4.0 AIR AND COMBUSTION

What is air really made of? Why do some metals rust and others don't? How do we obtain pure oxygen and nitrogen for industrial use? What happens when substances burn, and why is air pollution a global crisis? This chapter explores the composition of the atmosphere, the chemistry of combustion and rusting, and the impact of human activity on the air we breathe. These concepts link fundamental chemistry to real-world environmental issues.

4.1 COMPOSITION OF AIR

Air is not a single substance but a mixture of gases. Its composition is relatively constant near the Earth's surface (troposphere). The main components are:

• Nitrogen (N₂): Approximately 78% by volume.
• Oxygen (O₂): Approximately 21% by volume.
• Argon (Ar): Approximately 0.9% by volume (a noble gas).
• Carbon dioxide (CO₂): Approximately 0.04% (about 400 ppm) – variable due to human activity.
• Other gases: Neon, helium, methane, krypton, hydrogen, and water vapour (variable, up to about 1%).

4.1.1 Determining the Percentage of Oxygen in Air

Oxygen is the reactive component of air that supports combustion. Several experiments can determine its approximate percentage by volume. They all rely on reacting oxygen with another substance (like a metal) and measuring the volume decrease.

Method 1: Using Copper (Heated)

1. Air is passed back and forth over heated copper turnings in a hard glass tube.
2. Copper react