Chemistry - C3.2 - Water

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C3.2.1 - Hard Water

  • Soft water - lathers easily with soap - hard water needs more soap to effectively lather and wash: contains dissolved compounds that react with soap forming the insoluable solid scum - soapless detergents don't react with hard water to form scum
  • Compounds dissolve in water when it comes in contact with rocks - if compounds are Ca or Mg ions, they react to form scum: water is hard
  • When heated, one type of hard water produces the insoluable solid scale - can be deposited in kettles, boilers and pipes - reduces heating system efficiency and cause blockages
  • Ca compounds are good for health: help to develop strong bones and teeth, can reduce risk of heart disease
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C3.2.2 - Removing Hardness

  • Substances in soft water don't produce scum or scale - hard water can be softened by removing Ca and Mg ions
  • Some hard water affects heating - temporary hard water: softened by heating - removes ions and forms scale, permanent hard water: not soften and doesn't produce scale
  • Temporary hard water contains HCO3 -(aq) which decomposes when heated: 2HCO3 -(aq) -> CO3 2-(aq) + H2O(l) + CO2(g) - carbonate ions react with Ca and Mg ions - deposited as scale
  • To soften: precipitate out Mg and Ca ions: add washing soda (Na2CO3) - reacts with ions to form solid carbonates: can't react with soap
  • Ion-exchange column - packed with resin containing Na or H ions - when passed through resin, Mg and Ca ions attach to resin and replaced by Na or H ions that don't react with soap
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C3.2.3 - Water Treatment

  • Drinking water shouldn't contain any harmful substances and have low dissolved salts and microbes levels
  • Water from appropriate source can be treated and made safe to drink - treated by sedimentation and filtration to remove solids -> disinfection to kill microbes - chlorine often used
  • Water filters used to improve taste - often contain carbon and ion-exchange resin that remove some soluable substances and silver or other substances to prevent bacteria growth
  • Pure water made by distillation - requires large amounts of energy to boil and so expensive to do on large scale
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C3.2.4 - Water Issues

  • Advantages and disadvantages when treating water - must be considered before decisions are made - particulary important for public water treatment
  • Hardness depends on where you live - hard water causes problems in heating systems and with washing but has health benefits - if water isn't suitable for purpose, it can be treated or obtained somewhere else
  • Chlorine effective at killing microbes but is poisonous and can produce toxic compounds: use must be controlled
  • Fluoride compounds added to toothpaste ans water to prevent tooth decay - many arguements for and against it - eg. people haven't consented to it
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