Natural or treated water that does not contain a significant amount of dissolved minerals such as compounds of calcium or magnesium. It produces copious lather with a little amount of SOAP or detergent and does not form scales in boilers, heaters, and kettles.
Water that does not form an immediate lather with soap is called hard water. Hardness of water is due to the presence of soluble calcium, magnesium or iron compounds. The most common compounds are calcium bicarbonate Ca (HCO3)2, magnesium bicarbonate Mg (HCO3)2, calcium sulphate CaSO4 and magnesium sulphate MgSO4. The addition of soap forms an insoluble scum. The scum consists of insoluble salts of these metals. Removal of these salts from the solution makes the water soft. Water that forms an immediate lather with soap is called soft water. Such water does not have dissolved salts of calcium, magnesium and iron.
Types of Hardness:
Depending upon the behavior of water towards soap, hardness is divided into two types.
Hardness of water due to the presence of soluble bicarbonates of calcium and Magnesium is called temporary hardness. When water containing dissolved carbon dioxide passes over solid carbonates (chalk or limestone deposits etc.), these compounds get dissolved in water. Rainwater and distilled water are always soft because they do not have dissolved (soluble) salts.
This is due to the presence of chlorides and sulphates of calcium and magnesium. Such a hardness can be removed by the addition of washing soda. This removes both the temporary and the permanent hardness of water.
Problems with hard water:
The presence of certain metal ions in water causes a variety of problems. These ions interfere with the action of soaps. They also lead to build up of lime scale, which can foul plumbing, and galvanic corrosion. In industrial scale water softening plants, the effluent flow from re-generation process can precipitate scale that can interfere with sewerage systems.
The slippery feeling experienced when using soap with soft water occurs because soaps tend to bind to fats in the surface layers of skin, making soap molecules difficult to remove by simple dilution. In contrast, in hard-water areas the rinse water contains calcium and/or magnesium ions which form insoluble salts, effectively removing the residual soap from the skin but potentially leaving a coating of insoluble stearates on tub and shower surfaces, commonly called soap scum.
So it is very essential to remove the hardness of water which is known as softening of water.
METHODS OF SOFTENING OF WATER:
Temporary hardness is removed in the following ways:
By Boiling the Water:
On boiling, the soluble bicarbonate is decomposed into insoluble carbonate.
By adding slaked lime [Ca (OH) 2] to hard water, insoluble carbonates are formed. The insoluble calcium carbonate is the ‘fur’ (or scale) formed in kettles, boilers, pipes, etc.
Permanent hardness can be removed by the following methods:
Practical means for softening water rely on ion-exchange polymers or reverse osmosis. Other approaches include precipitation methods and sequestration by the addition of chelating) agents. Devices that claim to use magnetism or electricity as a “water softening” technique are fraudulent.
Conventional water-softening appliances intended for household use depend on an ion-exchange resin in which hardness ions mainly Ca2+ and Mg2+ are exchanged for sodium ions. Ion exchange devices reduce the hardness by replacing magnesium and calcium (Mg2+ and Ca2+) with sodium or potassium ions (Na+ and K+).”
Types of Ion Exchange Materials:
Ion exchange resins are organic polymers containing anionic functional groups to which the di-cations (Ca++) bind more strongly than mono-cations (Na+). Inorganic materials called zeolites also exhibit ion-exchange properties. These minerals are widely used in laundry detergents. Resins are also available to remove carbonate, bi-carbonate and sulphate ions which are absorbed and hydroxide ions released from the resin.
Regeneration of Ion Exchange Resins:
When all the available Na+ ions have been replaced by calcium or magnesium ions, the resin must be re-charged by eluting the Ca2+ and Mg2+ ions using a solution of sodium chloride or sodium hydroxide depending on the type of resin used. For anionic resins, regeneration typically uses a solution of sodium hydroxide or potassium hydroxide. The waste waters eluted from the ion exchange column containing the unwanted calcium and magnesium salts are typically discharged to the sewerage system.
Reverse osmosis is the other major technology for water softening. It is mainly deployed on a large scale rather than for individual domestic applications. As described by NSF/ANSI Standard 44, it “reverses, by the application of pressure, the flow of water in a natural process of osmosis so that water passes from a more concentrated solution to a more dilute solution through a semi-permeable membrane. Most reverse osmosis systems incorporate pre- and post-filters along with the membrane itself.”
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).
Distillation and rain water:
Since Ca2+ and Mg2+ exist as nonvolatile salts, they can be removed by distilling the water. Distillation is too expensive in most cases. Rainwater is soft because it is naturally distilled during the water cycle of evaporation, condensation and precipitation.