Water Hardness.

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Presentation transcript:

Water Hardness

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

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

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

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

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.

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

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.

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

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?