Hard water: temporary and permanent hardness

Water

1. Physical Properties

   • *colourless liquid at room temperature and pressure. *melting point is 0C. *boiling point 100C

2. Chemical test

   • anhydrous copper(II) sulphate-white solid changes to blue.

3. Hard water

   • Water that does not readily form a lather with soap.
     • Caused by Ca2+ ions and/or Mg2+ ions.
       • Soap is: Sodium stearate- reacts to form insoluble scum/solid.
         • Ca2+(or Mg2+) ions in water react with stearate ions in soap, producing insoluble scum called: Calcium stearate or(magnesium stearate).
   • Temporary Hardness
     • *Removed by boiling. *Caused by dissolved calcium hydrogen carbonate.
     • 1. Take a sample + add soap solution. 2.Shake mixture well-Should be lather. 3.Take another sample+boil. 4.Add soap solution. 5.Shake-should be lather.
   • Permanent Hardness
     • *Can't be removed by boiling. *Caused by dissolved calcium sulphate or MgSO4, or dissolved calcium chloride or MgCl2.
     • 5. No lather.

4. How temp hardness arises

   • Limestone(CaCO3) reacts with rainwater containing dissolved Co2 to form Calcium hydrogen carbonate solution.
     • CaCO3+H2O--->Ca(HCO3)2

5. Removing

   • Must remove dissolved Ca2+ ions.
     • Boiling
       • Dissolved Ca2+ ions in hard water removed as insoluble CaCO3. Ca(HCO3)2(aq)--->CaCO3(s)+CO2(g)+H2O(l)
     • Ion exchange
       • Dissolved Ca2+ ions in the hard water are removed + replaced by Na+ ions from ion exchange resin-Na+ ions do not cause hardness in water.
     • Hydrated sodium carbonate
       • Ca2+(aq)+CO3 (aq)--->CaCO3(s)

6. Adv.: Tastes better; brewing beer; tanning leather; calcium DisAdv.: Scum with soap; produces fur-(limescale) kettles less efficient + clogs pipes; dishwater salt needed to soften water=costly.