Identify evidence of chemical and physical changes in matter.
A physical change occurs when a substance changes properties but not composition. Phase changes and the formation or separation of mixtures are the standard examples. Ice melting is still H₂O; dissolving sugar still leaves sugar molecules.
A chemical change occurs when substances are transformed into new substances, typically with different compositions. The CED lists four kinds of observable evidence:
Note the CED's careful wording: these provide possible evidence that a chemical change has occurred. None of them is proof on its own. Boiling water produces bubbles of gas; mixing hot and cold water changes temperature; dissolving a colored salt changes the color of the liquid. Every one of those is a physical change.
The decisive test is always the same question: are there new substances present that were not there before? If yes, the change is chemical.
Symbolic conventions. Reactants on the left, products on the right, an arrow between. Phase labels — (s), (l), (g), (aq) — are expected and often carry points.
Classify each as a physical or chemical change and justify: (a) dry ice sublimes, (b) iron rusts, (c) a magnesium ribbon burns with a bright white light, (d) a purple KMnO₄ solution is diluted and appears pink.
(a) Physical. Solid CO₂ becomes gaseous CO₂. Only intermolecular interactions are overcome; the molecules themselves are unchanged.
(b) Chemical. Iron and oxygen form a new substance, iron(III) oxide, with a different composition and different properties.
(c) Chemical. The production of light and heat accompanies formation of a new substance, MgO, a white powder chemically distinct from the metal.
(d) Physical. The color changed, but no new substance formed — the same MnO₄⁻ ions are simply present at a lower concentration. This is exactly why color change is only "possible evidence".