4.4 Physical & Chemical Changes

Physical & Chemical Changes

Select from 8 scenarios to see before/after molecular views of physical vs. chemical changes. Bond-breaking flashes red in the before panel, bond-forming flashes green.

8 ScenariosBond Breaking/FormingBefore & AfterParticulate View
Topic 4.4

Physical and Chemical Changes

Explain the relationship between macroscopic characteristics and bond interactions for (i) chemical processes and (ii) physical processes.

This topic refines 4.1 by asking what is being broken. EK 4.4.A.1 gives the operational rule:

  • Processes involving the breaking and/or formation of chemical bonds are typically classified as chemical.
  • Processes involving only changes in intermolecular interactions, such as phase changes, are typically classified as physical.

Boiling water is physical: hydrogen bonds between molecules are overcome, while the O–H covalent bonds inside each molecule survive intact. That distinction — intermolecular vs. intramolecular — is the single most valuable sentence in this topic.

EK 4.4.A.2 then introduces a deliberate ambiguity you are expected to be able to argue: sometimes physical processes involve breaking chemical bonds. The CED's own example is dissolving a salt. Plausible arguments exist for classifying it either way, because dissolution breaks ionic bonds in the lattice and forms ion–dipole interactions between the ions and the solvent.

The AP Exam has asked students to take a position and defend it with evidence. Either answer can earn full credit if the reasoning names what is broken and what is formed. What loses credit is a bare label with no mechanism.

Energetic signature. Chemical processes generally involve much larger energy changes than physical ones, because covalent and ionic bonds are far stronger than intermolecular forces. Comparing magnitudes is legitimate supporting evidence.

Key points

  • Bonds broken/formed → chemical. Only intermolecular interactions changed → physical.
  • Boiling and melting never break covalent bonds within molecules.
  • Dissolving an ionic solid is a genuine borderline case; argue it with mechanism, not with a label.
  • Chemical changes generally involve much larger energy changes than physical ones.

Common mistakes

  • "Boiling breaks the bonds in water." The most penalized sentence in Unit 3/4.
  • Giving a classification with no justification. The skill for this topic is supporting a claim with evidence.
  • Assuming dissolving is always physical. The CED explicitly presents it as arguable.

Worked example

A student claims that dissolving NaCl in water is a chemical process. Evaluate this claim.

Support for "chemical": ionic bonds in the NaCl crystal lattice must be broken for dissolution to occur, and new ion–dipole interactions between Na⁺/Cl⁻ and water molecules are formed. Breaking and forming of bonds is the CED's own criterion for a chemical process. The dissolved solution also has different chemical behavior from the solid — it conducts electricity, and its ions can participate in precipitation reactions.

Support for "physical": no new chemical species are produced. The Na⁺ and Cl⁻ ions existed in the crystal and still exist in solution; they are simply solvated rather than locked in a lattice. The process is readily reversed by evaporating the water, recovering the original NaCl. The energy change is also modest compared with a typical reaction enthalpy.

Evaluation: the claim is defensible. EK 4.4.A.2 explicitly identifies salt dissolution as a case where plausible arguments can be made either way, because it involves breaking ionic bonds while forming ion–dipole interactions. A complete answer must identify those specific interactions rather than simply asserting a category.

Full notes for topic 4.4 →