Gold
is a metallic element with a characteristic yellow color,
but can also be black or ruby when finely divided, while
colloidal solutions are intensely colored and often
purple. These colors are the result of gold's plasmon
frequency lying in the visible range, which causes red
and yellow light to be reflected, and blue light to
be absorbed. Only silver colloids exhibit the same interactions
with light, albeit at a shorter frequency, making silver
colloids yellow in color.
It is the most malleable and ductile metal known; a
single gram can be beaten into a sheet of one square
meter, or an ounce into 300 square feet. Gold readily
forms alloys with many other metals. These alloys can
be produced to increase the hardness or to create exotic
colors. Adding copper yields a redder metal, iron blue,
aluminium purple, platinum metals white, and natural
bismuth together with silver alloys produce black. Native
gold contains usually eight to ten percent silver, but
often much more — alloys with a silver content
over 20% are called electrum. As the amount of silver
increases, the color becomes whiter and the specific
gravity becomes lower.
Gold is a good conductor of heat and electricity,
and is not affected by air and most reagents. Heat,
moisture, oxygen, and most corrosive agents have very
little chemical effect on gold, making it well-suited
for use in coins and jewelry; conversely, halogens
will chemically alter gold, and aqua regia dissolves
it.
Common oxidation states of gold include +1 and +3
(gold(III) or auric compounds). Gold ions in solution
are readily reduced and precipitated out as gold metal
by adding any other metal as the reducing agent. The
added metal is oxidized and dissolves allowing the
gold to be displaced from solution and be recovered
as a solid precipitate.
Recent research undertaken by Sir Frank Reith of
the Australian National University shows that microbes
play an important role in forming gold deposits, transporting
and precipitating gold to form grains and nuggets
that collect in alluvial deposits.
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