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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.
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 heavy, silver-white metal that is in its liquid state at room
temperature. Two other metals have rather low melting points are cesium,
28.5oC, and gallium, 28 oC. 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 -39oC to 357oC. 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.
Mercury is a good solvent for metals and many amalgams
exist. These include:
Mercury (I) chloride, is used as a fungicide, insecticide, and in the manufacture
of a type of electrical reference electrode, called a calomel
electrode.
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.
