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Water Hardness

Water Hardness. What causes it ?. Dissolved CO 2 makes water weakly acidic. (Carbonic Acid) This “acidic” water reacts with CaCO 3 in limestone and chalk to make Calcium Hydrogen Carbonate . CaCO 3 +H 2 O → CO 2 + Ca(HCO 3 ) 2 This causes TEMPORARY HARDNESS. Permanent Hardness .

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Water Hardness

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  1. Water Hardness

  2. What causes it ? • Dissolved CO2 makes water weakly acidic. (Carbonic Acid) • This “acidic” water reacts with CaCO3 in limestone and chalk to make Calcium Hydrogen Carbonate. • CaCO3 +H2O → CO2 + Ca(HCO3)2 • This causes TEMPORARY HARDNESS

  3. Permanent Hardness • This is caused by the presence of CaSO4 • This isn’t removed by boiling the water

  4. Removing Water Hardness • Temporary hardness can be removed by boiling. Ca(HCO3)2 decomposes to form calcium carbonate, water and carbon dioxide. • Ca(HCO3)2 → CaCO3 + H2O + CO2 • This process makes the water soft, but produces limescale, CaCO3

  5. Removing Water Hardness • Both types of hardness can be removed by: • Passing the water through an ion exchange column. • Adding Washing Soda (Sodium Carbonate)

  6. Ion Exchange Columns Specialised resin beads are seeded with sodium ions. This resin is placed in a column and packed loosely together so that water can flow slowly through it. Hard water containing Ca2+ ions is poured into the top of the column and allowed to trickle over the resin. The Ca2+ displace the Na+ ions from the resin beads. The Ca2+ ions “stick” to the beads and are removed from the water. NaHCO3 and Na2SO4 flow out of the base of the column. The water is softened.

  7. Washing Soda • Washing Soda reacts with Ca2+ ions to produce CaCO3, which precipitates out of solution. This precipitate is often called Limescale. • Lime scale is removed using weak acids, such as Ethanoic Acid (CH3COOH). • CH3COOH + CaCO3 → Ca(CH3COO)2 + CO2 +H2O 2

  8. Measuring Water Hardness • Madhur used soap solution to measure the level of water hardness. • The harder the water, the more soap solution required to make lasting lather. • Distilled water does not contain any water hardness and so is used as a control. • A calibration curve was first constructed using solutions of known Ca2+concentrations.

  9. Calibration Chart • This is produced plotting the amount of soap added to solutions known concentrations of Calcium ions. If a water sample took 100 drops of soap solution to produce a lather, how much Calcium does it contain ? 7.6 Mol/dm3

  10. Questions • Which of these acids would you use to de-scale a kettle? • Citric Acid • Sulphuric Acid • Hydrochloric Acid • Why is temporary hardness a problem for electric water heaters, but permanent hardness isn’t?

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