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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.

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Presentation on theme: "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."— Presentation transcript:

1 Water Hardness

2 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

3 Permanent Hardness This is caused by the presence of CaSO 4 This isnt removed by boiling the water

4 Removing Water Hardness Temporary hardness can be removed by boiling. Ca(HCO 3 ) 2 decomposes to form calcium carbonate, water and carbon dioxide. Ca(HCO 3 ) 2 CaCO 3 + H 2 O + CO 2 This process makes the water soft, but produces limescale, CaCO 3

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 Ca 2+ ions is poured into the top of the column and allowed to trickle over the resin. The Ca 2+ displace the Na + ions from the resin beads. The Ca 2+ ions stick to the beads and are removed from the water. NaHCO 3 and Na 2 SO 4 flow out of the base of the column. The water is softened.

7 Washing Soda Washing Soda reacts with Ca 2+ ions to produce CaCO 3, which precipitates out of solution. This precipitate is often called Limescale. Lime scale is removed using weak acids, such as Ethanoic Acid (CH 3 COOH). CH 3 COOH + CaCO 3 Ca(CH 3 COO) 2 + CO 2 +H 2 O 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 Ca 2+ 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 ? If a water sample took 100 drops of soap solution to produce a lather, how much Calcium does it contain ? 7.6 Mol/dm 3

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 isnt?


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