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You know dissolving hides a solid in a liquid, and evaporating brings it back. Some changes can be undone completely. Mia leaves a chocolate bar in the sun, and it melts into a puddle.
How does Mia get solid chocolate back?
Reversible Changes
Demonstrate that dissolving, mixing, and changes of state are reversible changes where no new materials are formed
What a learner can do afterwards
- Define a reversible change as one where the original materials can be recovered
- Give at least three examples of reversible changes: melting, freezing, dissolving, evaporating
- Explain how to reverse each example (e.g. freeze melted chocolate, evaporate a solution)
1 · Read
A reversible change is a change you can undo. Melt chocolate, then freeze it again, and you get chocolate back. Dissolve sugar in water, then let the water evaporate, and the sugar is still there. No new material forms. You get back exactly what you started with.
Try it together
Mia leaves a chocolate bar in the sun and it melts into a puddle. She puts the puddle in the freezer. An hour later, it is solid chocolate again, the same chocolate she started with.
A change is reversible if you can get the exact same material back. If the change makes something completely new instead, it is not reversible, and no amount of cooling or waiting will bring the first material back. Notice the word can. A change stays reversible even when nobody actually reverses it: ice left in a warm room melts and usually nobody freezes it again, but freezing would work, so melting is still a reversible change.
Melting, freezing, dissolving, and evaporating are reversible changes: you can get the original material back, and no new material forms.
Readings 'Water's round trip' and 'Can it come back?' are original to this dataset.
2 · Watch
3 · Play
Take it off screen
Print a worksheetA4 with an answer key page for grown-ups. No screen, no internet.
Where it sits
Where this leads
Jobs that lean on this skill. Follow one to see everything it is built on.
Then practise
24 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.