Silver articles become black on prolonged exposure to air. This is due to the formation of

$(a)$ $A$ substance $X$,an oxide of a metal,is used extensively in the cement industry. This element is found in our bones also. On treatment with water,it forms a solution which turns red litmus blue. Identify $X$ and also write the chemical reaction involved.
$(b)$ Choose a metal from the following metals which reacts only with hot water: Sodium,magnesium,iron. Mention the products formed during the reaction.

What is the chemical formula of rust?

Which method is used for the refining of metals having low melting points?

State whether the following statement is true or false:
Those minerals which contain a very high percentage of a particular metal and from which the metal can be profitably extracted are called ores.

Which of the following gases is of brown colour?