Lesson 15
Reversible and Nonreversible Changes
Learners distinguish between changes that can be undone, like water freezing and thawing, and changes that cannot, like burning. The key question is whether the fundamental particles of the substance have been altered.

Key Ideas
  • Changes between solid, liquid, and gas states (melting, freezing, evaporation, condensation) are REVERSIBLE. The substance can return to its original form when conditions change back.
  • Some changes go all the way. When wood burns, paper chars, an egg cooks, or metal rusts, the particles that made up the original substance break apart and form entirely new substances. No amount of heating or cooling will bring back what was there before. These are NONREVERSIBLE changes.

Optional Home Activities


Vocabulary
  • Reversible change: A change where the basic particles stay the same and the process can go backward. The original substance can be restored.
  • Nonreversible change: A change where the basic particles are broken apart and form new substances. The original substance cannot be restored.
  • Physical change: A change in the form or appearance of a substance without changing what it fundamentally is. State changes are physical changes.
  • Chemical change: A change that produces one or more new substances with different properties from the original. Burning, cooking, and rusting are chemical changes.

Discussion Questions
  • Cooking an egg is nonreversible because the particles form new substances. Does that mean the original particles are gone? Where did they go?
  • 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?

Hands-On Activity: Reversible vs. Nonreversible Comparison

Supply List
  • A small amount of butter or an ice cube
  • A refrigerator or freezer
  • One hard-boiled egg and one raw egg (or just one hard-boiled egg to examine)
  • A small piece of paper and something to burn it with (teacher demo only, outdoors or over a sink)
  • Notebook and pencil

Instructions
  • Melt a small piece of butter in a warm spot or briefly in a microwave. Observe it go from solid to liquid. Place the melted butter in the freezer and wait. What happens?
  • Discuss: was the butter change reversible? How do you know?
  • Examine the hard-boiled egg. The inside has been changed by heat. Ask: could you un-cook this egg by cooling it down? Why not?
  • Teacher demo: carefully burn a small piece of paper. Observe the ash. Ask: could you get the original paper back by cooling the ash down?
  • Make a chart with two columns. List three examples of reversible changes and three examples of nonreversible changes. For each one, write one sentence explaining what happened to the particles.
  • Discuss: what is the key difference between a reversible and a nonreversible change at the particle level?

Sources:

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L15: Reversible and Nonreversible Changes by Selene

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