Question

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

Answer 1

The given question is an example of thermal decomposition reaction. When copper nitrate is heated in a test tube, initially the water molecules are evaporated. Further heating, the copper nitrate decomposes to form a reddish brown gas nitrogen dioxide and oxygen. We will elaborate this with chemical reactions and try to understand the process.

Complete answer:

The thermal decomposition of copper II is an experiment performed in a lab.
We take a test tube and we add copper crystals to it.
Then we heat the test tube containing the compound on flame.
The balanced chemical reaction taking place can be written using the chemical formula as:
$2Cu{(N{O_3})_2}\xrightarrow{{heat}}2CuO + {O_2} + 4N{O_2}$
The copper (II) nitrate decomposes on heating to form copper oxide, which is black in color.
Two gases are liberated. One is a colorless gas – oxygen. The other one is a reddish brown colored gas – nitrogen dioxide.
Hence, the term X asked in the question is nitrogen dioxide gas.
The brown gas formed dissolves in water to form an acidic solution. The balanced chemical reaction can be written as –
$2N{O_2} + {H_2}O \to HN{O_3} + HN{O_2}$
We have to remember that the aqueous solution of nitrogen dioxide gas should be less than the pH 7 because the non-metal oxides show acidic property.

Note: We have to remember that the thermal decomposition reaction involves breaking of a certain compound in two or more compounds on heating. This particular thermal decomposition reaction on heating activates at temperature above $170^\circ C$. The brown gas formed, $N{O_2}$ dissolves in water to form an acidic solution. The pH of this solution is below 7.