Physical Changes

Definition: A physical change alters the form or appearance of a substance but doesn't change its chemical composition.

Examples:

* Melting ice: Water (H₂O) changes from solid to liquid, but it remains chemically the same.

* Boiling water: Water changes from liquid to gas (steam), but the chemical composition remains H₂O.

* Crushing a can: The shape of the can changes, but the metal itself remains aluminum.

* Dissolving sugar in water: Sugar molecules disperse in water but their chemical structure remains unchanged.

* Cutting paper: The paper is divided into smaller pieces, but the paper itself is still paper.

Key characteristics:

* Usually reversible.

* No new substances are formed.

* Often involve changes in state (solid, liquid, gas).

Chemical Changes

Definition: A chemical change results in the formation of new substances with different chemical properties.

Examples:

* Burning wood: Wood reacts with oxygen, producing ash, carbon dioxide, and water. This results in the formation of new substances with different properties than wood.

* Rusting iron: Iron reacts with oxygen and water to form iron oxide (rust), a new substance with different properties.

* Baking a cake: The ingredients (flour, sugar, eggs, etc.) undergo chemical reactions during baking, forming a new substance with a different texture and flavor.

* Digesting food: Enzymes break down food molecules into smaller molecules, changing their chemical composition.

* Exploding dynamite: Dynamite explodes due to a chemical reaction, forming new gases and releasing energy.

Key characteristics:

* Usually irreversible.

* New substances are formed with different chemical properties.

* Often accompanied by changes in color, temperature, or gas production.

It's important to remember that sometimes it can be difficult to classify a change as purely physical or purely chemical. Some changes involve both physical and chemical processes.