뒤로Physical and Chemical Changes in Matter
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Physical and Chemical Changes
Physical Changes
Physical changes involve alterations in the physical state or appearance of a substance without changing its chemical composition. These changes are typically reversible and do not result in the formation of new substances.
Dissolving of solute
Cutting, sorting, tearing, breaking material
Mixing substances
Crushing a substance
Example: Dissolving sugar in water is a physical change because the sugar can be recovered by evaporating the water.
Other examples: Wood burning (not a physical change), Iron rusting (not a physical change), Dynamite exploding (not a physical change)
Chemical Changes
Chemical changes result in the formation of one or more new substances with new chemical bonds and properties. These changes are usually not easily reversible.
Rusting of metals
Metabolism of food
Color change
Burning of material
Reactions that alter chemical structure
Example: Cooking an egg is a chemical change because new substances are formed and the process cannot be reversed by simple physical means.
Other examples: Melting wax (not a chemical change), Condensing water vapor (not a chemical change), Carving a piece of wood (not a chemical change)
Practice: Identifying Physical Changes
Mix lemon juice with half of the refrigerator: Not a physical change (implies a chemical reaction).
Turning potassium CaCO3 neutralizes stomach acid: Chemical change.
Sugar caramelizing when heated in a skillet: Chemical change.
Tearing a piece of paper: Physical change.
Reversible & Irreversible Changes
Reversible Changes
A reversible change is one that can be undone to restore the original structure of a compound. Most physical changes and phase changes are reversible.
Phase changes: Melting, freezing, condensation, evaporation, sublimation, and deposition.
Example:
Bond Forming | Bond Breaking |
|---|---|
Gas condensation → Liquid freezing → Solid | Solid melting → Liquid vaporization → Gas |
deposition | sublimation |
Irreversible Changes
Irreversible changes are permanent and cannot be undone to restore the original structure. Chemical changes are the most common example.
Example:
Examples: Reversible vs. Irreversible
Process | Type |
|---|---|
Dissolving sugar in water | Reversible |
Cooking an egg | Irreversible |
Making a solution from purple dye | Reversible |
Adding salt to boiling water | Reversible |
Mixing oil and water | Reversible |
Practice: Identifying Reversible Changes
Making a chocolate bar from hot solidifying in the kitchen: Reversible change.
Changing water to vapor and hydrogen gas: Irreversible (if chemical change).
Baking a cake from a recipe: Irreversible (chemical change).
