Temporary Hard Water Carbon Dioxide in the air dissolves in rain water forming the week acid, carbonic acid. CO2 (g) + H2O (I) → H2CO3 (aq) When this dilute acidic solutions falls on rocks containing calcium carbonate e.g. limestone, soluble calcium hydrogencarbonate is formed. As a result there is calcium ions dissolved in water and the water is hard. H2CO3 (aq) + CaCO3 (s) → Ca(HCO3)2 (aq) The Calcium Hydrogencarbonate is easily decomposed on heating. The calcium carbonate produced is insoluble and is precipitated out. Ca(HCO3)2 (aq) → CaCO3 (s) + CO2 (g) + H2O (l) As calcium ions are removed, the hard water is softened. Calcium Carbonate is deposited on the heating vessel. Permanent Hard Water Cannot be softened by boiling Carbonates are not the only calcium or magnesium containing compounds found in rocks, gypsum, for example contains calcium sulphate. When rain passes over these rocks, calcium and magnesium ions can dissolve in the water and make it hard. These compounds do not decompose on heating, the ions stay in the water and it remains hard. CaSO4 (s) + aq → Ca2+ (aq) + SO42- (aq) Boiling can be used but only for temporary hard water.
Temporary Hard Water Carbon Dioxide in the air dissolves in rain water forming the week acid, carbonic acid. CO2 (g) + H2O (I) → H2CO3 (aq) When this dilute acidic solutions falls on rocks containing calcium carbonate e.g. limestone, soluble calcium hydrogencarbonate is formed. As a result there is calcium ions dissolved in water and the water is hard. H2CO3 (aq) + CaCO3 (s) → Ca(HCO3)2 (aq) The Calcium Hydrogencarbonate is easily decomposed on heating. The calcium carbonate produced is insoluble and is precipitated out. Ca(HCO3)2 (aq) → CaCO3 (s) + CO2 (g) + H2O (l) As calcium ions are removed, the hard water is softened. Calcium Carbonate is deposited on the heating vessel. Permanent Hard Water Cannot be softened by boiling Carbonates are not the only calcium or magnesium containing compounds found in rocks, gypsum, for example contains calcium sulphate. When rain passes over these rocks, calcium and magnesium ions can dissolve in the water and make it hard. These compounds do not decompose on heating, the ions stay in the water and it remains hard. CaSO4 (s) + aq → Ca2+ (aq) + SO42- (aq) Boiling can be used but only for temporary hard water.
g-c-s-e-chemistry-notes.docx