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When an object changes such that the substance doesn't change, but, only its observable state changes, the change is called a physical change. Physical changes are reversible. That is, just like heating ice gave us back water, in other physical changes, we can get back the original substance if we reverse what we initially did.

When an object changes such that the substance itself changes, the change is called a chemical change. Some chemical changes are reversible, some are irreversible.