Chemistry- Hard & Soft Water

Hard and soft water

  • Hard water is water that doesn’t lather easily soap. Soft water does lather easily with soap. Hardness in water is caused by dissolved calcium or magnesium ions.
  • There are two types of hardness in water: temporary and permanent.
  • Temporary hardness can be removed by boiling- permanent cannot.
  • Temporary hardness is caused by dissolved calcium hydrogen carbonate.
  • Permanent hardness is caused by dissolved calcium/magnesium sulphate or dissolved calcium/magnesium chloride.
  • Calcium/magnesium ions in the hard water react with the soap to form scum.

Temporary Hard water: Limestone (CaCO3) reacts with rainwater containing dissolved carbon dioxide to form calcium hydrogen carbonate:
                                   

 H2O (l) + CO2 (g) + CaCO3 (s) = Ca(HCO3)2 (aq)

Permanent Hard water: Rainwater dissolves rock containing calcium sulphate (Gypsum CaSO4). Because these compounds don’t decompose on heating, the calcium or magnesium ions remain in the water and it stays hard 

CaSO4 (s) + aq = Ca2+ (aq) + SO42- (aq)

Softening Hard Water

1. Boiling (but only for temporary hard water):

Ca(HCO3)2 = CaCO3 (s) + CO2 + H2O

CaCO3 is insoluble in water and won’t cause hard water.

 

2. Adding washing soda: (hydrated sodium carbonate)

Dissolved calcium or magnesium ions are removed due to a reaction with carbonate ions from the washing soda to form insoluble CaCO3 (or MgCO3):

 

Ca2+ (aq) + CO32- (aq) = CaCO3 (s)

 

3. Ion Exchange:

Dissolved calcium or magnesium ions are removed and replaced by sodium ions from the ion exchanger

Advantages of Hard Water

  • Tastes better
  • Better for brewing beer
  • Good for tanning leather
  • It provides calcium for healthy bones and teeth.

Disadvantages of Hard Water

  • Produces a scum with soap, which wastes soap
  • It provides lime scale. This makes kettles less efficient and clogs up pipes.

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