Which of the following gases is insoluble in water?

$2 \, mL$ each of concentrated $HCl$,$HNO_3$,and a mixture of concentrated $HCl$ and concentrated $HNO_3$ in the ratio of $3:1$ were taken in test tubes labeled as $A$,$B$,and $C$. $A$ small piece of metal was put in each test tube. No change occurred in test tubes $A$ and $B$,but the metal dissolved in test tube $C$. The metal could be:

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

Identify the elements with the following properties and arrange them in increasing order of their reactivity:
$(a)$ An element which is a soft and reactive metal.
$(b)$ The metal which is an important constituent of limestone.
$(c)$ The metal which exists in a liquid state at room temperature.

Which of the following statements is incorrect?

Which one of the following figures correctly describes the process of electrolytic refining?