Section 9.2 hardness in water

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Information about Section 9.2 hardness in water

Published on March 9, 2014

Author: brownm7


    Hard water is water that will not easily form a lather with soap. Hardness is caused by the presence of Ca2+ or Mg2+ ions dissolved in the water. These ions react with the soap to form a scum rather than a lather. The concentration of the Ca2+ ions is greater than the concentration of any other metal ion in our water. Water hardness is usually expressed in ppm CaCO3

 Soaps (e.g. Sodium Stearate)  Soap is a Sodium salt of Stearic Acid, C17H35COONa) which is a long chain fatty acid O NaHO  palmitic acid Sodium Stearate Soap scum is formed when the Ca 2+ ion (or Mg2+ ion) reacts with the soap. This causes an insoluble salt (calcium stearate) to precipitates to form the scum. 2C17H35COONa + Ca2+   (C17H35COO)2Ca↓ + 2Na+ Soap actually softens hard water by removing the Ca 2+ ions from the water

 Temporary Hardness  Permanent Hardness

Temporary Hardness is hardness that can be removed by boiling. Temporary hardness is caused by: ◦ Calcium Hydrogencarbonate Ca(HCO3)2 ◦ Magnesium Hydrogencarbonate Mg(HCO3)2

Carbonic Acid Limestone + Carbonic Acid Calcium Hydrogen Carbonate (aq)

When boiled Ca(HCO3)2 heat CaCO3 + CO2 + H2O Ca2+ (and Mg2+) ions are removed from the water and now form the precipitate CaCO3 at the bottom of the kettle etc. This is referred to as limescale. This can damage kettles and hot pipes.

 Hard water does cause soap scum, clogs pipes and clogs boilers as limescale

  Permanent Hardness is hardness that cannot be removed by boiling. Permanent hardness is caused by ◦ Calcium Chloride CaCl2 ◦ Magnesium Chloride MgCl2 ◦ Calcium Sulfate CaSO4 ◦ Magnesium Chloride Mg SO4

 Ca2+ + Na2R → CaR + 2Na+ The Hardness causing calcium and magnesium ions are removed from the water and replaced by sodium ions, which do not cause hardness

Ion exchange resins with a mixture of a cation exchanger and anion exchanger remove all the ions from water.

M+ + HR → MR + H+ All the metal ions in the water are replaced by hydrogen ions A- + ROH → RA + OHAll the anions in the water are replaced by hydroxide ions The hydrogen ions and hydroxide ions to give water H+ + OH- → H2O This mixed resin replaces all the dissolved salts with water molecules The water is said to be DEIONISED

Deionised water is not quite as pure as distilled water Deionised water had all the dissolved ions in the water removed Distilled water has all the dissolved solids and dissolved gases removed

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