CHAPTER 3
Metals and Non-metals
You should know some facts about Metals and Non-metals
Anodising is a process of forming a thick oxide layer of
aluminium. Aluminium develops a thin oxide layer
when exposed to air. This aluminium oxide coat makes
it resistant to further
corrosion. The resistance can be improved further by making
the oxide layer
thicker. During
anodising, a clean
aluminium article is made the anode
and is electrolysed with dilute sulphuric acid. The oxygen gas evolved
at the anode reacts with aluminium to make a thicker protective oxide layer. This oxide
layer can be dyed easily
to give aluminium articles an attractive finish.
Aqua regia, (Latin
for ‘royal water’) is a freshly prepared mixture of concentrated hydrochloric
acid and concentrated nitric acid in the ratio of 3:1. It can dissolve gold,
even though neither of these acids can do so alone. Aqua regia is a highly corrosive,
fuming liquid. It is one of the few reagents
that are able
to dissolve gold
and platinum.
Pure
gold, known as 24-carat gold, is very soft. It is, therefore, not suitable for making
jewellery. It is alloyed with either silver or copper
to make it hard. Generally, in India, 22
carat gold is used for making ornaments. It means that 22 parts
of pure gold are
alloyed with 2 parts of either copper or silver.
The wonder of ancient Indian metallurgy
The iron pillar near the Qutub Minar in Delhi was made around 400 BC by the
ironworkers of India. They had developed a process which prevented wrought
iron from rusting. This is likely because of the formation of a thin film of
magnetic oxide (Fe3O4)
on the surface, as a result
of finishing treatment given to the pillar, painting
it with a mixture of different salts,
then heating and quenching. The iron pillar
is 8 m high and weighs 6 tonnes (6000 kg).
