question_answer 27)

                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 still used for making cooking utensils. (d) Carbonate and sulphide ores are usually converted into oxides during the process of extraction. 

Answer:

(a) These metals placed at the bottom of the activity series thus are very little reactive in nature. Gold and platinum are even known as noble metals. They are not affected by air, water and even by chemicals. Since they have bright lustre, jewellery can be made from them. (b) All the three metals react with water producing lot of heat. As a result, hydrogen evolved catches fire. They cannot be kept in air also 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  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 un reactive and can be used in 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 the oxide from by processes of calcination and roasting.