Chemical & Physical Change
Quick Notes
- Physical change: changes in physical properties (state, shape, appearance) but no change in chemical composition.
- Examples: melting, freezing, dissolving, mixing, boiling
- Chemical change: formation of new substances with different chemical composition.
- Evidence of chemical change includes:
- Color change
- Temperature change (heat/light)
- Gas formation (bubbling, fizzing)
- Precipitate formation (solid forming from solution)
Full Notes
Matter can undergo changes that are classified as either physical or chemical. Understanding the difference is fundamental to recognizing what kind of transformation has taken place.
Physical Changes
In a physical change, the composition of the substance stays the same, but its form or appearance changes. No new substances are created.
Common examples include:
- Phase changes: melting, freezing, evaporation, condensation, sublimation
- Mixing substances without a chemical reaction (e.g., sugar dissolving in water)
- Changes in size or shape, such as cutting or grinding
In all of these cases, the chemical identity of the substance is unchanged.
Example Water changing state is an example of a physical change as the composition of the substance (ice, liquid water, and steam) remains the same – H2O.
Chemical Changes
A chemical change (chemical reaction) results in the formation of one or more new substances with different chemical compositions and properties.
Indicators of a chemical change may include:
- Color change (e.g., clear to blue or colorless to yellow)
- Formation of a gas (bubbling or fizzing, not just boiling)
- Precipitate formation (a solid forming in a liquid solution)
- Heat or light given off (temperature rise or flame, glowing)
- Change in odor (often due to formation of a new substance)
ExampleMixing a weak acid with sodium hydrogen carbonate results in carbon dioxide gas being produced (effervescence observed), signalling a chemical change.
Look for observable evidence, but remember that not every color change or temperature change automatically means a chemical reaction occurred. Think about whether the chemical identity of the substance is actually changing.
Summary
- Physical changes alter the appearance or state of a substance but leave its composition unchanged.
- Chemical changes create new substances with different chemical properties.
- Evidence such as color change, temperature change, gas production, or precipitate formation can help identify a chemical change.
- Being able to distinguish between these types of changes is essential for analyzing both lab results and real-world processes in chemistry.