Lesson 15: Reversible and Nonreversible Changes
Whether a change can be undone is a different question from whether new substances have formed. Melting, freezing, and burning provide examples for comparing reversibility with physical and chemical change.
Key Ideas
- A reversible change can be undone under the right conditions.
- Some changes only affect shape, size, or state. Other changes make new substances.
- Melting ice and boiling water change water’s state. The material is still water.
- Burning paper makes new substances. Cooling the ashes does not turn them back into the original paper.
- Whether we can undo a change depends on what changed and the methods available. A change that makes no new substance may still be hard to undo.
Vocabulary
- Reversible Change: A change that can be undone to get the original material or state back.
- Nonreversible Change: A change that cannot easily be undone using the methods available.
Discussion Questions
- Burning paper forms new substances. What happens to the atoms, and why does cooling the products not rebuild the original paper?
- Rusting is nonreversible, but iron can be smelted from ore. Does that make rusting reversible after all, or is smelting a completely different process?
- If you could reverse any one nonreversible change in the real world, which would be most useful and why?