Open the Chemical sciences strand with the big question: "When something changes, can we always change it back?" Today's answer is about changes you can undo.
Define a reversible change: the substance changes form or appearance, but it's still the same substance, and you can get it back. Model with a quick demo of ice melting and refreezing, thinking aloud: "Solid water became liquid water. It looks different, but it's still water — and cooling it turns it back to ice. Same substance the whole time."
Introduce the key vocabulary as you go: substance (what something is made of), physical change, dissolve. Model dissolving sugar in warm water and ask: "Has the sugar gone? Or is it still there, just spread out?" — pointing toward the idea that the sugar could be recovered by evaporating the water.
Set up three hands-on stations and rotate groups through them. As a class, do the first station together so the recording routine is clear.
- Melting: hold an ice cube / warm chocolate under a lamp — observe and describe.
- Dissolving: stir sugar or salt into warm water — where did it go?
- Evaporating: discuss what would be left if the water dried up.
At each station, groups record: what they observed, and whether the change could be undone. Pause the class to ask: "What's the same about all three changes?" — drawing out that the substance stays the same.
Students complete their observation sheet independently, classifying each station's change and justifying it:
- describe what they observed at each station;
- state whether each change is reversible;
- for each, explain how you could reverse it (or why you know the substance is unchanged).
Exit ticket: students write a sentence using "reversible change" and "substance" correctly — e.g. "Melting is a reversible change because the substance is still the same."