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.