The articles made of silver metal become dark on prolonged exposure to air. This is due to the formation of a layer of its :

85 Which of the following metals are extracted by the electrolysis of their molten chlorides? 86 Rock salt is an ore of one of the following metals. This metal is : 88 A sulphide ore is converted into metal oxide by the process of : 89 The metal which can be extracted from pyrolusite ore is :