Correct Option
The correct option is (a) 1 only.
Explanation
Air is a mixture of various gases, water vapour, and particulate matter that forms the Earth's atmosphere. While the major gaseous components remain relatively stable in the lower atmosphere, other constituents like water vapour and dust particles exhibit significant spatial and temporal variations.
Statement-wise Analysis
- Statement 1 is Correct: Air is composed of a mixture of gases, primarily Nitrogen (approximately 78%) and Oxygen (approximately 21%). It also contains small amounts of Carbon dioxide, Argon, Helium, Ozone, and other gases. Additionally, air contains varying amounts of water vapour and solid particles such as dust and smoke.
- Statement 2 is Incorrect: The concentration of dust particles in the air is not constant. It varies significantly from place to place and time to time. For instance, the amount of dust is typically higher in industrial areas, urban centers, or during dry seasons compared to rural areas or after rainfall.
- Statement 3 is Incorrect: Water vapour is a fundamental component of the hydrological cycle (water cycle). Water evaporates from oceans and land surfaces to form water vapour, which rises, cools, and condenses to form clouds. This process eventually leads to precipitation, returning water to the surface. Without water vapour, the water cycle cannot function.
Key Takeaway: Air is a dynamic mixture containing gases, water vapour, and particulate matter. While the ratio of major gases is fairly constant, components like water vapour and pollutants (dust, smoke) vary based on geographical location and environmental conditions.