Permanent Hard Water

• Permanent hardness is caused by dissolved calcium sulfate or magnesium sulfate.
• • Permanent hard water cannot be removed by boiling the water - two possible ways.
• WASHING SODA

  • Contains sodium carbonate (Na2CO3) - when added to permanent hard water, the carbonate ions react with dissolved calcium ions, forming a precipitate of calcium carbonate (limescale - insoluble solid).
  • Large quantities of washing soda used makes this an expensive method and leads to formation if limescale, a problem.
• ION EXCHANGE COLUMN

  • Hard water continually passes through the column (resin coated in a high concentration of sodium ions). Each calcium (or magnesium) ion is exchanged for two sodium ions.
  • Technique is more convenient as it’s continuous and more suitable for large scale treatment.
  • ion exchange columns are expensive but resin can be regenerated by passing concentrated sodium chloride solution through it - so cheaper in long run.
• • Sodium carbonate reacts with calcium / magnesium ions to form insoluble calcium / magnesium carbonates (limescale) and sodium ions (don't cause harm).
  • Ca^2+ / Mg^2+ (aq) + Na2CO3 (aq) -> CaCO3 / MgCO3 (s) + 2Na^+ (aq)