chemistry 3 . 2

chemistry 3 . 2

1. soft water

   • lathers easily with soap

2. hard water

   • contains dissolved compounds that reacts with soap to form an insoluble product called scum
     • scum is formed when water which contains dissolved calcium or magnesium ions from certain rocks, reacts with the soap
       • calcium compounds are good for health; strong bones and teeth and reduces the risk of heart disease
   • temporary hard water
     • produces the insoluble product called scale when its heated
       • scale can be deposited in kettles, boilers and pipes. This reduces the efficiency of heating systems

3. removing hardness

   • temporary hard water hardness can be removed by boiling
     • - temporary hard water contains hydrogencarbonate ions
       • - the hydrogen carbonate ions decompose when heated to form carbonate ions, water and carbon dioxide
         • the carbonate ions react with the calcium and magnesium ions in the water to produce precipitates of magnesium carbonate and calcium carbonate that are deposited as scale
   • permanent hard water hardness can be removed by...
     • sodium carbonate reacts with the calcium ions and magnesium ions in the water
       • this forms solid calcium carbonate and magnesium carbonate
         • these carbonates can't react with soap
     • 1) adding sodium carbonate (washing soda)
     • 2) ion-exchange collum
       • the column has a resin containing sodium or hydrogen ions
         • when hard waiter is passed through the resin, calcium and magnesium ions become attached to the resin
           • the sodium or hydrogen ions replace the calcium/magnesium ions in the water

4. water treatment

   • phosphates and nitrates and microbes are harmful
     • microbes can cause diseases like cholera and dysentery
       • distillation works by boiling water and condensing the steam to form pure water
         • fluoride can be added to drinking water because it helps to reduce tooth decay
           • chlorine is also added to prevent disease
             • however chlorine can react with other natural substances in the water to produce toxic by-products which can cause cancer
   • water from reservoirs go through water treatment works
     • 1) the water passes through a mesh screen
       • 2) chemicals are added to make the solids and microbes stick together and fall to the bottom
         • 3) the water is filtered through gravel beds to remove all solids
           • 4) the water is chlorinated to kill microbes
     • even after this process, some people still aren't satisfied
       • you can buy filters that contain carbon or silver to remove substance from the tap water
         • carbon removes the chlorine taste
         • silver kills bacteria