Physical versus Chemical Change

Interactive molecular-level visualization comparing physical and chemical changes through five animated scenarios. Physical changes: (1) thermal expansion and contraction (particles vibrate and spread/contract), (2) dissolving salt in water (ions separate but remain intact), (3) cutting hair (long molecules simply separated). Chemical changes: (4) combustion of methane (CH₄ + 2O₂ → CO₂ + 2H₂O, bonds break and reform), (5) rusting of iron (4Fe + 3O₂ → 2Fe₂O₃, new substance forms). Each scenario shows molecular transformations via canvas animation with: play/pause controls, manual slider to scrub through reaction stages, reset button, dynamic captions explaining each stage, chemical equations, and element legend. Dark theme with colour-coded atoms (H, C, O, Fe, Na, Cl). Teaches the distinction: physical changes are usually reversible with molecules unchanged; chemical changes form new substances with rearranged atoms and are difficult to reverse.