Potable water is conveyed through lead or iron pipes. Some waters passing through iron pipes lead to the growth of vegetation, which rapidly corrodes the iron, and in time the pipes may be completely choked. Soft waters more than hard are likely to attack iron. Lead is rapidly attacked by distilled, or rain, water in the presence of air, forming lead hydroxide, Pb(OH)2, which is appreciably soluble, or forms a colloidal solution. The action is due partly to dissolved oxygen, and partly to free carbonic acid. Hard water has much less action on lead than soft water.
Expt. 4. - Two pieces of clean lead pipe are placed in two beakers containing distilled water and tap-water, respectively, the metal being only partly covered. Allow the beakers to stand for a few hours. The distilled water rapidly becomes turbid, whilst the tap-water (if hard) remains clear. Pour off the liquids, and add hydrogen sulphide water. Compare the brown or black colorations, due to lead sulphide. The water should not be filtered, as dissolved lead hydroxide is retained to some extent by filter-paper. Bicarbonates in water (temporary hardness) reduce the action on lead; free carbonic acid (e.g., in rain water) increases the action. Peaty waters, containing organic acids, act rapidly on lead or zinc, unless neutralised by lime.
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