Pure and Impure Substances - Keystage Three Chemistry

Pure Substances

• Definition: Made of one substance only, which can be an element or a compound.
• Examples:
  • Pure Element:
    • Example: Oxygen (O2)
    • Contains only oxygen atoms, which exist in pairs (O2).
  • Pure Compound:
    • Example: Water (H2O)
    • Contains only water molecules if pure.
• Characteristics:
  • Have specific melting and boiling points.
  • Melting and boiling point graphs show flat lines at specific temperatures.

Impure Substances (Mixtures)

• Definition: Made of two or more substances, which can be elements or compounds.
• Examples:
  • Air: Mixture of various gases (oxygen, water vapor, carbon dioxide, nitrogen).
  • Milk: Mixture of proteins, sugars, lipids, etc.
  • Sea Water: Mixture of water and various salts.
  • Fruit Juice: A mixture of various ingredients.
  • Paint: Various ingredients mixed together.
• Characteristics:
  • Melt and boil over a range of temperatures (not a flat line in graphs).

Key Concepts

• Melting and Boiling Points of Pure Substances:
  • Water: Melts at 0°C and boils at 100°C
  • Graphs show a flat line at these temperatures during phase changes.
• Melting and Boiling Points of Impure Substances:
  • Occur over a range of temperatures.
  • Example: Impure water (e.g., with salt) does not have a specific melting/boiling point.

Summary

• Pure substances contain only one type of element or compound and have specific melting and boiling points.
• Impure substances are mixtures and melt/boil over a range of temperatures.
• Using graphs, we can identify if a substance is pure or impure based on its melting and boiling behavior.