Review of Chemical Equations
Chemical equations show the products that the combination of reactants yields, but the equation doesn’t show how the reactants form those products. What we don’t see are the underlying chemical and physical changes that allow molecules to rearrange to change properties or create new substances. All it does is represent a chemical reaction!
Chemical Changes
Generally, chemical changes involve intramolecular (literally meaning “inside molecule”) bonds. This includes breaking and/or forming ionic or covalent bonds between elements during a chemical reaction. Let’s look at a chemical reaction using Lewis Dot Diagrams to visualize it.
The synthesis reaction above shows an interaction between two magnesium atoms and an oxygen molecule. The bonds that are broken are red, and the bonds formed are green. Here, we can see that the oxygen molecule's covalent bonds had to break, and then new ionic bonds formed between the magnesium and oxygen to form two molecules of MgO.
Note that ionic bonds are not represented like covalent bonds with lines as shown in the above diagram. They are rather expressed with brackets and charges around the brackets to show the transfer of electrons.
Remember that chemical changes are usually accompanied by a chemical reaction to show this breaking and forming of bonds. Examples of chemical changes include:
- 🔥 Burning: The chemical reaction that occurs when a substance reacts with oxygen to produce heat and light. Think about combustion reactions!
- 🧱 Rusting: The chemical reaction that occurs when iron reacts with water and oxygen to form iron oxide (rust).
- 🥦 Digestion: The chemical reactions that occur in the body to break down food into nutrients that can be absorbed and used by cells.
Physical Changes
Physical changes are usually intermolecular, such as phase changes. Some examples are freezing water and cutting paper.
The ice’s molecules maintain the same atomic structure (H2O), but more hydrogen bonds between each water molecule are formed. Conversely, the paper’s molecules keep the same atomic structure, but the interaction between the paper molecules is altered, or broken.
Since physical changes are only a transformation that changes a substance's properties without undergoing a chemical change, they are reversible.
However, some reactions can go either way and sometimes physical processes involve the breaking of chemical bonds. For example, the reaction between salt and water, or the dissolution of salt in water, involves breaking the bonds between ions, but it also creates ion-dipole interactions between the water and ions.
The easiest way to distinguish between a chemical and physical bond is to think about what forces/bonds are breaking or forming. If intramolecular bonds, such as covalent and ionic bonds, are being broken and formed, there must be a change at a molecular level. However, if intermolecular forces are involved and changing, it is simply a physical change.
Review Activity
Name whether each scenario describes a chemical or physical change.
- Burning a match
- Iron rusting over time
- Mixing two powders
Answers
- Burning a match is a chemical change
- Iron rusting is a chemical change
- Mixing two powders is a physical change
🎥 Watch AP Chemistry teacher Wes Winter discuss physical and chemical changes, as well as introduce chemical reactions.
Vocabulary
The following words are mentioned explicitly in the College Board Course and Exam Description for this topic.
| Term | Definition |
|---|---|
| bond interactions | The forces between atoms or molecules, including chemical bonds and intermolecular forces, that determine the properties and behavior of substances. |
| chemical process | A transformation in which substances are converted into different substances through the breaking and forming of chemical bonds. |
| dissolution | The process by which a solute dissolves in a solvent to form a solution, involving the breaking of bonds or interactions in the solute and formation of new interactions with the solvent. |
| intermolecular interactions | Forces between molecules, such as hydrogen bonding, dipole-dipole forces, and London dispersion forces, that affect the physical and chemical properties of substances. |
| ion-dipole interactions | Attractive forces between an ion and a polar molecule, where the charged ion interacts with the partial charges on the polar molecule. |
| ionic bonds | Chemical bonds formed between positively and negatively charged ions through electrostatic attraction. |
| macroscopic characteristics | Observable properties of matter that can be seen and measured without a microscope, such as color, state, temperature, and solubility. |
| phase transition | The process by which a substance changes from one state of matter to another (solid, liquid, or gas). |
| physical process | A change in the state or properties of matter that does not alter the identity of the substances involved. |
Frequently Asked Questions
What's the difference between a physical change and a chemical change?
