Hard and Soft Water - Water Softening
Pure water or water containing dissolved sodium salts easily gives lather with soap. Such a water sample is called 'soft water' and is suitable for use. On the other hand, if the sample contains dissolved calcium or magnesium salts, it is hard to get lather with soap (sodium stearate). This is because of the formation of an insoluble scum of metal (calcium or magnesium) stearate.
(where M = Ca or Mg)
This kind of water sample is called 'hard water' and is not suitable for use.
Hardness in water is of two types-temporary and permanent. Temporary hardness is due to the presence of bicarbonates of calcium or magnesium. It can be removed by boiling the water when carbonates formed are precipitated out.
In an alternative method, carbonates can be precipitated and subsequently removed by the addition of a calculated amount of slaked lime (Clarke's method).
Permanent hardness is caused when calcium or magnesium salts (usually chloride or sulphate) other than bicarbonates are present. This type of hardness (along with temporary hardness) can be removed by the addition of sodium hydroxide, carbonate or sodium phosphate when corresponding insoluble salts of calcium and magnesium are formed and removed.
M2+ + 2NaOH 2Na+ + M(OH)2
M2+ + Na2CO3 2Na+ + MCO3
M2+ + (NaPO3) n 2Na+ + Na(n-2) Ma(PO3) n
Ion exchange resins are also used extensively to soften the water. In this method, hard water is passed through a column of artificial zeolites (complex sodium aluminosilicates, abbreviated as Na+Z-) or synthetic cation exchange resins (giant organic compounds with an acidic group attached, abbreviated as H+R-) when Ca2+ or Mg2+ ions present in hard water are exchanged for Na+ or H+ ions.
To prepare de-ionized water (water without any ions, cations or anions), the sample may be further passed through anion exchanger when the anions , etc.) are exchanged for OH- ions.
A typical ion exchanger is shown in the below figure:
Water is extensively used for making steam in industries. It should be free from chemical impurities otherwise they will be deposited within the boiler and corrode it. Suitable reagents may also be added in the boiler for cleaning and proper functioning. In some industrial preparations and in the laboratory, distilled or de-ionized water (using ion exchangers) is needed.