why are lead and mercury called heavy metals
Heavy metals Heavy metals are generally defined as metals with relatively high densities, atomic weights, or atomic numbers. The criteria used, and whether metalloids are included, vary depending on the author and context. In metallurgy, for example, a heavy metal may be defined on the basis of density, whereas in physics the distinguishing criterion might be atomic number, while a chemist would likely be more concerned with chemical behaviour. More specific definitions have been published, but none of these have been widely accepted. The definitions surveyed in this article encompass up to 96 out of the 118 known chemical elements; only mercury, lead and bismuth meet all of them. Despite this lack of agreement, the term (plural or singular) is widely used in science. A density of more than 5Â g/cm3 is sometimes quoted as a commonly used criterion and is used in the body of this article.
Plumbing Present-day water-supply systems use a network of high-pressure pumps, and pipes in buildings are now made of copper,[25] brass, plastic (particularly cross-linked polyethylene called PEX, which is estimated to be used in 60% of single-family homes[26]), or other nontoxic material. Due to its toxicity, most cities moved away from lead water-supply piping by the 1920s in the United States,[27] although lead pipes were approved by national plumbing codes into the 1980s,[28] and lead was used in plumbing solder for drinking water until it was banned in 1986.[27] Drain and vent lines are made of plastic, steel, cast-iron, or lead.[29][30]