• Pure Substances: Matter with distinct properties and a composition that does not vary from sample to sample.
  • Elements: Substances that cannot be decomposed into simpler substances (e.g., Fe, O$_2$).
  • Compounds: Substances composed of two or more elements acting as a single unit (e.g., H$_2$O, NaCl).
• Mixtures: Combinations of two or more substances where each retains its chemical identity.
  • Homogeneous Mixtures (Solutions): Uniform throughout (e.g., Air, Saltwater, Steel).
  • Heterogeneous Mixtures: Not uniform; components are visible (e.g., Concrete, Soil, Oil and Water).

• Physical Properties: Can be observed without changing the identity and composition of the substance.
  • Examples: Color, odor, density, melting point, boiling point, hardness.
• Chemical Properties: Describe the way a substance may change, or react, to form other substances.
  • Examples: Flammability, toxicity, acidity, reactivity with water.

• Intensive Properties: Independent of the amount of substance present. Used to identify substances.
  • Examples: Density, temperature, boiling point.
• Extensive Properties: Depend on the quantity of the sample.
  • Examples: Mass, volume, energy content.

"Matter is neither created nor destroyed during a chemical reaction."

• The total mass of reactants must equal the total mass of products.
• Discovered by Antoine Lavoisier.

"Energy cannot be created or destroyed; it can only be converted from one form to another."

• The total energy of the universe is constant.
• First Law of Thermodynamics.