The brown gas prepared by the action of concentrated nitric acid on copper is an equilibrium mixture of dinitrogen tetraoxide and nitrogen dioxide : `N_(2)O_(4(g))overset(Endothermic)(hArr)2NO_(2(g))`
Which one of the following changes would result in a darkening of the colour?

Options

A. Increase in temperature
B. Increase in pressure
C. Addition of a catalyst
D. Removal of dinitrogen tetra oxide by liquefaction

Answer

To solve the question regarding the equilibrium mixture of dinitrogen tetraoxide (N₂O₄) and nitrogen dioxide (NO₂), we need to analyze the given reaction and the effects of various changes on the equilibrium position.

### Step-by-Step Solution:

1. **Identify the Reaction**: The equilibrium reaction is given as:
\[ N_2O_4(g) \overset{\text{Endothermic}}{\rightleftharpoons} 2NO_2(g) \]
Here, N₂O₄ is a colorless gas, while NO₂ is a brown gas. The presence of NO₂ gives the mixture its brown color.

2. **Understand the Color Change**: The question asks for a change that would result in a darkening of the color. Darkening of the color indicates an increase in the concentration of NO₂, as it is responsible for the brown color.

3. **Analyze Temperature Changes**: Since the reaction is endothermic (ΔH is positive), increasing the temperature will favor the formation of products (NO₂). According to Le Chatelier's principle, if we increase the temperature of an endothermic reaction, the equilibrium shifts to the right to absorb the added heat, thus producing more NO₂.

4. **Analyze Pressure Changes**: The reaction involves 1 mole of N₂O₄ producing 2 moles of NO₂. Increasing the pressure would favor the side with fewer moles of gas, which is the reactant side (N₂O₄). Therefore, increasing pressure would shift the equilibrium to the left, resulting in less NO₂ and a lighter color.

5. **Conclusion**: The change that would result in a darkening of the color (increasing the concentration of NO₂) is increasing the temperature.

### Final Answer:
**Increasing the temperature will result in a darkening of the color.**

---