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The purest water which can be obtained is almost, but not quite, a non-conductor of electricity. After allowing for the effects of traces of conducting impurities, a slight conductivity, due to the ions of water itself, remains. The ionisation of water into hydrogen ions and hydroxide ions is very small, and a state of equilibrium is set up: H2O <=> H˙ + OH'. To pass a current of 1 ampere through a centimetre cube of pure water at 18° would require a potential gradient of about a million volts, i.e., the electrodes would have to be connected with 500,000 accumulator cells in series. The ionisation of water proceeds only to the extent of 1 mol of water ionised in ten million litres (1010 c.c.).
If 1 mol of hydrochloric acid is dissolved in water so that the total volume of solution is 1 litre, the conductivity of the water is increased nearly ten millionfold. This great increase in conductivity is due to the ionisation of the hydrochloric acid: HCl <=> H˙ + Cl', the ions of which are present in very great numbers as compared with the ions of water. The acid, in fact, exists in the solution almost completely in the form of ions. Hydrochloric acid is a strong electrolyte, water is a very weak electrolyte.
Most acids, bases, and salts, such as hydrochloric acid, sulphuric acid, caustic potash, lime, common salt, copper sulphate, and alum, give conducting solutions with water, and are electrolytes. Pure sugar, urea, alcohol, and most organic compounds, do not give conducting solutions with water: they are non-electrolytes. Since acids may be regarded as hydrogen salts, and bases as salts containing the hydroxide radical, OH, the results described may be summarised in the statement that most salts are electrolytes, whilst substances which are not salts are non-electrolytes.
All acids give the hydrogen ion in aqueous solution. Dry liquefied hydrogen chloride does not redden dry litmus, or act on zinc or marble, and it is almost a perfect insulator. In solution it behaves as an acid, since then the hydrogen ion is formed.
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