Hardness of Water

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  • Created by: Jo Wells
  • Created on: 10-04-13 15:10
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  • Hardness of water
    • hard water makes SCUM and SCALE
      • whereas soft water makes a lather
      • dissolved calcium & magnesium ions in the water react with the soap to make scum = insoluble
        • to get a lather you need to use more soap
      • when heated the water forms a scale (mostly calcium carbonate)
        • on the inside of kettles, pipes & boilers
          • reducing efficiency - can eventually block pipes
          • scale is also a bit of a thermal insulator - e.g. a kettle will take longer to boil = less efficient
    • hardness is caused Ca2+ & Mg2+ ions
      • rain falling on some types of rocks (e.g. limestone, chalk & gypsum) can dissolve some compounds
        • like magnesium sulfate (soluble) & calcium sulfate (a bit soluble)
    • Ca2+ ions are good for healthy teeth and bones
    • studies have found people who live in hard water areas are at less risk of developing heart disease than those who live in soft water areas
    • removing the dissolved Ca2+ & Mg2+ ions makes water soft
      • there are 2 kinds of hardness
        • temporary
          • caused by the hydrogen -carbonate ion (HCO3
          • can be removed by boiling
            • when heated the calcium hydrogen -carbonate decomposes to form insoluble calcium carbonate
              • this is the 'limescale' formed in kettles
              • as well as water & CO2
        • permanent
          • can't be removed by boiling
          • both can be softened/ removed by =
            • temporary
              • caused by the hydrogen -carbonate ion (HCO3
              • can be removed by boiling
                • when heated the calcium hydrogen -carbonate decomposes to form insoluble calcium carbonate
                  • this is the 'limescale' formed in kettles
                  • as well as water & CO2
            • both can be softened by adding washing soda (sodium carbonate, Na2CO3)
              • the added carbonate ions react with the Ca2+ & Mg2+ ions to make an insoluble precipitate of calcium carbonate & magnesium carbonate
                • the Ca2+ Mg2+ are no longer dissolved in the water so they can't make it hard
            • both can be removed by running water through 'ion exchange columns'
              • the columns contain sodium (or hydrogen) ions & they 'exchange' them for Ca or Mg ions in the water running through them
    • titration can be used to compare the hardness of water samples
      • method =
        • 1. fill burette with 50cm3 of soap solution
          • 2. put 50cm3 of 1st water sample in a flask
            • 3. add 1cm3 of soap solution to flask using burette
              • 4. put bung in flask & shake for 10 seconds
                • 5. repeat 3 & 4 until a lather is formed
                  • 6. record how much soap was needed (in cm3)
                    • 7. repeat 1-6 with other samples
                      • 8. boil fresh samples of each type of water (10 mins) & repeat 1-7
                  • where the bubbles cover the surface for at least 30 secs
            • Untitled
      • results
        • distilled water has little or no hardness
        • should be able to tell whether temporary or permanent hardness
  • the columns contain sodium (or hydrogen) ions & they 'exchange' them for Ca or Mg ions in the water running through them

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