Mercury is a silvery liquid; its density is 13.546 at 20°C, its molecular weight is 200.61, its boiling point is 356.9°C, and its freezing point is —38.87°C. It has an appreciable vapor pressure (Othmer and Steinden 1967), and reacts with halogens, sulfur, and oxygen, to give corresponding halides, sulfides, and oxides. It does not react with water, alkali, or weak acids. It is oxidized by concentrated nitric acid, releasing nitric oxide, and by hot concentrated sul-furic acid, releasing sulfur dioxide. It forms alloys called amalgams with metals and with ammonium ions. It solubility in air-free water is 20-30 ppb at 30°C, but this increases in the presence of air, chlorides, and alkali (Linke).

The mercurous ion is the univalent form of mercury and it is diatomic. It disproportionates in the presence of sulfide, hydroxy, and cyanide ions (Linke), according to equation 8.3(23).

Mercury(I) salts have low solubility except for the nitrate, chlorate, and perchlorate. They behave as strong electrolytes. Mercury(I) is the mercurous or univalent form of mercury and shows no tendency to form covalent bonds.

Mercury(II), the bivalent form of mercury, forms predominantly covalent bonds and is readily complexed by inorganic and organic ligands such as HgCl|—, HgSf— and Hg(CN)4—. Mercury(II) forms oxides, sulfides and all the common salts.

Mercury(II) forms organomercury compounds in which there is at least one C---Hg bond (as distinguished from mercury salts of organic acids and organic complexes). The organomercury bond is relatively stable. These compounds are moderately insoluble in water and may be decomposed by hydrolysis or oxidation. Metallic mercury and its inorganic mercury compounds are converted to organomer-cury compounds as well as the reverse, by geochemical and biochemical processes.

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