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Understanding Pure Substances and Mixtures

Apr 29, 2025

Pure Substances and Mixtures

Learning Objectives

  • Distinguish between pure substances and mixtures.
  • Identify mixtures as homogeneous or heterogeneous.

Pure Substances

  • Definition: Contains only one kind of matter.
  • Types:
    • Element: Single type of atom.
    • Compound: Made of molecules that are combinations of different atoms.
  • Characteristics:
    • Fixed, definite set of properties.
    • Examples:
      • Lead (Pb) - Element, consists of lead atoms.
      • Oxygen gas (O2) - Element, consists of oxygen molecules.
      • Water (H2O) - Compound, consists of water molecules.
      • Ammonia (NH3) - Compound, consists of ammonia molecules.

Mixtures

  • Definition: Combination of two or more pure substances.
  • Characteristics:
    • Can be separated into component pure substances.
    • Substances retain individual properties.
    • Example: Mixing sodium and chlorine forms sodium chloride (table salt), different from each element's properties.

Types of Mixtures

Heterogeneous Mixture

  • Composition is not uniform.
  • Example: Vegetable soup - varying amounts of components in each spoonful.

Homogeneous Mixture (Solution)

  • Composition is uniform.
  • Behaves as a single substance.
  • Example: Salt dissolved in water - appears as a single substance.

Examples and Exercises

  • Example 3.5.3:

    • Soda water - Homogeneous mixture.
    • Iron filings and sulfur - Heterogeneous mixture.
  • Exercise 3.5.3: Classify mixtures

    • Human body - Heterogeneous mixture.
    • An amalgam - Homogeneous mixture.

Categorizing Matter

  • Figure 3.5.1: Relationships between types of matter and separation methods.
  • Phase Definition: Part of a sample with uniform composition and properties.
  • Pure substance or homogeneous mixture = single phase.
  • Heterogeneous mixture = multiple phases (e.g., oil and water layers).

Examples

  • Example 3.5.1: Classify substances

    • Filtered tea - Homogeneous mixture.
    • Orange juice - Heterogeneous mixture.
    • Compact disc - Heterogeneous mixture.
    • Aluminum oxide - Pure compound.
    • Selenium - Element.
  • Exercise 3.5.1: Classify substances

    • White wine - Homogeneous mixture.
    • Mercury - Element.
    • Ranch-style salad dressing - Heterogeneous mixture.
    • Table sugar - Compound.

Additional Examples

  • Example 3.5.2: Categorize matter

    • Saltwater - Homogeneous mixture.
    • Soil - Heterogeneous mixture.
    • Water - Compound.
    • Oxygen - Element.
  • Exercise 3.5.2: Categorize matter

    • Coffee - Homogeneous mixture.
    • Hydrogen - Element.
    • An egg - Heterogeneous mixture.

Summary

  • Matter Categories:
    • Pure substances: Constant composition and properties.
    • Mixtures: Physical combinations of elements/compounds.
  • Mixtures Types:
    • Homogeneous: Not visually distinguishable.
    • Heterogeneous: Visually distinguishable.

Key Takeaways

  • Pure substances: Single element or compound.
  • Mixtures: Combinations of different substances.
  • Homogeneous mixtures: Uniform, not visually distinct.
  • Heterogeneous mixtures: Non-uniform, visually distinct.

Contributors

  • Stephen Lower, Marisa Alviar-Agnew, Henry Agnew.