On heating blue coloured powder of copper nitrate in a boiling tube, copper oxide (black), oxygen gas and a brown gas 'X ' is formed. What is (X)?
A. $NO$
B. ${{N}_{2}}{{O}_{3}}$
C. ${{N}_{2}}O$
D. $N{{O}_{2}}$

Hint: We need to write down the balanced equation for the reaction mentioned in the question. Once we are done, we can find the answer as to which is the basin gas that is formed.

Complete answer:
On heating blue coloured power of copper nitrate in a boiling tube, copper oxide, which is black in colour, oxygen gas, which is a colourless gas and another brown gas is produced.
The chemical reaction involved and the chemical equation is:
\[2Cu{{(N{{O}_{3}})}_{2}}_{(s)}\xrightarrow{heat}2Cu{{O}_{(s)}}+4N{{O}_{2}}_{(g)}+{{O}_{2}}_{(g)}\]
Here, 2 moles of blue copper nitrate when heated, forms 2 moles of black copper oxide, 1 mole of clear oxygen gas, and 4 moles of brown nitrogen dioxide gas.
From the reaction we can see that the products that are formed are copper oxide (black), oxygen (colourless) and the brown colour gas that is produced is nitrogen dioxide.
So we can say that the name of X is nitrogen dioxide. Therefore, the correct answer is Option D.
The colour of nitrogen dioxide is reddish brown which decomposes in water to form nitric acid and nitric oxide. Here is the equation:
\[HN{{O}_{2(aq)}}+{{H}_{2}}{{O}_{(l)}}\xrightarrow{{}}N{{O}_{2}}{{^{-}}_{(aq)}}+{{H}_{3}}{{O}^{+}}_{(aq)}\]

Note:
Please balance the reaction properly and take into consideration the colour of the gas. If the reaction is not balanced properly, you may get your answer as ${{N}_{2}}O$ which is a colourless gas and is thus not the right answer.