The Chemistry of Air

Air is a mechanical mixture of gases that surrounds the earth. It forms the atmosphere and is essential for life on earth. Unlike compounds, the components of air are not chemically bonded and can be separated by physical methods.

1. Natural Constituents of Air

The atmosphere is composed of several gases in relatively constant proportions, although some components like water vapour and pollutants vary based on location and environmental conditions.

• Nitrogen (N₂): Approximately 78% by volume. It is chemically inert and acts as a diluent for oxygen, slowing down combustion and respiration.
• Oxygen (O₂): Approximately 21% by volume. It is essential for respiration and combustion.
• Carbon (IV) Oxide (CO₂): Approximately 0.03% by volume. It is essential for photosynthesis in plants.
• Noble Gases (Argon, Neon, etc.): Approximately 1% (mostly Argon). These are chemically unreactive.
• Water Vapour: Variable concentration depending on humidity and temperature.

2. Evidence that Air is a Mixture

Air is classified as a mixture rather than a compound for the following reasons:

• Physical Separation: The constituents can be separated by physical means such as fractional distillation of liquid air.
• Retained Properties: Each gas in air retains its individual chemical properties (e.g., oxygen still supports combustion).
• Variable Composition: The proportion of gases in air is not fixed; it varies slightly from place to place (e.g., more CO₂ in industrial areas, more water vapour in coastal regions).
• No Energy Change: There is no heat or light evolved or absorbed when the constituents of air are mixed together.
• No Chemical Formula: Air cannot be represented by a single chemical formula.

3. Separation of Air Components

The industrial separation of air is achieved through fractional distillation of liquid air. The process involves several steps:

• Purification: Air is filtered to remove dust. It is then passed through concentrated sodium hydroxide (NaOH) to remove CO₂ and through silica gel or concentrated H₂SO₄ to remove water vapour. This prevents these substances from freezing and clogging the machinery later.
• Liquefaction: The remaining air (mostly N₂ and O₂) is cooled and compressed until it liquefies at -200°C.
• Fractional Distillation: The liquid air is allowed to warm up gradually in a fractionating column. Nitrogen boils off first (B.P. -196°C), followed by Argon (B.P. -186°C), and finally Oxygen (B.P. -183°C).

4. Variation in Air Composition

While the major gases remain fairly constant, variations occur due to:

• Altitude: Air becomes thinner (less dense) at higher altitudes.
• Industrial Activity: Higher concentrations of CO₂, SO₂, and Nitrogen oxides are found in urban areas.
• Photosynthesis and Respiration: In areas with dense vegetation, O₂ levels may be slightly higher during the day.
• Humidity: Coastal areas have higher water vapour content than deserts.

5. Uses of Atmospheric Gases

• Oxygen: Used in hospitals for artificial respiration, in welding (oxy-acetylene flame), and in the manufacture of steel.
• Nitrogen: Used in the manufacture of ammonia (Haber process), as a refrigerant (liquid nitrogen), and for food preservation to prevent oxidation.
• Argon: Used to fill electric light bulbs to prevent the tungsten filament from oxidizing.
• Neon: Used in advertising glow signs (neon lights).
• Helium: Used in filling weather balloons because it is light and non-flammable.
• Carbon (IV) Oxide: Used in fire extinguishers and in the manufacture of aerated (fizzy) drinks.