|Atomic mass||200.59 amu|
Mercury has been known since ancient times. The chemical symbol, Hg, is taken from the Latin, hydrargyrus, meaning "liquid silver". Evidence shows that the Chinese were using mercury before 2000 B.C. Ancient Egyptian tombs contain vials of mercury. One of the world's best known mercury mines (the Almaden mine in Spain) has been in continuous operation since 400 B.C.
Mercury is a heavy, silver-white metal that is in its liquid state at room temperature. Two other metals have rather low melting points are cesium, 28.5°C, and gallium, 28°C. Both are liquid in hot weather. When mercury is cooled to its freezing point, it looks and feels like lead. Many uses of mercury arise from this liquid range of -39°C to 357°C. This property combined with the fact that mercury expands and contracts evenly with changes in temperature, it is an ideal material for liquid thermometers and barometers. Mercury's fluidity and conductivity make it useful for electrical switches in thermostats. At high pressures, mercury vapour can be excited electrically to emit the bright white light seen in sports stadium and highway lights.
Virtually all mercury is derived from cinnabar, or mercury sulfide (HgS). Red cinnabar is so rich in mercury content that droplets of elemental mercury can be found in samples of the ore. The ore is heated with a reducing agent (such as oxygen, iron, and quicklime, CaO) and the mercury vapour is led away into vertical columns of water where the mercury liquefies. Since mercury is quite dense (13.5 g/ml), mercury collects at the bottom while most impurities float on the surface where they can be scraped away.
Mercury is a good solvent for metals and many amalgams exist. These include:
- amalgam electrodes used in the Daniell cell.
- sodium/mercury amalgam in the Chloralkali Process for the production of Cl2 from sea water.
In view of mercury's toxicity, past contamination of waste water from the Chloralkali Process and the disposal of old mercury batteries are serious environmental concerns.
Mercury (I) chloride, is used as a fungicide, insecticide, and in the manufacture of a type of electrical reference electrode, called a calomel electrode.