A reversible change is a physical change that can be undone. A final substance can be converted back to the original substance without creating any new material. Freezing, melting, dissolving, boiling, evaporating and condensing are reversible changes.
The chemical composition of a substance in a reversible reaction remains the same. Freezing water into ice is reversible because the ice can be melted back to water. Salt dissolved in water is reversible because salt water can be boiled and the water evaporated, leaving behind salt.
A chemical change that cannot be undone is an irreversible change. A new material is created that cannot be returned to the original material. Burning wood is an example of an irreversible change.