Overview
This lecture explains the differences between pure substances and mixtures, including definitions, properties, separation methods, and examples.
Pure Substances
- Pure substances have a constant, definite composition throughout.
- They can be elements (one type of atom, e.g., helium, iron, gold, silver) or compounds (two or more types of atoms bonded together, e.g., water, carbon dioxide, sugar).
- The composition of a pure substance cannot be varied.
- Compounds can be broken down into their elements by chemical processes.
Mixtures
- Mixtures are combinations of two or more pure substances.
- Mixtures have variable composition; the ratio of components can be changed (e.g., different concentrations of salt water).
- Mixtures can be separated by physical processes such as boiling, evaporation, filtration, or centrifugation.
- Examples include air, brass (copper and zinc), soda, milk, seawater, soil, wood, and wine.
- Alloys (e.g., brass, sterling silver, 14-karat gold) are always mixtures.
Separating Substances
- Physical processes (e.g., evaporation, boiling) separate mixtures into their components.
- Chemical processes (e.g., electrolysis) separate compounds into simpler substances or elements.
Classification Examples
- Helium, hydrogen gas, water, sugar, carbon dioxide, iron, gold, and silver are pure substances.
- Air, brass, salt water, sugar water, rubbing alcohol (isopropyl alcohol + water), soda, milk, sterling silver, seawater, wood, soil, and wine are mixtures.
- Homogeneous mixtures have uniform composition (e.g., air, brass); heterogeneous mixtures do not (e.g., soil, wood).
Key Terms & Definitions
- Pure Substance — matter with constant, definite composition (element or compound).
- Mixture — combination of two or more pure substances with variable composition.
- Element — substance made of one type of atom.
- Compound — substance made of two or more different atoms chemically bonded.
- Molecule — particle made of two or more atoms bonded together.
- Alloy — mixture of metals (e.g., brass, sterling silver).
- Homogeneous Mixture — uniform composition throughout.
- Heterogeneous Mixture — non-uniform composition.
- Electrolysis — chemical process using electricity to break down compounds.
Action Items / Next Steps
- Practice classifying given substances as pure substances or mixtures.
- Review examples of physical and chemical separation methods.