Final answer:
Physical changes only affect a substance's appearance, not its chemical composition. Chemical changes generate a whole new substance with a different chemical formula. Chemical changes are also called chemical reactions.
Step-by-step explanation:
A physical change is one that changes only the appearance of a substance, not its chemical composition. For example, if you chop up a cucumber, it's still a cucumber: its shape has changed, but not what it's made of. A chemical change, on the other hand, changes the substance into an entirely new one with a new chemical formula. If you burn a piece of wood, the resulting ash and smoke are chemically different from the original wood. A chemical change is also called a 'chemical reaction.'
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