Here's why:
* Physical changes only alter the *form* or *appearance* of a substance, not its chemical composition. The molecules themselves remain the same.
* Chemical changes involve the breaking and forming of new chemical bonds, resulting in the creation of entirely different substances with new properties.
Examples of Physical Changes:
* Melting ice: Water (H₂O) changes state from solid to liquid, but the chemical composition remains the same.
* Boiling water: Water changes state from liquid to gas, but the chemical composition remains the same.
* Cutting paper: The paper is divided into smaller pieces, but the chemical composition of the paper remains the same.
Examples of Chemical Changes:
* Burning wood: Wood reacts with oxygen to produce ash, carbon dioxide, and water. New compounds are formed.
* Rusting iron: Iron reacts with oxygen to form iron oxide (rust), a new compound.
* Baking a cake: The ingredients undergo chemical reactions to form a new compound, the cake.
Key takeaway: Physical changes are reversible (usually) and don't create new substances, while chemical changes are typically irreversible and involve the formation of new substances.
