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.
|
