Illustrate how copper metal can give different products on reaction with $HNO_3$.

Concentrated nitric acid is a strong oxidizing agent. It is used for oxidizing most metals. The products of oxidation depend on the concentration of the acid,temperature,and also on the material undergoing oxidation.
With dilute $HNO_3$:
$3Cu + 8HNO_{3(dilute)} \to 3Cu(NO_3)_2 + 2NO + 4H_2O$
With concentrated $HNO_3$:
$Cu + 4HNO_{3(conc)} \to Cu(NO_3)_2 + 2NO_2 + 2H_2O$