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Gases in the Atmosphere Overview

May 26, 2025

Lecture Notes: Gases in the Atmosphere

Overview

  • Discussion on the composition of air, gases in the atmosphere, reactions involving these gases, and relative information.
  • Focus on unpolluted and dry air, excluding pollutants and water vapor.

Composition of Air

  • Nitrogen: 78.1%
  • Oxygen: 21.0%
  • Argon: 0.9%
  • Carbon Dioxide (CO2): 0.04%
  • Water Vapor: Ranges from 0-4%, depending on the area.
  • Noble Gases: Helium, Neon, Krypton, Xenon; present in small percentages.

Practical Experiments

Oxygen Content in Air Using Copper

  • Setup: Two gas syringes and a silica tube with copper fillings.
  • Procedure:
    1. Start with 100 cm³ of air.
    2. Heat silica tube and pass air over heated copper.
    3. Measure volume change to determine oxygen content.
  • Outcome: Approximately 1/5 of the air is oxygen (~21% by volume).
  • Safety: Wear eye protection, avoid breaking syringes.

Oxygen Content Using Iron

  • Setup: Conical flask with wet iron fillings connected to a gas syringe.
  • Procedure:
    1. Measure initial air volume.
    2. Leave for a week to allow iron to rust.
    3. Measure change in volume to calculate oxygen content.
  • Outcome: Similar results as copper experiment (~21% oxygen).
  • Notes: Ensure enough iron is present to react with oxygen.

Oxygen Content Using Phosphorus

  • Setup: Bell jar over phosphorus in water.
  • Procedure:
    1. Ignite phosphorus and seal jar.
    2. Measure water level change to determine oxygen content.
  • Outcome: Confirms oxygen content in air (~21%).
  • Precautions: Requires quick sealing post-ignition.

Burning Reactions and Oxides

Combustion of Elements

  • Magnesium: Burns with bright white flame; forms magnesium oxide (white ash).
  • Sulfur: Burns with bright blue flame; forms sulfur dioxide (poisonous gas).
  • Hydrogen: Burns with pale blue flame; forms water. Mixtures explode if not controlled.

Types of Oxides

  • Metal Oxides:
    • Ionic compounds.
    • Basic oxides, react with acids to form salts.
    • Some dissolve in water to form alkaline solutions.
  • Non-Metal Oxides:
    • Covalent compounds.
    • Acidic oxides, react with bases to form salts.
    • Often soluble in water forming acidic solutions (e.g., SO3 to H2SO4).

Carbon Dioxide and Global Warming

  • Produced by burning fossil fuels.
  • Greenhouse Effect:
    • Earth absorbs UV, re-emits as IR radiation.
    • CO2 absorbs IR, heats atmosphere.
  • Impact: Melting polar ice caps, rising sea levels, extreme weather events.

Molecular Structures

  • Water: Polar molecule with slight charges.
  • Carbon Dioxide: Linear; dipoles cancel, making it a greenhouse gas.
  • Oxygen & Nitrogen: Non-polar, not greenhouse gases due to lack of dipoles.

Summary/Conclusion

  • Air contains a mixture of gases with specific volume percentages.
  • Practical experiments demonstrate oxygen content (~21%) in air.
  • Understanding combustion and oxide formation is crucial for various applications.
  • Carbon dioxide plays a significant role in climate change through the greenhouse effect.

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