Irreversible Changes

Activities to try

1. 1

   New-substance detector bench with failed reversals

   Why it works: Irreversibility is detected by evidence that a new substance formed, and experienced through reversal attempts that fail — not memorised as a definition.

   1. Trigger a change at each station: pour vinegar onto bicarbonate of soda and watch it froth; move a time-lapse slider so a shiny nail blooms orange then crumbles to rust; observe a guarded flame turn white paper into blackened char, grey ash and smoke (burning kept as a supervised teacher demonstration — learners observe, they do not ignite anything).
   2. For each change, fill in an evidence-of-new-substance checklist: gas/fizz? colour change? heat or light given off? smell? new solid? (Expect different changes to show different signs — e.g. vinegar+bicarb shows gas but no heat, while burning gives off heat and light.)
   3. Try a simple reversal for each change (e.g. cooling, drying, scraping or sieving; for simulated stations, a 'reverse' control) and record that it does not bring back the originals — e.g. a stamp reading 'can't undo: a new substance was made (here, a gas)'. Remember that a simulated control only shows the result someone programmed; the real evidence is the signs you observed.
   4. Compare side by side with a reversible change (melting chocolate that sets again when cooled) and state the rule in your own words: when the signs show a new material formed, the change usually can't easily be reversed.
2. 2

   Predict–Observe–Explain: the balloon and the bottle

   Why it works: A gas appearing with no heating is strong evidence of a newly-made substance, which distinguishes an irreversible chemical change from mere mixing or boiling.

   1. Before mixing, predict what a balloon stretched over a bottle containing a little baking soda will do when vinegar is tipped in — and say WHY.
   2. Tip in the vinegar, observe the balloon inflate, and explain what the gas tells you about whether a new substance was made.
   3. Answer probing questions: burning toast, a firework, and rusting iron — what sign in each shows a new substance formed? If nothing was heated, why did a gas appear? How is this different from stirring sand into salt, which you CAN separate again? If someone says 'it's just hidden, not changed', what evidence shows it is really new?
3. 3

   Sort and match: reversible vs irreversible by its sign

   Why it works: Classifying a change requires justification by an observed sign, not just a label — and a separable mixture is not an irreversible change.

   1. Sort a set of changes into reversible and irreversible, explaining the reason for each.
   2. For each irreversible change, match it to the evidence-of-new-substance sign that justifies the classification (e.g. acid+bicarb → gas produced; rusting → new solid / colour change; burning → heat/light given off + ash as a new solid).
   3. Contrast a genuine mixture (sand stirred into salt, separable) with a change that made a new substance, and state why one can be undone and the other cannot.