AP® Chemistry review sheet from Aim for Five (aimforfive.com/chem/units/4/4-4)
Unit 4 · Topic 4.4
4.4 Physical and Chemical Changes
At the particle level, a chemical change breaks and forms chemical bonds, while a physical change only changes the attractions between particles. Some processes, like dissolving a salt, sit on the border and can reasonably be argued either way.
Key terms
- chemical bond
- intermolecular forces
- phase change
- dissolution
- ion-dipole interaction
Bonds versus intermolecular forces
Topic 4.1 sorted changes by what you observe. This topic explains them by what happens to particles.
Chemical processes break and form chemical bonds: covalent bonds within molecules, ionic bonds between ions or metallic bonds. When methane burns, C–H and O=O bonds break and C=O and O–H bonds form, making new substances.
Physical processes change only intermolecular forces. When water boils, the hydrogen bonds between molecules are overcome, but every O–H bond inside each molecule stays intact. For molecular substances, melting, freezing, boiling and condensing are all physical for this reason.
Any process can be written as an equation, physical or chemical. Comparing the formulas on each side is a quick check: if every formula is the same and only the state symbols change, as in H₂O(s) → H₂O(l), the change is physical.
Comparing the energies
Covalent bonds are usually much stronger than intermolecular forces. Boiling water takes about 41 kJ per mole at 100 °C, but breaking the O–H bonds in a mole of water would take roughly 900 kJ. That's why heating water to boiling doesn't decompose it into hydrogen and oxygen; you need something like electrolysis to do that.
The borderline cases
Dissolving an ionic solid is the classic gray area. When NaCl dissolves, the ionic bonds holding the lattice together are broken, and new ion-dipole attractions form between the ions and water molecules. Bonds break, which sounds chemical. But no new substance forms: the Na⁺ and Cl⁻ ions are the same ions, and evaporating the water gives back NaCl. That sounds physical.
Either classification can earn credit if you support it with the right reasoning. Argue 'chemical' by pointing to the ionic bonds broken. Argue 'physical' by pointing to the same ions being present before and after, with no new substance and an easily reversed process.
Dissolving a molecular solid, like sugar, is clearly physical: only IMFs between sugar molecules are replaced by IMFs between sugar and water, and the molecules stay intact. Dissolving HCl gas in water, on the other hand, breaks covalent H–Cl bonds and makes H₃O⁺ and Cl⁻ ions, so it's usually considered chemical.
How to argue a classification
- State what happens to the particles: which bonds or IMFs are broken and which form.
- State whether the particles at the end are the same as or different from those at the start.
- Support your claim with evidence, such as a new property, a gas or a precipitate, or the ability to recover the starting substance by a physical process.
Worked examples
Try each one yourself first, then open the solution.
- Example 1
Classifying with particle-level reasoning
Classify each process as physical or chemical, and explain at the particle level: (a) I₂(s) → I₂(g); (b) 2H₂O(l) → 2H₂(g) + O₂(g); (c) C₁₂H₂₂O₁₁(s) → C₁₂H₂₂O₁₁(aq).
Show the solutionHide the solution
- Step 1: (a) Solid iodine turning to vapor: I₂ molecules separate, overcoming dispersion forces. The I–I bond stays intact. Physical.
- Step 2: (b) Electrolysis of water: O–H bonds break and new H–H and O=O bonds form, making new substances. Chemical.
- Step 3: (c) Dissolving sugar: attractions between sugar molecules are replaced by hydrogen bonds between sugar and water. No covalent bonds break. Physical.
Answer: (a) physical; (b) chemical; (c) physical
- Example 2
Arguing the borderline case
Write a short argument that dissolving KBr in water is a physical process, then a short argument that it is a chemical process.
Show the solutionHide the solution
- Step 1: Physical: KBr(s) → K⁺(aq) + Br⁻(aq). The K⁺ and Br⁻ ions are the same particles that made up the solid; no new substance forms, and evaporating the water recovers solid KBr.
- Step 2: Chemical: dissolving breaks the ionic bonds that hold K⁺ and Br⁻ in the crystal lattice and forms new ion-dipole interactions with water, so bonds are broken during the process.
- Step 3: Both are defensible, because the definition depends on whether you focus on bonds or on substances.
Answer: Physical: same ions before and after, no new substance. Chemical: ionic bonds in the lattice are broken and new ion-dipole interactions form.
Common mistakes
- Saying boiling or melting breaks covalent bonds. Only IMFs are overcome.
- Claiming there's one correct answer for dissolving a salt. The exam accepts either with good reasoning.
- Treating dissolving sugar like dissolving salt. Sugar's molecules stay intact; no bonds break.
On the exam
- Expect to justify a classification. The point is earned by the reasoning: name the bonds or IMFs involved and say whether new substances form.
- Watch the wording 'intramolecular' (inside molecules: bonds) versus 'intermolecular' (between molecules).
Connected topics
Videos
Check yourself
4 questions on 4.4 Physical and Chemical Changes. Pick an answer to see if you got it, and why.
Which of the following processes involves breaking covalent bonds?
Which of the following correctly describes the changes in particle interactions when solid NaCl dissolves in water?
Which of the following processes overcomes only intermolecular forces, without breaking any covalent, ionic or metallic bonds?
Dissolving sodium chloride in water is sometimes described as a physical change and sometimes as a chemical change. Which of the following statements best supports describing it as a chemical change?
0 of 4 answered