Physical vs Chemical Changes

Activities to try

1. 1

   Change classifier: macro stage synced to a particle window

   Why it works: The only difference that matters is whether the substance's particles stayed the same or atoms regrouped into new substances; mass is conserved either way because atoms are only rearranged (counting every atom involved, including any gas exchanged with the surroundings). A changed appearance (state, colour, a solid 'disappearing') is not by itself evidence of a chemical change.

   1. Trigger melting wax: observe the macro stage turn liquid, and in the particle window watch the same wax molecules loosen and slide while staying whole.
   2. Check the before/after property panel ('same substance') and the mass-balance readout; label the change physical.
   3. Trigger burning magnesium: observe the macro stage flare white, and in the particle window watch magnesium atoms combine with oxygen atoms drawn from the surrounding air into a new white compound (magnesium oxide).
   4. Check the property panel ('new substance, new properties') and confirm that when every atom — including the oxygen taken from the air — is counted, the closed-system mass balance still holds; note that on an open bench balance the solid product would weigh more than the original magnesium, provided all of the product is collected, because it has gained that oxygen; label the change chemical.
   5. Classify a dissolving case: observe the solid seem to disappear on the macro stage while the particle window shows the same particles dispersing among the water particles, not new substances forming; label it physical despite the changed appearance.
2. 2

   Predict-Observe-Explain: dissolving versus displacement

   Why it works: Pre-observation reasoning exposes whether a learner classifies by 'new substance formed?' or by a weaker reversibility/appearance test. The dissolving case is the key discriminator: its dramatic appearance change tempts an appearance-based classifier to call it chemical even though no new substance forms, whereas the displacement reaction genuinely is chemical — so committing to a rule before observing reveals which criterion the learner is actually using.

   1. Before observing, predict for adding blue (hydrated) copper sulfate crystals to warm water whether a NEW substance will form, and explain why.
   2. Before observing, predict for dropping an iron nail into copper sulfate solution whether a NEW substance will form, and explain why.
   3. Observe both: the blue copper sulfate crystals dissolve (blue solution, crystals recoverable by gentle evaporation/crystallisation, no new substance); the iron nail turns copper-coloured and the blue fades (a new substance forms).
   4. Compare each prediction with what happened, and revise any rule that relied on reversibility or appearance rather than 'was a new substance made?'.
   5. Answer probes: why is dissolving sugar physical but rusting chemical? What signs show a new substance has formed? Does total mass change in a sealed flask, and why not? Is baking a cake physical or chemical, and what is your evidence?
3. 3

   Classify and justify the rule

   Why it works: A robust classifier rests on 'was a new substance made?' (often signalled by colour change, gas, temperature change, or precipitate, though these signs alone are not proof, since boiling releases gas and some dissolving changes temperature or colour), not on 'is it reversible?'. State changes and dissolving stay physical because the same substance is preserved, and mass is conserved in chemical changes because atoms are only regrouped.

   1. Sort three changes into physical and chemical: (a) ice melting to water, (b) sugar dissolving in tea, (c) a nail rusting.
   2. State the RULE used — what has to happen for a change to count as chemical.
   3. Interpret before/after property data for further cases to decide whether a chemical reaction occurred.
   4. For a chemical change occurring in a sealed flask, state whether total mass changes and justify using atom conservation.