These metals placed at the bottom of the activity series are very little reactive in nature. Gold and platinum are known as noble metals. They are not affected by air, water and by chemicals. Since they have bright lustre, jewellery can be made from these metals.
(b) There are reactive metals placed high in the activity series. In air, their surface gets tarnished due to presence oxygen, water vapours and carbon dioxide in air. With water, these react violently to evolve so much heat that is not possible to handle them. These metals are generally kept under kerosene which does not contain air and water.
(c ) When exposed to air, the metal changes its oxide called aluminium oxide `(Al_(2)O_(3))`. It gets deposited over the surface of the metal and forms a protective coating on the surface. Due to the presence of this layer, the metal becomes unreactive and can be used in making cooking utensils.
(d) Both carbonate and sulphide ores of metals cannot be directly reduced to metallic state. However, metal oxides can be easily reduced with coke or other reducing agents. Both are therefore, converted into their respective oxides by calcination process (for carbonate ores) and by roasting process (for sulphide ores). For chemical equations, consult section 3.11. Metal oxides can be easily reduced to metallic form with coke (C ) or any other suitable reducing agent. Therefore, carbonates and sulphides are converted to the oxide form by process of calcination and roasting and are not directly reduced.
