The reddish-brown coloured gas formed when nitric oxide is oxidised by air is

Which one of the following is correct with respect to basic character?

During the borax bead test with $CuSO_4$,a blue-green colour of the bead was observed in the oxidising flame due to the formation of:

In the ring test of $NO_{3}^{-}$ ion,$Fe^{2+}$ ion reduces nitrate ion to nitric oxide,which combines with $Fe^{2+}_{(aq)}$ ion to form a brown complex. Write the reactions involved in the formation of the brown ring.

What is produced when $Ag$ is heated with concentrated $HNO_3$?

In a cyclotrimetaphosphoric acid molecule,how many single and double bonds are present?