Its constituents retain their individual properties and variable proportions.
Explanation
In chemistry, matter is classified into elements, compounds, and mixtures. A compound consists of two or more elements chemically bonded in a fixed ratio, resulting in a substance with properties entirely different from its constituents. A mixture consists of substances physically combined where no chemical bonding occurs, and the ratio of components can vary.
Justification for Correct Option:
- Retention of Properties: In atmospheric air, the constituent gases (Nitrogen, Oxygen, Argon, Carbon Dioxide, etc.) do not lose their individual identities. For example, Oxygen in the air supports combustion and respiration, just as pure Oxygen would. This indicates that the components are not chemically combined.
- Variable Proportions: While the proportion of major gases (Nitrogen and Oxygen) is relatively stable, the concentration of other components like water vapor, dust particles, and industrial pollutants varies significantly depending on the location, altitude, and weather conditions. This variability is a characteristic of mixtures, whereas compounds have a fixed chemical formula.
Analysis of Incorrect Options:
- It has a fixed boiling point. is incorrect: Pure compounds have a sharp, fixed boiling point. Air (when liquefied) boils over a range of temperatures because its different components (Nitrogen, Oxygen) have different boiling points. This property allows them to be separated via fractional distillation.
- It is homogeneous in nature throughout the stratosphere. is incorrect: While air is often considered a homogeneous mixture (solution) in the lower atmosphere, homogeneity alone does not define a compound. Furthermore, the composition of the atmosphere changes with altitude (e.g., the ozone layer in the stratosphere), making it heterogeneous on a larger scale.
- It is essential for the survival of biological life. is incorrect: The biological necessity of air is a functional attribute, not a chemical criterion for classification.
Key Takeaway:
Atmospheric air is a mechanical mixture of gases because its constituents are not chemically bonded, they retain their individual physical and chemical properties, and their proportions can vary (especially water vapor and aerosols).