Lecture on Physical and Chemical Changes

Key Concepts

Physical Change: The chemical identity remains the same. Alteration in form or appearance without forming a new substance.
Chemical Change: The chemical identity changes, resulting in a new substance.

Examples

Physical Changes

Crumpling Paper
- Paper remains paper; no change in chemical identity.
Freezing Water
- Liquid water (H2O) changes to solid ice (H2O); chemical identity remains the same.
Boiling/Vaporization
- Liquid water changes to steam (gas); still H2O.
Melting Ice
- Solid ice turns into liquid water; same chemical identity (H2O).
Sublimation of Dry Ice
- Solid CO2 turns directly into gas; no liquid phase.
Condensation
- Gas turns back into liquid.
Deposition
- Gas transitions directly to solid (e.g., CO2 gas to dry ice).
Copper Wire Formation
- Copper metal is physically altered into wire; remains copper.

Chemical Changes

Burning Paper
- Paper turns to CO2 and H2O; new substances are formed.
Electrolysis of Water
- Water breaks into hydrogen and oxygen gases; new substances formed.
Burning Gasoline
- Hydrocarbons in gasoline turn into CO2 and H2O upon combustion.
Rusting of Iron
- Iron reacts with oxygen to form iron oxide, a new substance.
Zinc Reacting with Acid
- Corrosion is a chemical change where zinc forms new compounds.
Tarnishing of Silver
- Silver reacts with sulfur to form silver sulfide, a new substance.

Indicators of Chemical Change

Color Change
- Indicates a chemical reaction, as in tarnishing of silver.
Formation of Gas
- Observed in many chemical reactions such as combustion.
Formation of Precipitate
- A solid forms from a liquid mixture, indicating a new substance.

Summary

Understanding whether a change is physical or chemical is key in chemistry. Physical changes affect the form but not the chemical composition, while chemical changes result in new substances with different properties and compositions. This distinction is crucial in identifying the processes and outcomes of different reactions and transformations.