Please explain this question.

Solution: 

17) A reversible change is the one in which a substance that is undergoing the change can be recovered in its original form.

For example, when ice cubes are kept at room temperature for some time, they melt to give water. If the same tray containing water is kept in a freezer, then the water turns into ice. As the ice cubes are re-obtained when the tray is kept in the freezer, the change is reversible.

The change in which a substance that is undergoing the change cannot be re-obtained is known as an irreversible change. Hence, it can be said that an irreversible change is the one in which a substance that is undergoing the change cannot be recovered in its original form. 

The examples of reversible and irreversible changes are asked in the question, so you can now easily understand the concept behind those examples.

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