exist as nonvolatile salts, they can be removed by distilling the water, but distillation is too expensive in most cases (rainwater is soft because it is, in effect, distilled). [edit] Ion-exchange resin devicesIon-exchange materials contain sodium ions (Na+) that are readily replaced by hardness ions such as Ca2+ and Mg2+. Ion exchange resins are organic polymers containing anionic functional groups to which the Na+ is bound. Minerals called zeolites also exhibit ion-exchange properties; these minerals are widely used in laundry detergents. [edit] Method of operationMain article: Ion chromatography The water to be treated passes through a bed of the resin. Negatively charged resins absorb and bind metal ions, which are positively charged (2RNa(s)+M2+(aq)=R2M(s)+2Na+(aq)(M=Mg or Ca)). The resins initially contain univalent (1+) ions, most commonly sodium, but sometimes also hydrogen (H+) or potassium (K+). Divalent calcium and magnesium ions in the water replace these univalent ions, which are released into the water. The "harder" the water, the more hydrogen, sodium or potassium ions are released from the resin and into the water. Resins are also available to remove carbonate, bi-carbonate and sulphate ions which are absorbed and hydroxyl ions released from the resin. Both types of resin may be provided in a single water softener. This method is called the ion exchange method. [edit] RegenerationThe resin's capacity is gradually exhausted and eventually it contains only divalent ions (e.g., Mg2+ and Ca2+ for cation exchange resins, and SO42- for anion exchange resins). At this stage, the resin must be regenerated. If a cationic resin is used (to remove calcium and magnesium ions) then regeneration is usually effected by passing a concentrated brine, usually of sodium chloride or potassium chloride, or hydrochloric acid solution through them. For anionic resins, regeneration typically uses a solution of sodium hydroxide (lye) or potassium hydroxide. The salts used for regeneration are released into the soil or sewer. In industrial scale water softening plants, the effluent flow from re-generation process can precipitate scale that can interfere with sewerage systems. [edit] Lime softeningMain article: Lime softening [edit] Chelating agentsMain article: Chelation Chelators are used in chemical analysis, as water softeners, and are ingredients in many commercial products such as shampoos and food preservatives. Citric acid is used to soften water in soaps and laundry detergents. A commonly used synthetic chelator is ethylenediaminetetraacetic acid (EDTA).