Water

Water

1. Hard Water

   • Water that lathers easily with soap is said to be soft water
   • Hard water uses more soap to produce a lather and to wash effectively
   • This is because hard water contains dissolved compounds that react with soap to form an insoluble solid called scum
     • Scum
   • Other detergents (called soapless detergents) do not react with hard water to form scum
   • When water is in contact with rocks, some compounds dissolve
   • If the water contains dissolved calcium or magnesium ions, these will react with soap to form scum, so the water is hard
   • When it is heated, one type of hard water called temporary hard water, produces an insoluble solid called scale
     • Temporary Hard Water
   • Scale can be deposited in kettles, boilers and pipes reducing efficiency because of blockages
   • Calcium compounds are good for our health, helping to develop strong bones and teeth and may reduce the risk for heart disease

2. Removing Hardness

   • Soft water may contain dissolved substance, but it does not contain dissolved salts that react with soap to produce scum
   • Also, it does not produce scale when it is heated
   • Hard water can be made soft by removing the dissolved calcium and magnesium ions that react with soap
   • Some types of hard water are affected by heating while others are not
   • Temporary hard water is softened by boiling, because when it is heated the calcium and magnesium compounds form insoluble scale and this removes them from the water
     • Temporary Hard Water
   • Permanent hard water is not softened by boiling and does not produce scale when it is heated
     • Permanent Hard Water
   • Temporary hard water contains hydrogen carbonate ions HCO3-(aq)
     • the hydrogen carbonate ions decompose when heated to produce carbonate ions, water and carbon dioxide
     • 2HCO3-(aq) ---- CO32-(aq) + H2O(l) + CO2(g)
   • The carbonate ions react with calcium ions and magnesium ions in the water to produce precipitates of calcium carbonate that are deposited as scale
   • One method of softening either type of hard water is by precipitating out the ions that cause hardness
     • this can be done by adding washing soda, which is sodium carbonate
     • The sodium carbonate reacts with calcium ions and magnesium ions in the water to form solid calcium carbonate and magnesium carbonate that cannot react with soap
   • Another method is to use an ion-exchange column packed with resin containing sodium or hydrogen ions
     • When hard water is passed through the resin, calcium and magnesium ions become attached to the resin, and sodium or hydrogen ions take their place in the water
     • Sodium ions and hydrogen ions do not react with the soap

3. Water Treatment

   • Drinking water should not contain any harmful substances and should have sufficiently low levels of dissolved salts and microbes
   • Water from an appropriate source can be treated to make it safe to drink
   • Water is often treated by sedimentation and filtration to remove solids
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   • This is followed by disinfection to kill microbes in the water
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   • Chlorine is often used to kill microbes in drinking water
   • Water filters can be used to improve the taste of water
   • They often contain carbon and an ion-exchange resin that remove some soluble substances and silver or another substance to prevent the growth of bacteria
   • Pure water can be made by distillation
   • This requires a large amount of energy to boil the water so it would be very expensive to do on a large scale

4. Water Issues

   • The hardness of water supplied depends on where you live
   • Hard water causes problems in heating systems and with washing, but if used for drinking has health benefits
   • If the water is not suitable for a particular purpose, you can treat the water or use an alternative supply
   • Chlorine is particularly effective in killing microbes in water so that it is safe to use
   • However, chlorine is poisonous, and it can produce other toxic compounds so must be carefully controlled to minimise the risk
   • Fluoride compounds are added to toothpastes and to water supplies to help prevent tooth decay
   • AGAINST FLUORIDE: people should be able to chose to take extra fluoride or not