(a) Platinum, gold and silver are used to make jewellery because of its bright shiny surface, malleable and ductile nature. These properties are causes of Jewellery being very custrous, drawn into wires and sheets to make jewellery designs more efficient.
(b) Metals like sodium and potassium are stored under oil because they are very reactive in nature, they react with oxygen present in air. Thus to prevent their oxidation they are kept in the oil.
OR
Silver articles become black when kept in open for some time, whereas copper vessel lose their shiny brown surfaces and gain a green coat when kept in open because silver articles reacts with sulphur compounds such as hydrogen suphide present in the air to form silver sulphide (Ag2S) whereas copper reacts slowly with CO2 and water present in the air to form green coating of mixture of copper carbonate and copper hydroxide.