A physical change alters a substance’s macroscopic form or intermolecular interactions without making/breaking its chemical bonds—think phase changes (ice melting → liquid water), dissolving where only ion–dipole/hydration and other intermolecular forces change, or mixing. A chemical change involves breaking and/or forming chemical bonds and producing new substances (e.g., H2 + Cl2 → 2 HCl). The CED rule of thumb: bond breaking/formation → chemical; only changes in intermolecular forces → physical (4.4.A.1). Note the gray area: dissolving NaCl breaks ionic lattice energy but forms ion–dipole interactions, so it can be argued either way (4.4.A.2). For AP exam answers, name whether bonds are broken/formed and cite relevant interactions (ionic/covalent bond, ion-dipole, hydrogen bonding, lattice energy). For a quick study refresher, check the Topic 4.4 study guide (https://library.fiveable.me/ap-chemistry/unit-4/physical-vs-chemical-changes/study-guide/klo7E1MlmIr8w2mRq9jA) and practice problems (https://library.fiveable.me/practice/ap-chemistry).
How do I know if breaking bonds makes something a chemical or physical process?
If a process breaks or forms chemical bonds (ionic or covalent) it’s typically a chemical change; if it only changes intermolecular forces (H-bonding, ion-dipole, dispersion) it’s a physical change. That’s the CED rule: bond breaking/formation ⇒ chemical; only intermolecular changes ⇒ physical (Topic 4.4, EK 4.4.A.1). Watch for gray areas: dissolution of an ionic solid (like NaCl in water) involves breaking ionic lattice bonds and forming ion–dipole interactions, so you can plausibly call it physical (dissociation/solvation) or chemical—explain your reasoning on the exam. Use terms: lattice energy, hydration energy, dissociation. For extra practice and clear examples, review the Topic 4.4 study guide (https://library.fiveable.me/ap-chemistry/unit-4/physical-vs-chemical-changes/study-guide/klo7E1MlmIr8w2mRq9jA) and try practice problems (https://library.fiveable.me/practice/ap-chemistry).
Why is melting ice considered a physical change if you're breaking hydrogen bonds?
Melting ice is a physical change because you’re changing the phase (solid → liquid) by overcoming intermolecular interactions (hydrogen bonds) without making or breaking covalent bonds inside each H2O. The O–H covalent bonds in each water molecule stay intact; what changes is the pattern and strength of hydrogen bonding between molecules. That fits the CED rule: chemical processes typically involve breaking/forming chemical bonds, while physical processes involve only changes in intermolecular forces (4.4.A.1). Note the nuance in the CED: some “physical” processes (like dissolving an ionic solid) can involve breaking ionic bonds, so classification can depend on what you count as a chemical-bond change (4.4.A.2). For AP review, see the Topic 4.4 study guide (https://library.fiveable.me/ap-chemistry/unit-4/physical-vs-chemical-changes/study-guide/klo7E1MlmIr8w2mRq9jA) and try practice problems (https://library.fiveable.me/practice/ap-chemistry).
Can someone explain why dissolving salt in water could be either physical or chemical?
Both answers can be correct depending on how you justify it. Physical-process view (common): solid NaCl simply dissociates into Na+ and Cl– and those ions become solvated by water (ion–dipole intermolecular interactions). No new covalent species form, so macroscopic change = salt disappears but composition of species (ions) is conserved → usually called a physical change (CED 4.4.A.1: only intermolecular interactions change). Chemical-process view: dissociation requires breaking the ionic lattice (ionic “bonds”) and forming many new ion–dipole interactions (new interactions form). If you argue bond breaking/formation, you can classify it as a chemical process (CED 4.4.A.2). Compare lattice energy vs hydration (hydration) energy and ΔH to support your claim. For the AP exam, explicitly link macroscopic evidence to bond/intermolecular changes in your explanation (LO 4.4.A). For a quick refresher, see the Topic 4.4 study guide (https://library.fiveable.me/ap-chemistry/unit-4/physical-vs-chemical-changes/study-guide/klo7E1MlmIr8w2mRq9jA) and use practice problems (https://library.fiveable.me/practice/ap-chemistry) to practice framing your justification.
What are intermolecular interactions and how are they different from chemical bonds?
Intermolecular interactions are the forces between separate molecules or ions—things like hydrogen bonding, dipole–dipole, London dispersion, and ion–dipole interactions. They’re generally much weaker than chemical bonds and govern macroscopic behaviors like boiling/melting points, solubility, and phase changes. Chemical bonds (ionic or covalent) are intramolecular—the strong attractions that hold atoms together within a molecule or ionic lattice. Breaking or forming chemical bonds changes chemical identity and is a chemical process; changing only intermolecular interactions (e.g., vaporization, melting) is usually a physical process (CED 4.4.A.1). One caution from the CED (4.4.A.2): some “physical” processes can involve bond breaking (dissolving an ionic solid breaks ionic bonds and forms ion-dipole solvation), so you must justify whether bonds are actually broken. On the exam, use that bond-breaking criterion to classify changes. For a quick review, see the Topic 4.4 study guide (https://library.fiveable.me/ap-chemistry/unit-4/physical-vs-chemical-changes/study-guide/klo7E1MlmIr8w2mRq9jA) and try practice problems (https://library.fiveable.me/practice/ap-chemistry).
