The reason why A returned to its original color but B did not is because; A chemical change occurred in substance A, and a physical change occurred in substance B.
A chemical change results in a change in the "composition" of a substance. When the composition of the substance changes, the change is often irreversible.
A physical change does not alter the composition of a substance hence a physical change is easily reversible.
Since A did not return to its former color, it underwent a chemical change while B underwent a physical change since it returned to its former color.
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