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Water : Water, Natural |
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The following division of natural waters is convenient: (1) rain, (2) river, (3) spring, or deep well, (4) sea and (5) mineral. The impurities in natural water are of two kinds: (1) suspended, both mineral and organic; (2) dissolved, both solids (mineral and organic), and gases. These are present in amounts varying considerably with the particular source of the water. Rain water always contains impurities, especially if deposited near, or in, towns where coal is burnt. Dissolved atmospheric gases (oxygen, nitrogen, carbon dioxide), and sodium chloride, derived from sea-spray carried inland by winds, are invariably present. Nitrous and nitric acids, produced by electrical discharges (lightning), are nearly always present as ammonium nitrite and nitrate, and sometimes free ammonia occurs. In towns, sulphuric acid, from the combustion of coal (which contains iron pyrites, FeS2), is present. Suspended impurities, chiefly soot from fuel smoke, are usually contained in rain. The free sulphuric acid may be neutralised by lime-water, or by allowing the water to stand over limestone. Melted snow contains similar impurities. River water is rain water which has percolated through surface-soil, and taken up salts, organic matter, and suspended matter such as clay. The dissolved matter is especially marked when the water has passed through limestone or calcareous soil (i.e., soil rich in calcium carbonate) because the carbonic acid present in the rain, produced from atmospheric carbon dioxide: CO2 + H2O <=> H2CO3, dissolves the carbonates of calcium and magnesium, forming unstable soluble bicarbonates. These are readily decomposed on boiling the water, with precipitation of the insoluble carbonates and evolution of carbon dioxide: CaCO3 + H2CO3 <=> Ca(HCO3)2; MgCO3 + H2CO3 <=> Mg(HCO3)2. Expt. 1. - Pass a stream of carbon dioxide (washed from acid spray by passing through a wash-bottle containing water) into lime-water. This at first becomes turbid, owing to the formation of insoluble calcium carbonate: Ca(OH)2 + CO2 = CaCO3+H2O. On continued passage of the gas, the precipitate redissolves, producing calcium bicarbonate: Ca(HCO3)2, (or CaO,2CO2 + H2O, i.e., containing twice as much CO2, for the same weight of lime, as the carbonate, CaO,CO2). On boiling, the clear liquid again becomes turbid, and calcium carbonate is precipitated. The reaction is reversible: CaCO3 + H2O + CO2 <=> Ca(HCO3)2. If an equal volume of lime-water is added to the clear bicarbonate solution, turbidity is produced, and nearly insoluble calcium carbonate precipitated: Ca(HCO3)2 + Ca(OH)2 = 2CaCO3 + 2H2O (or, omitting water: CaO,2CO2 + CaO = 2CaO,CO2). The filtrate is practically free from calcium salts.
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