Which of the following acids is called 'the king of chemicals'?

Give reasons for the following:
$(a)$ Generally,metals are not found in their free state.
$(b)$ Metals are good conductors of heat.

Give reasons for the following:
$(i)$ Ionic compounds have high melting and boiling points.
$(ii)$ Ionic compounds are soluble in water.
$(iii)$ Ionic compounds are solid and somewhat hard.
$(iv)$ Ionic compounds conduct electricity in the molten state.
$(v)$ Metals have a tendency to lose electrons.

Write chemical equations for the reactions taking place when:
$(i)$ Iron reacts with steam.
$(ii)$ Magnesium $(Mg)$ reacts with dilute $HCl$.
$(iii)$ Copper is heated in air.

$A$ 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.

Metals are refined by using different methods. Which of the following metals are refined by electrolytic refining?
$(i)$ $Au$
$(ii)$ $Cu$
$(iii)$ $Na$
$(iv)$ $K$