Physical changes and chemical changes are two fundamental types of transformations in matter, and they differ in several key ways:
Physical Change:
- Definition: A physical change is a change that affects one or more physical properties of a substance without altering its chemical composition.
- Examples: Common examples include changes in state (e.g., melting ice to water, boiling water to steam), changes in shape (e.g., breaking a glass), and changes in size (e.g., tearing paper).
- Reversibility: Many physical changes are reversible (e.g., freezing and melting water), although some may not be easily reversible (e.g., crumpling paper).
- Chemical Composition: The chemical composition of the substance remains the same before and after the change. For example, water remains H₂O whether it is in solid, liquid, or gaseous form.
- Energy Changes: Physical changes may involve energy changes (e.g., heat absorption or release during phase changes), but do not generally involve the breaking or forming of bonds.
Chemical Change:
- Definition: A chemical change is a change that results in the formation of one or more new substances with different chemical properties and compositions.
- Examples: Common examples include rusting of iron, burning wood, digestion of food, and the reaction between vinegar and baking soda.
- Reversibility: Chemical changes are often not easily reversible (e.g., burning wood turns it to ash and smoke, which cannot be easily converted back to wood).
- Chemical Composition: The chemical composition of the substances involved changes; for instance, in the combustion of hydrogen, hydrogen (H₂) reacts with oxygen (O₂) to form water (H₂O).
- Energy Changes: Chemical changes typically involve significant energy changes, including the release or absorption of heat, light, or gas.
In summary, the main difference lies in whether the chemical identity of the substance changes. Physical changes do not alter the chemical composition, while chemical changes result in new substances.