BackPhysical and Chemical Changes: Classification and Examples
Study Guide - Smart Notes
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Physical Changes
Definition and Characteristics
Physical changes are transformations in the state or appearance of a substance that do not alter its chemical composition. These changes are typically reversible and do not result in the formation of new substances.
Examples of Physical Changes: Dissolving, chopping, cutting, tearing, breaking, mixing, and crushing.
Key Features:
No new substances are formed.
Often involve changes in state (solid, liquid, gas) or form.
Usually reversible.
Example: Dissolving sugar in water is a physical change because the sugar retains its chemical identity and can be recovered by evaporation.
Chemical Changes
Definition and Characteristics
Chemical changes involve a transformation that alters the chemical composition of a substance, resulting in the formation of one or more new substances. These changes are often irreversible under normal conditions.
Examples of Chemical Changes: Burning, metabolism of food, color change, odor change, and chemical reactions.
Key Features:
New substances with different properties are formed.
Often accompanied by energy changes (heat, light, sound).
Usually irreversible.
Example: Cooking an egg is a chemical change because the proteins in the egg undergo denaturation and cannot return to their original state.
Reversible and Irreversible Changes
Definitions
Reversible Change: A process that can be undone, restoring the original substance. Most physical changes are reversible.
Irreversible Change: A process that cannot be undone by simple physical means. Most chemical changes are irreversible.
Phase Changes
Phase changes are physical changes involving the transition between solid, liquid, and gas states. These changes are typically reversible and involve the making or breaking of intermolecular bonds, not chemical bonds.
Bond Forming | Bond Breaking |
|---|---|
Gas → Liquid: condensation Liquid → Solid: freezing Gas → Solid: deposition | Solid → Liquid: melting or fusion Liquid → Gas: vaporization Solid → Gas: sublimation |
Examples of Reversible and Irreversible Changes
Reversible: Dissolving sugar in water, mixing oil and water, melting a chocolate bar.
Irreversible: Adding lemon juice to cabbage juice (color change), adding citrus to baking powder (gas formation).
Practice and Applications
Physical Change Examples:
Milk sours when left out of the refrigerator (chemical change).
Tums (containing CaCO3) neutralizes stomach acid (chemical change).
Sugar caramelizing when heated on a skillet (chemical change).
Teasing a piece of wool (physical change).
Reversible Change Example: Melting a chocolate bar and then re-solidifying it in the freezer.
Key Terms and Definitions
Physical Change: A change in the state or appearance of matter without altering its chemical composition.
Chemical Change: A process in which one or more substances are converted into new substances with different properties.
Reversible Change: A change that can be undone, returning the substance to its original form.
Irreversible Change: A change that cannot be undone by simple physical means.
Phase Change: The transition of matter from one state (solid, liquid, gas) to another.
Relevant Equations
Phase change example:
