On heating graphite with conc. `HNO_(3)` repetedly, a yellow mass is obtained which is called

P_2O_5 heated with conc. HNO_3 gives

On heating with conc. HNO_3 , proteins give yellow colour. This test is called

Proteins when heated with conc. HNO_3 give a yellow colour. This is

On heating copper turnings with conc. HNO_(3) , a brown coloured gas is evolved which upon cooling dimerises. Identify the gas.