The key difference between a physical change and a chemical change lies in whether the substance's fundamental identity is altered: a physical change affects appearance or form, while a chemical change results in the formation of new substances.
Physical Change
A physical change involves altering the form or appearance of a substance, but not its chemical composition. The molecules themselves remain the same.
- Definition: A change that affects the form or appearance of a substance but does not change its chemical composition.
- Key Feature: No new substances are formed.
- Reversibility: Often reversible.
- Examples:
- Melting ice (solid water becomes liquid water).
- Boiling water (liquid water becomes gaseous water - steam).
- Cutting paper.
- Dissolving sugar in water (the sugar and water molecules are still present, just mixed).
- Crushing a can.
Chemical Change
A chemical change, also known as a chemical reaction, involves the rearrangement of atoms and molecules to form new substances with different properties.
- Definition: A change that results in the formation of new substances with different chemical compositions and properties.
- Key Feature: New substances are formed with different properties.
- Reversibility: Often irreversible (requires another chemical reaction to reverse).
- Indicators: Chemical changes are often accompanied by observable signs, such as:
- Change in color.
- Formation of a precipitate (a solid forming from a solution).
- Production of a gas (bubbles).
- Change in temperature (release or absorption of heat).
- Emission of light.
- Examples:
- Burning wood (wood is converted into ash, carbon dioxide, and water).
- Rusting of iron (iron reacts with oxygen to form iron oxide).
- Cooking an egg (the egg white and yolk undergo irreversible changes).
- Baking a cake (ingredients combine and react to form a new substance).
- Neutralization reaction (acid + base -> salt + water).
Summary Table
| Feature | Physical Change | Chemical Change |
|---|---|---|
| Definition | Change in form or appearance | Formation of new substances |
| Composition | Remains the same | Changes |
| New Substances | No | Yes |
| Reversibility | Often Reversible | Often Irreversible |
| Energy Change | Usually small | Often significant (heat, light absorbed or released) |
| Examples | Melting, boiling, dissolving, crushing, tearing | Burning, rusting, cooking, reacting with acids/bases |
In essence, a physical change is a change of state or form, while a chemical change is a change in the very substance itself.
