$2 \, mL$ 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:

What happens when
$(a)$ $ZnCO_{3}$ is heated in the absence of oxygen?
$(b)$ $A$ mixture of $Cu_{2}O$ and $Cu_{2}S$ is heated?

Which gas is useful for inhibiting bacterial growth?

Which metal exists in a liquid state at room temperature?

Explain the following statements:
$(a)$ Most metal oxides are insoluble in water but some of these dissolve in water. What are these oxides and their solutions in water called?
$(b)$ At ordinary temperature,the surfaces of metals such as magnesium,aluminium,and zinc are covered with a thin layer. What is the composition of this layer? State its importance.
$(c)$ Some alkali metals can be cut with a knife.

In the electrolytic refining of copper metal,what is used as the cathode?