If copper is kept open in air,it slowly loses its shining brown surface and gains a green coating. It is due to the formation of

$2 \, mL$ 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 are the constituent elements of brass?

What is thermit reaction? How is it used to join the railway tracks or cracked machine parts?

State whether the following statement is true or false:
Iron does not burn on heating,but iron filings burn vigorously when sprinkled in the flame of a burner.

Explain the following:
$(a)$ $NaCl$ is not a conductor of electricity in solid state,whereas it does conduct electricity in aqueous solution as well as in molten state.
$(b)$ Iron articles are galvanised.
$(c)$ Metals like $Na, K, Ca$ and $Mg$ are never found in their free state in nature.