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Water Hardness

Water Hardness. Limescale deposits in hot water pipes and on filament in kettles CaCO 3. Water Hardness. Hard water is water that will not easily form a lather with soap.

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Water Hardness

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  1. Water Hardness

  2. Limescale deposits in hot water pipes and on filament in kettles CaCO3

  3. Water Hardness • 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

  4. Where do the ions that cause water Hardness come from? • Mg+ ions come from dolomite rock • MgCO3 • Ca+ ions come from Limestone rock

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

  6. Modern detergents • Made from crude oil • Not affected by hard water ions • Too severe on skin

  7. Soft water • Is water that lathers easily • No Ca 2+ or Mg 2+ ions

  8. Types of Hardness • Temporary Hardness • Permanent Hardness

  9. Temporary 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

  10. Causes of Temporary Hardness • Temporary Hardness can be removed by Boiling • Rainwater slightly acidic due to CO2 (carbonic acid) • Acidic rainfall will dissolve limestone (CaCO3) • Forming soluble Calcium Hydrogen Carbonate Ca(HCO3)2 and Mg (HCO3)2 • If heated a reaction occurs in which the Ca ions form insoluble CaCO3 • Pg 264 book for equations (must know)

  11. Water Cycle

  12. Causes of Temporary Hardness Limestone + Carbonic Acid Carbonic Acid Calcium Hydrogen Carbonate (aq)

  13. Temporary Hardness= Rain to Acidic Rain to Hard water to Lime scale carbonic acid H20 + CO2 = H2CO3 CaCO3 + H2CO3 = Ca(HCO3 )2 (soluble) Ca(HCO3)2 + heating = CaCO3 + CO2 + H2O insoluble CaCO3 destroys heating elements and blocks pipes Limestone + carbonic acid = calcium Hydrogencarbonate Insoluble soluble

  14. Temporary Hardness 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.

  15. Why Be Concerned About Hard Water? • Hard water does cause soap scum, clogs pipes and clogs boilers as limescale

  16. Permanent hardness • Permanent Hardnessis 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

  17. Hard water disadvantages • Limescale is deposited on heating elements which reduces their efficiency e.g. washing machines and kettles • Limescale blocks pipes • Soap not as efficient • Produces scum

  18. Hard water Advantages • Source of calcium which is good for teeth and bones • Some people believe it tastes nicer • Makes better tasting beer

  19. To soften the hard water it is necessary to remove the calcium and magnesium ions • Addition of washing soda • Distillation • Ion exchanges • Deionisers

  20. Addition of washing soda • Washing soda is hydrated sodium carbonate (Na2CO3.10H2O) • Calcium ions+ carbonate ions=calcium carbonate (insoluble) • Washing soda is usually included in clothes detergent • Ca2+ + CO3- = CaCO3 (precipitate)

  21. Distillation • Distilling water removes all insoluble and soluble particle • Including calcium and magnesium ions • Hence water is softened • Too expensive • Pure water formed

  22. Ion exchangers • Swaps the hardness ions with ions that don’t cause hardness • Sodium ions used as swap for mg and Ca ions • Cation Exchange • Uses filters • Not as pure as distillation

  23. Removal of Hardness by Ion Exchange • 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

  24. Deionised water • To make deionised water it must be passed over a mixed bed resin (cation and anion resin) • RH (cation) + Na+ = RNa + H+ • ROH (anion) + CL- = RCL + OH- • H+ + OH- = H2O • Only ions removed so product not as pure as distillation

  25. Deionisers • Removes all ions from water • As water passes through all the positive ions are replaced with H+and negative ions replaced with OH-

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

  27. Deionised Water M+ + HR → MR + H+ All the metal ions in the water are replaced by hydrogen ions A- + ROH → RA + OH- All 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

  28. Purer Dissolved and suspended solids removed Not as pure Dissolved gases present Non-ionic molecules present Distilled vs Deionised

  29. Deionised Water and Distilled Water Deionised water is not quite as pure as distilled water Deionised water has all the dissolved ions in the water removed Distilled water has all the dissolved solids and covalent compounds e.g. dissolved gases removed

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