Overview
This lecture explains the difference between physical and chemical changes, providing key examples to help distinguish between the two in various scenarios.
Physical Changes
- A physical change does not alter the chemical identity of a substance.
- Crumpling paper is a physical change because the paper remains the same substance.
- Freezing water turns liquid water into ice, both of which are H₂O, making it a physical change.
- Boiling and vaporizing water (liquid to steam) is a physical change since both are H₂O.
- Melting ice into liquid water is a physical change; only the state changes, not the composition.
- Condensation (gas to liquid) and sublimation (solid to gas, e.g., dry ice) are physical changes.
- Deposition (gas directly to solid) is also a physical change.
- Pulling copper into thin wire is a physical change; the chemical identity remains copper.
- Changes in state (solid, liquid, gas) are always physical changes.
Chemical Changes
- A chemical change results in a new substance with a different chemical identity.
- Burning paper converts it into carbon dioxide and water vapor—a chemical change.
- Electrolysis of water splits H₂O into hydrogen and oxygen gases—a chemical change.
- Burning gasoline produces carbon dioxide and water, indicating a chemical change.
- Rusting (corrosion) of iron forms iron oxide, a new substance (chemical change).
- Color changes in a solution usually indicate a chemical change due to a chemical reaction.
- Reacting zinc with acid is a chemical change (corrosion).
- Silver tarnishing (forming black silver sulfide) is a chemical change; both color and identity change.
- Corrosion or burning always indicates a chemical change.
Key Terms & Definitions
- Physical Change — A change where the chemical identity of a substance remains the same.
- Chemical Change — A change that produces a new substance with a different chemical identity.
- Sublimation — Direct transition from solid to gas without becoming liquid.
- Deposition — Direct transition from gas to solid, skipping the liquid state.
- Ductility — The ability of a metal to be drawn into thin wires.
- Malleability — The ability of a metal to be hammered into sheets.
- Corrosion — Chemical reaction that degrades metals, forming new compounds.
Action Items / Next Steps
- Review examples of physical and chemical changes.
- Learn and understand key terms: sublimation, deposition, ductility, malleability, corrosion.
- Practice distinguishing between physical and chemical changes in everyday situations.