Give reasons for the following:
(a) Platinum, gold and silver are used to make jewellery.
(b) Sodium, potassium and lithium are stored under oil.
(c) Aluminium is a highly reactive metal but is still used for making cooking utensils.
(d) Carbonate and sulphide ores are usually converted into oxides during the process of extraction.
(a) These metals are placed at the bottom of the activity series and are least reactive in nature. Gold and platinum are known as noble metals. They are not affected by air, water or even by chemicals. Since they have lustre, jewellery can be made from them.
(b) All three metals react with water producing lots of heat. As a result, the hydrogen evolved catches fire. They cannot be kept in air because air contains moisture or water vapours. These are kept under kerosene to avoid contact with both air and water.
(c) When exposed to air, the metal changes into its oxide called aluminium oxide (Al2O3). It gets deposited over the surface of the metal and forms a protective coating on the surface. Due to the presence of this layer, aluminium becomes unreactive and can be used for making cooking utensils.
(d) 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 their oxide form by processes of calcination and roasting.