Question

When concentrated sulphuric acid was added to an unknown salt present in a test tube a brown gas (A) was evolved. This gas intensified when copper turnings were added to this test tube. On cooling, the gas (A) changes into a colourless solid (B). Why does gas (A) change to solid on cooling?

Answer 1

Hint: This question is based on the laboratory experiment. First, identify the compounds A and B. After the identification write down the chemical reaction occurred in this experiment. The compound A, and B are related to the nitrogen. So, now we will determine the reason.

Complete step-by-step answer:
Now, we will discuss the experiment step by step.
First, we know there is a reaction of concentrated sulphuric with an unknown salt. The chemical reaction can be written as:

MNO$_3$ + H$_2$SO$_4$ $\rightarrow$ MHSO$_4$ + HNO$_3$

In this reaction, M represents the metal forming an unknown salt. Here, nitric acid formed will further react and the reaction will be:

4HNO$_3$ $\rightarrow$ 4NO$_2$ $\uparrow$ + O$_2$ + 2H$_2$O

Here, nitric oxide formed is the brown gas. We can say that brown fumes of gas are formed.
According to the question, now further brown gas intensifies on the addition of copper turnings, then the reaction will be

3Cu + 8HNO$_3$ $\rightarrow$ 3Cu(NO$_3$)$_2$ + 2NO$\uparrow$ + 4H$_2$O

In this reaction, we can see that there is formation of cupric nitrate, and the nitrogen oxide is evolved.
We can say that the nitrogen oxide evolved will react with the oxygen, so the reaction can be written as:

2NO + O$_2$ $\rightarrow$ NO$_2$

Now, further it is cooled, and it means that the temperature will be low. Thus, the reaction is

2NO$_2$ $\rightleftharpoons$ N$_2$O$_4$

In this reaction, we can see that there is formation of dinitrogen tetroxide; i.e. colourless solid, and it is compound (B).
Here, the compound A changes to solid on cooling; and the reason is that the compound A is an unstable molecule.
Now, for the stabilisation of molecules it changes to the compound B.
In the last we can conclude that the solid formed is due to the stabilization of molecules.

Note: Don’t get confused while writing the chemical reactions. Just move step by step according to the question. The confusion could happen why compound A is unstable. Then the reason for unstable nature is an odd electron molecule, so there is dimerization of compound A.