(a) Platinum, gold and
silver are used to make jewellery because of their bright shiny surface and
high resistance to corrosion. Also they have high malleability and ductility.
potassium and lithium are stored under oil to prevent their reaction with
oxygen, moisture and carbon dioxide of air so as to protect them.
Aluminium metal forms a thin layer of aluminium oxide all over its surface under
the action of moist air. This layer prevents the metal underneath from further
corrosion. It is cheap, easily available, malleable and ductile. Therefore, it
is used to make utensils for cooking.
It is easier to obtain a metal from its oxides as compared to its sulphides and
carbonates. So, prior to reduction, metal carbonate and sulphides must be converted
into metal oxides. A carbonate ore is converted into oxide by calcination
whereas a sulphide ore is converted into oxide by roasting.
When copper vessels are exposed to moist air, they form a green coating of
basic copper carbonate [CuCO3. Cu(OH)2].
The sour substances such as lemon or tamarind juice contain
acids. Lemon juice contains citric acid and tamarind contains tartaric acid.
These acids dissolve the coating of copper oxide or basic copper carbonate
present on the surface of tarnished copper vessels and make them shining
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