June 14th, 2007
Hardness is a term to describe water that doesn’t allow soaping action, in other words no bubbles (lather) and no cleaning. The term ’soaping action’ refers to the formation of micelles by negative ions joined to long carbon chains that dissolve in the fat or grease leaving the negative charges outside the micelle structure. These can then bond to the water molecules. In this way the hydrophobic grease is made to dissolve and lifts off the fabric or skin.
Hardness is due to Ca2+ or Mg2+ ions dissolved in the water. These ions react with the long negative ions of soap (stearate ions C17 H35 COO-) to make an insoluble grey compound which we call ’scum’. As the reaction removes the necessary soap ions from the water it prevents lathering and the soap cannot perform its function.
Hardness can be removed by various means depending on the other ion that is dissolved with the Mg2+ or Ca2+ ions.
There are two subcategories of hardness:
1. Temporary
2. Permanent
Temporary hardness is due to dissolved calcium (or magnesium) hydrogen carbonate Ca(HCO3)2, it can be removed by boiling the water, when the hydrogen carbonate ions decompose forming insoluble calcium (or magnesium) carbonate:
Ca(HCO3)2 –> CaCO3 + CO2 + H20
Permanent hardness (due to dissolved calcium and magnesium sulphates and chlorides) must be removed by another method either:
1. ion exchange using resins etc.
2. chemical precipitation
Ca2+ + CO3(2-) –> CaCO3(ppt)
use of substances such as washing soda (sodium carbonate) do just that.икони
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