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Removing Hardness Part 2: Ion E xchange

Removing Hardness Part 2: Ion E xchange. Learning Objectives : Explain origins of temporary and permanent hardness Understand the chemistry of softening water Describe how ion exchange can be used to remove hardness from water. Water Hardness. Water is a good SOLVENT .

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Removing Hardness Part 2: Ion E xchange

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  1. Removing Hardness Part 2: Ion Exchange Learning Objectives: Explain origins of temporary and permanent hardness Understand the chemistry of softening water Describe how ion exchange can be used to remove hardness from water

  2. Water Hardness • Water is a good SOLVENT. • Water from the tap, in streams, sea etc is NOT PURE, it contains many dissolved substances, e.g. oxygen, carbon dioxide, salts and minerals from rocks. • Hardness is caused by compounds of group 2 metals, e.g. calcium carbonate from LIMESTONE and calcium sulfate from GYPSUM

  3. Carbonate hardness (Temporary) Air contains carbon dioxide, which dissolves in water to form carbonic acid aWEAK ACID Water + carbon dioxide  carbonic acid H2O (l) CO2 (g) H2CO3 (aq) It DISSOCIATES to form: H2CO3 (aq)H+(aq)+ HCO3-(aq) (carbonic acid) (hydrogen carbonate ion)

  4. Chemical weathering of Limestone • When carbonic acid trickles through limestone, there is a chemical reaction, INSOLUBLE calcium carbonate is changed into SOLUBLE calcium hydrogen carbonate. carbonic acid + calcium carbonate  calcium hydrogen carbonate + water + carbon dioxide 2H2CO3 (aq) + CaCO3 (s)  Ca(HCO3)2 (aq) + H2O (l) + CO2 (g) • This reaction is REVERSIBLE (how stalactites and stalagmites are formed) • Soluble calcium hydrogen carbonate causes temp hardness and can be removed by HEATING (insoluble calcium carbonate, LIMESCALE formed)

  5. Permanent Hardness • Cannot be removed by heating, needs to removed CHEMICALLY • Washing soda (sodium carbonate.) Carbonate ions bind to calcium in the water, solid calcium carbonate is PRECIPITATED. (e.g. Calgon) • Ion exchange. (e.g. dishwasher salt unit)

  6. Na+ ions replace Ca2+ ions to soften the water SOFT WATER OUT HARD WATER IN Calcium ions in hard water Ion exchange resin When all the sodium ions have been washed out, the column can be recharged by running salt water (sodium chloride) through it

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