Recall yesterday's reversible changes (same substance, can undo). Pose the contrast: "Some changes make something completely new — and you can't get the original back." Define an irreversible change (also called a chemical change): a new substance is made, and the change can't be undone.
Model the four signs that a new substance has formed, listing them on the board:
Think aloud through burning toast: "The bread turns black, gives off smoke, and smells different. Can I turn the burnt toast back into fresh bread? No — a new substance has formed. That's irreversible." Do the same with rusting and cooking an egg.
Run the bicarbonate-soda + vinegar reaction in groups (goggles on). Guide observation with the signs checklist:
- Did you see a gas? (fizzing/bubbles)
- Was there a colour or temperature change?
- Is there a new substance you couldn't separate back out?
Then show image sets / short clips of burning, rusting and cooking. For each, ask the class to call out which signs of chemical change are visible and conclude: reversible or irreversible? Build a shared two-column list on the board.
Students work independently:
- list the signs of chemical change they observed in the reaction;
- explain why the reaction can't be undone (a new substance formed);
- sort four given changes (e.g. melting ice, burning wood, dissolving salt, rusting nail) into reversible / irreversible with a reason for each.
Exit ticket: the sorted four-change list with one-sentence justifications — this previews the Lesson 7 assessment task.