Page 50 - Science Class 08
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precipitate is melted in order to remove all its word–karat–means a bean. It was once used to
impuri es. Pure gold is then melted and made weigh precious stones in Arabia.
into gold bars. This gold is 99.99% pure. It regards to the purity of gold, carat means
Proper es : Gold is duc le as well as malleable. 1/24 of the total weight of the gold in the alloy
Being so , it can be shaped into any form. used for ornaments.
v It resists rust and other chemicals as also For example, 22 carat gold means that it has 22
air cannot tarnish it. parts of pure gold and 2 parts of the other
v It has to be combined with some other metal used to alloy the gold. Pure gold is said to
metals before being used to make be 24 carat gold.
ornaments. Pla num
v It melts at 1064.4°C and boils at 2807°C. Pla num is a dense, stable and rare metal that
v Its atomic number is 79 and density is 19. is o en used in jewellery for its a rac ve,
32 grams per cu.cm at 20°C. silver-like appearance, as well as in medical,
electronic and chemical applica ons due to its
v It can dissolve in aqua regia — mixture of
various and unique chemical and physical
hydrochloric acid and nitric acid.
proper es.
Use of Gold
This costly metal was discovered in 1557 by an
Gold is used as under: Italian scien st, though its use began about the
v It is used to make ornaments a er being year 1750 AD. Its ores were discovered in
combined with other metals like copper. America by Spaniards who gave it the name
v It is used as interna onal money for trade plata meaning silver in Spanish.
and exchange. Ores of pla num occur in the beds of gold
White gold is the gold alloyed with pla num or bearing sands. So, miners called it white gold.
silver. These ores contain 55-60% pure pla num.
Some ores contain sulphur and arsenic in them.
v It is used for studding gems, rings and
Pla num is mined in Japan, Russia, Canada,
precious stones.
Australia and South Africa.
v It is used to make gold foils and gold leaves
Proper es
which are used in le ering and for filling
tooth-cavi es. The proper es of pla num are:
v Two other forms of gold, sponge gold and v It is the most valuable metal and its symbol
gold pallets are also used in den stry. is Pt.
v Gold-precipitate is used to make ruby glass. v Its atomic number is 78 and mel ng point
is 1772°C.
v Gold laces are made by twining fine gold
strips, round silk, etc. v It is one of the heaviest substances known
so far. Its weight is 21 mes the weight of
Purity of Gold
the water equal to it.
The term used to express the purity of gold is
v It can be shaped and worked out like silver
carat. This term comes from the Arabic
and gold.
50 Science-8
