Pure Substances, Mixtures, and the Composition of Air

Pure Substances and Mixtures

  • Scientific Definition of Pure: A substance consisting exclusively of a single chemical substance (e.g., pure water contains only water molecules).

  • Definition and Properties of a Mixture:

    • Formed when two or more distinct particles are physically combined.

    • Can consist of elements, compounds, or both.

    • Exists as solids, liquids, or gases.

Composition of Air

  • Air as a Gaseous Mixture:

    • Nitrogen: 78%78\%

    • Oxygen: 21%21\%

    • Trace Components: 1%1\% (carbon dioxide, argon, water vapour, and other gases)

Variations in Atmospheric Composition

  • Short-Term Variations:

    • Weather: Water vapour fluctuates based on conditions.

    • Biological: Respiration adds CO2CO_2; plant food production removes CO2CO_2.

    • Human Activity: Burning fossil fuels and pollution increase atmospheric CO2CO_2.

  • Long-Term Changes: Atmospheric composition changed over millions of years (e.g., oxygen levels were historically much lower).