GCSE C3 Chapter 2

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  • Created by: emma998
  • Created on: 14-03-14 16:13

Hard water

  • Soft water is water that does not produce scale when heated or scum when mixed with soap
  • Hard water reacts with soap to produce scum and some types produce scale when heated
  • When water is in contact with rocks some compounds dissolve
  • The water is hard if it contains calcium or magnesium ions
  • Temporary hard water produces an insoluble solid called scale when heated
  • Scale is deposited on kettles and heating systems reducing their efficiency and causing blockages
  • Calcium compounds are good for health
    • They can reduce the risk of heart disease
    • They help to develop strong bones and teeth
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Removing hardness

Temporary hard water can be softened by heating:

  • Temporary hard water contains hydrogen carbonate ions
  • They decompose when heated to produce carbonate ions, water and carbon doixide
  • 2HCO3 -  =  CO3 2-  +  CO2  + H2O
  • The carbonate ions react with the magnesium and calcium ions 
  • They produce solid precipitates of Ca or Mg carbonates - deposited as scale
  • Ca 2+(aq)  +  CO3 2-(aq)  =  CaCO3(s)

Washing soda

  • Washing soda contains sodium carbonate and is mixed with the water
  • The calcium and magnesium ions react with the sodium carbonate
  • Produces solid precipitates of Ca/Mg carbonates which can be filtered from water
  • Sodium remains in the water

Ion exchange collumn

  • Collumn packed with resin containing sodium or hydrogen ions
  • As water passes through, Ca and Mg ions attach to resin
  • Sodium / hydrogen ions replace the Mg or Ca ions in the water
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Water treatment

  • Drinking water is not pure water: it just contains low levels of salts and microbes andcontains nothing harmful
  • Water treatment
    • Suitable source
    • Filtration to remove solids
    • Chlorine added to kill bacteria
  • Some people choose to treat their water further using a water filter
    • Often contain carbon and an ion exchange collumn to remove soluble substances and hardness
    • Can contain silver particles to prevent the growth of bacteria
  • Pure water can be made by distillation. It is an expensive process which requires a large amount of energy.
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Water issues

  • Water treatment has advantages and disadvantages
  • Advantages:
    • Kills bacteria in water
    • Removes hardness so doesn't effect heating systems and pipes
  • Disadvantages:
    • Chlorine is poisonous and it ca produce other toxic compounds and therefore use must be carefully controlled to minimise risks
    • Calcium ions are important for developing strong bones and teeth and can reduce the risk of heart disease
  • Flourides can be added to water supplies because it helps to prevent tooth decay however there are many ehtical implications and arguments against this because people believe they should have the choice as to whether or not they wish to take eaxtra flouride.
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