Problem 65 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following oxides of nitrogen is a coloured gas? (a) \(\mathrm{N}_{2} \mathrm{O}\) (b) NO (c) \(\mathrm{N}_{2} \mathrm{O}_{4}\) (d) \(\mathrm{NO}_{2}\)

Step-by-Step Solution

Answer

The colored gas is \(\mathrm{NO}_{2}\).

Step 1: Understanding the Task

The exercise asks us to identify which of the given nitrogen oxides is a colored gas. We have four options to examine: \(\mathrm{N}_{2} \mathrm{O}\), NO, \(\mathrm{N}_{2} \mathrm{O}_{4}\), and \(\mathrm{NO}_{2}\).

Step 2: Analyzing \(\mathrm{N}_{2} \mathrm{O}\) (Nitrous Oxide)

The first option, \(\mathrm{N}_{2} \mathrm{O}\), also known as nitrous oxide or laughing gas, is a colorless gas. Since we are looking for a colored gas, this option is not correct.

Step 3: Analyzing NO (Nitric Oxide)

The second option, NO, is called nitric oxide. It is a colorless gas as well, so this does not satisfy the requirement of being colored.

Step 4: Analyzing \(\mathrm{N}_{2} \mathrm{O}_{4}\) (Dinitrogen Tetroxide)

\(\mathrm{N}_{2} \mathrm{O}_{4}\) is generally a colorless or pale yellow gas. This oxide can exist in equilibrium with \(\mathrm{NO}_{2}\) and is not vividly colored on its own, thus it does not meet the criteria of being a colored gas.

Step 5: Analyzing \(\mathrm{NO}_{2}\) (Nitrogen Dioxide)

\(\mathrm{NO}_{2}\) is a reddish-brown gas. It is known for its distinctive color and is often observed as a polluting brown haze over cities. This matches the criteria for being a colored gas.

Key Concepts

Nitrous Oxide (N2O)

Nitrous Oxide, commonly known as laughing gas, is a compound with the chemical formula \(\mathrm{N}_2\mathrm{O}\). It is a colorless, non-flammable gas at room temperature with a slightly sweet odor and taste. Medical and dental professionals widely use it as an anesthetic and analgesic due to its sedative properties.

• Properties: Despite its use in anesthesia, nitrous oxide is not reactive and is considered relatively safe in controlled doses.
• Uses: Apart from medical applications, it is also used in the food industry as a propellant, particularly in whipped cream dispensers, and in automotive racing to increase engine power.

Though nitrous oxide has many applications, it does not have a color, making it irrelevant to tasks focused on identifying colored gases.

Nitric Oxide (NO)

Nitric Oxide, represented by the formula \(\mathrm{NO}\), is another oxide of nitrogen that is colorless when in its gaseous state. It is a significant intermediary in various chemical reactions, particularly those related to combustion and atmospheric processes.

• Formation: Nitric oxide is naturally formed in our bodies and plays a crucial role in cell signaling. Additionally, it forms during the burning of fossil fuels and is emitted as a pollutant.
• Reactivity: Although colorless, it is quite reactive, especially with oxygen, where it further oxidizes to form nitrogen dioxide \(\mathrm{NO}_2\).

Due to its lack of color, nitric oxide does not fit the criteria for colored gas identification, but its transformation into other oxides makes it environmentally significant.

Nitrogen Dioxide (NO2)

Nitrogen Dioxide, known chemically as \(\mathrm{NO}_2\), stands out due to its distinctive reddish-brown color. It is a highly visible pollutant and a significant component of the brown atmospheric haze often found over urban areas.

• Characteristics: \(\mathrm{NO}_2\) is known for its acrid smell and irritating effects on the respiratory tract, making it a concern in air quality management.
• Environmental Impact: The presence of \(\mathrm{NO}_2\) in the atmosphere contributes to the formation of tropospheric ozone and can lead to acid rain, due to its ability to form nitric acid \(\mathrm{HNO}_3\) when dissolved in water.

Understanding \(\mathrm{NO}_2\) is crucial for environmental and public health, marking it as the colored gas among the oxides analyzed in this context.

Colored Gas Identification

Colored gases are unique due to their distinct appearances and can serve as important markers in chemical analysis and environmental monitoring. Identifying a gas by color is useful when addressing air quality concerns, pollution sources, or conducting chemical reactions in a lab setting.

• Example: As noted with \(\mathrm{NO}_2\), its reddish-brown color provides a visual cue that is useful in identifying and measuring pollution levels in urban settings.
• Role in Analysis: The color of a gas can indicate specific reactions or processes taking place, especially when changes in concentration or environmental conditions occur.

Identifying gases by color simplifies some complex environmental science challenges, providing clear, observable characteristics that can aid in quick decision-making and policy development.