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

$A$ metal $M$ does not liberate hydrogen from acids but reacts with oxygen to give a black coloured product. Identify $M$ and the black coloured product,and also explain the reaction of $M$ with oxygen.

What is the chemical formula for potassium dichromate?

Write chemical equations for the reactions taking place when:
$(i)$ iron is strongly heated in air.
$(ii)$ lead carbonate is calcined.
$(iii)$ chromium oxide $(Cr_{2}O_{3})$ is heated with aluminium powder.

What are the constituent elements of brass?

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.