Physical and Chemical Changes
Explain physical and chemical changes through the interactions and species that change.
AP Chemistry Topic 4.4 · CED printed p. 81 / PDF p. 88. This teaching, practice and feedback were added for the AP edition; original author material remains separately traceable.
Before you start
- Moles and molarity
- Ionic charges and molecular formulas
- Intermolecular versus intramolecular interactions
- Ideal gas equation with kelvin temperature
The useful distinction is what interactions change. When water boils, H2O molecules separate as attractions between molecules are overcome; O–H bonds within each molecule remain. When water decomposes, O–H bonds are broken and H–H and O=O bonds form. At the bench both processes can involve gas, but the particle explanations are very different. A phase change usually alters intermolecular interactions. A chemical change usually alters the bonded combinations of atoms.
Salt dissolution is a good reminder that categories are tools, not substitutes for explanation. Ions separate from an ionic lattice, water molecules reorganize, and ion–dipole attractions form. Calling it physical because the same ions can be recovered is defensible; calling attention to broken ionic interactions can motivate a chemical description. The CED allows discussion of this boundary. Give the actual interaction account and state your classification criterion. Never say that NaCl dissolves into neutral Na and Cl atoms or that the water O–H bonds must break.
Let's work through it
Compare boiling water, dissolving NaCl, and forming AgCl from aqueous ions.
- Boiling: disrupt intermolecular hydrogen-bond interactions, keeping H2O molecules intact. This is a physical phase change.
- NaCl dissolution: separate lattice ions and form hydrated ions with ion–dipole attractions. Explain these changes explicitly instead of insisting that no interactions change.
- AgCl precipitation: solvated Ag+ and Cl− enter a new solid lattice. A chemically distinct solid phase forms from the supplied ions; describe the new lattice and loss/reorganization of hydration interactions.
Try it from memory
Pause & retrieve
Try it before you peek.
Write your answer and the reason from memory. Then compare with the worked answer. Your self-assessment is a study decision, not an automatically graded score.
Question 1
What bonds stay intact when liquid ethanol evaporates?
See every recall question with its answer
These references preserve each prompt and its answer for direct evidence links. Try the interactive questions above before opening the worked reasoning.
Retrieval 1
What bonds stay intact when liquid ethanol evaporates?
Worked answer and reason
Its intramolecular C–C, C–H, C–O and O–H bonds stay intact. Intermolecular attractions are overcome.
A molecule’s identity persists through vaporization.
Retrieval 2
Why is “nothing breaks during dissolution” too strong?
Worked answer and reason
Separating ions disrupts the ionic lattice and rearranges water–water interactions; ion–dipole interactions then form.
Physical classification does not mean all interactions are unchanged.
Catch the tempting mistake
- Boiling breaks covalent bonds in water.
- Water vapor still contains H2O; the main change is intermolecular separation.
- All salts make molecules when dissolved.
- An ionic salt supplies hydrated ions according to its formula and charges.
Put it to work
Give the chemical reason as well as the calculation. Worked solutions are on a separate page.
- Unit 4 problem 7 · ap synthesis · multiple
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Scientific calculator
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For the teacher
- Accept a reasoned salt-dissolution classification that correctly identifies interactions.
- Ask which evidence could distinguish water vapor from H₂/O₂ products.
Source objectives and scope
Source edition: ap-chemistry-ced-fall2024-local-b5dfe867. See this topic in the course outline →
- 4.4.A
- Explain the relationship between macroscopic characteristics and bond interactions for: i. Chemical processes. ii. Physical processes.
- 4.4.A.1
- Processes that involve the breaking and/or formation of chemical bonds are typically classified as chemical processes. Processes that involve only changes in intermolecular interactions, such as phase changes, are typically classified as physical processes.
- 4.4.A.2
- Sometimes physical processes involve the breaking of chemical bonds. For example, plausible arguments could be made for the dissolution of a salt in water, as either a physical or chemical process, involves breaking of ionic bonds, and the formation of ion-dipole interactions between ions and solvent.
Science references for this teaching
- College Board AP Chemistry Course and Exam Description, Fall 2024 — Exact unit learning requirements and exclusions; supplied PDF is pinned by the integration catalog.
- OpenStax Chemistry 2e: Writing and Balancing Chemical Equations — Scientific check: conserved atoms/charge and complete/net ionic conventions.
- OpenStax Chemistry 2e: Classifying Chemical Reactions — Scientific check: precipitation, oxidation-number conventions, proton transfer; AP exclusions applied separately.
Glossary and deeper reading · 7 refreshers
These editorial refreshers supplement the reading. Definitions and examples are added guidance; original source wording and recorded answers remain in their original context.
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Mole
A mole is an amount of substance containing exactly 6.02214076 × 10²³ specified entities. The entities can be atoms, molecules, ions, or formula units, so the counted entity must be named. For example, one mole of oxygen molecules contains two moles of oxygen atoms.
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Molarity
Molarity is the amount of a solute in moles divided by the final solution volume in liters. It describes concentration rather than the total amount of solute. For example, 0.10 mole dissolved to make 0.50 liter of solution gives a molarity of 0.20 mole per liter.
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Molecular formula
A molecular formula gives the actual number of each kind of atom in a molecule. It can be a whole-number multiple of the empirical formula. For example, glucose has molecular formula C₆H₁₂O₆ and empirical formula CH₂O.
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Ideal gas law
The ideal gas law relates pressure, volume, amount, and absolute temperature in an idealized gas model. It treats particles as having negligible volume and no intermolecular attractions. For example, at fixed amount and temperature, doubling the container volume halves the ideal gas pressure.
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Phase change
A phase change alters a substance's physical state without changing its chemical identity. It involves a change in particle arrangement and interactions and can transfer energy without raising temperature during a pure-substance transition at fixed pressure. For example, vaporizing liquid water separates water molecules into a gas.
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Ion
An ion is an atom or group of atoms with a net electric charge. Ordinary ion formation changes electron count without changing the element's proton count. For example, a sodium atom losing one electron becomes a sodium ion with charge +1.
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Crystal lattice
A crystal lattice describes the repeating arrangement of structural sites in a crystalline solid. The occupants can be ions, atoms, or molecules depending on the material. For example, an ionic lattice places opposite charges near one another throughout a sodium chloride crystal.