On heating blue coloured powder of copper $(II)$ nitrate in a boiling tube,copper oxide (black),oxygen gas and a brown gas $X$ is formed.
$(a)$ Identify the type of reaction.
$(b)$ What could be the $pH$ range of an aqueous solution of the gas $X$?

(N/A) This is a thermal decomposition reaction.
$(b)$ The brown gas $X$ is nitrogen dioxide $(NO_2)$. When nitrogen dioxide dissolves in water,it forms nitric acid $(HNO_3)$ and nitrous acid $(HNO_2)$,resulting in an acidic solution. Since the solution is acidic,the $pH$ range of the aqueous solution of gas $X$ is less than $7$.