When a vapour, at atmosphere pressure was gradually heated from `25^(circ)C` its colour wasa found to deepen at first and then to fade as the temperature was raised above `160^(circ)C`. At `600^(circ)C`, the vapour was almost colourless, but its colour deepended when the pressure was raised at this temperature. The vapour was:

Options

A. the bromine.
B. a mixture of nitrogen dioxide and dinitrogen tetraoxide.
C. pure nitrogen dioxide .
D. pure dinitrogen tetraoxide.

Explanation

To solve the question regarding the vapor that deepens in color and then fades as the temperature increases, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding the Initial Conditions**:
- The vapor is at atmospheric pressure and is heated from 25°C.
- Initially, the color deepens, indicating a chemical change or reaction is occurring.

**Hint**: Consider what types of substances might change color with temperature.

2. **Identifying the Temperature Threshold**:
- The color deepens until the temperature reaches 160°C, at which point it starts to fade.
- This suggests that there is a specific reaction or phase change occurring at this temperature.

**Hint**: Look for substances that undergo significant changes at around 160°C.

3. **Considering Possible Substances**:
- The question suggests that the vapor could be nitrogen dioxide (NO2) or dinitrogen tetroxide (N2O4).
- Dinitrogen tetroxide (N2O4) is known to dissociate into nitrogen dioxide (NO2) at elevated temperatures.

**Hint**: Recall the equilibrium between N2O4 and NO2 and how temperature affects this equilibrium.

4. **Analyzing the Color Change**:
- At lower temperatures, N2O4 is present and is colorless; as the temperature increases, it dissociates into NO2, which is brown, leading to a deepening of color.
- Above 160°C, the reaction shifts towards NO2, and the color fades as the temperature increases further.

**Hint**: Think about how the concentration of each species affects the observed color.

5. **Observing the Behavior at 600°C**:
- At 600°C, the vapor is almost colorless, indicating that the concentration of NO2 is low.
- When the pressure is increased at this temperature, the color deepens again, suggesting a shift back to N2O4.

**Hint**: Apply Le Chatelier's principle to understand how pressure changes affect the equilibrium.

6. **Applying Le Chatelier's Principle**:
- According to Le Chatelier's principle, increasing the pressure favors the side with fewer moles of gas.
- In the reaction: 2NO + O2 ⇌ 2N2O4, increasing pressure will shift the equilibrium to the left, favoring the formation of N2O4, which is colorless.

**Hint**: Consider how the number of moles of gas on each side of the reaction influences the equilibrium shift.

7. **Conclusion**:
- The vapor that fits all these observations is dinitrogen tetroxide (N2O4), as it explains the color changes with temperature and pressure.

**Final Answer**: The vapor was dinitrogen tetroxide (N2O4).