I'm confused about phase changes - are we breaking bonds or not?
Short answer: usually no—phase changes (melting, boiling, subliming) are physical processes because you’re changing intermolecular interactions, not making or breaking chemical bonds. For example, when H2O(l) → H2O(g) you overcome hydrogen bonds between molecules (endothermic) but the H–O covalent bonds inside each molecule stay intact (CED 4.4.A.1: intermolecular vs. chemical bonds). Caveat: some “physical” processes can involve bond-breaking. Dissolving an ionic solid (like NaCl) breaks ionic lattice attractions and forms ion–dipole interactions with water; you can reasonably argue whether that’s purely physical or has chemical-bond implications (CED 4.4.A.2). For AP exam questions, focus on whether covalent/ionic bonds are broken or only intermolecular forces change—that’s what earns the point. Want practice distinguishing cases? Check the Topic 4.4 study guide (https://library.fiveable.me/ap-chemistry/unit-4/physical-vs-chemical-changes/study-guide/klo7E1MlmIr8w2mRq9jA) and hit the practice problems page (https://library.fiveable.me/practice/ap-chemistry).
Why does my textbook say some physical processes can break chemical bonds?
Short answer: your book is right—most physical processes only change intermolecular interactions, but some (like dissolution or dissociation) can break chemical bonds or ionic attractions even though we still call them “physical.” The CED’s Essential Knowledge 4.4.A.1 says bond breaking/formation is typically a chemical process, but 4.4.A.2 explicitly allows that physical processes can sometimes break bonds (e.g., NaCl(s) → Na+(aq) + Cl-(aq)). Whether you label it physical or chemical depends on perspective: dissolving an ionic lattice requires overcoming lattice energy (breaking ionic bonds) and gaining solvation/hydration energy (forming ion–dipole interactions). If net changes are only in intermolecular forces, it’s physical; if covalent bonds are cleaved and new covalent bonds form, it’s chemical. For more examples and AP-style framing, see the Topic 4.4 study guide (https://library.fiveable.me/ap-chemistry/unit-4/physical-vs-chemical-changes/study-guide/klo7E1MlmIr8w2mRq9jA) and the Unit 4 overview (https://library.fiveable.me/ap-chemistry/unit-4). For extra practice, try problems at (https://library.fiveable.me/practice/ap-chemistry).
How do I tell the difference between breaking chemical bonds vs intermolecular forces?
Think “what changes at the atomic-bond level?” Chemical processes break or form chemical bonds (ionic or covalent)—new substances form, with different formulas and often different energy changes. Physical processes change how molecules interact (intermolecular forces: H-bonds, dipole–dipole, London dispersion) but don’t change molecular identity—e.g., melting, boiling, most dissolving of molecular solutes. Quick checks you can use: - Did the chemical formula change? Yes → chemical (bonds broken/formed). No → probably physical (IMFs changed). - Did new substances appear (gas evolution, color change, precipitate, permanent energy release/absorb)? That suggests chemical. - Think about scale: breaking covalent bonds needs much more energy than overcoming IMFs (phase changes). Ionic lattice dissociation (like salts dissolving) is a gray area—ionic bonds are broken but products are solvated ions, so you can justify either view; AP accepts that nuance (CED 4.4.A.2). If you want practice identifying examples, check the Topic 4.4 study guide on Fiveable (https://library.fiveable.me/ap-chemistry/unit-4/physical-vs-chemical-changes/study-guide/klo7E1MlmIr8w2mRq9jA) and more practice problems at (https://library.fiveable.me/practice/ap-chemistry).
What happens at the molecular level during a physical change?
At the molecular level a physical change rearranges how particles interact without making or breaking covalent (or new strong) bonds between atoms. That usually means changing intermolecular forces—e.g., melting or boiling weakens/overcomes hydrogen bonding, dipole–dipole, or London dispersion forces so molecules move farther apart; freezing or condensation strengthens them. In dissolution the solvent–solute intermolecular interactions (solvation, hydration) replace crystal lattice or ionic attractions; sometimes ionic bonds are disrupted (think NaCl dissolving) but no new chemical bonds form between atoms, so composition stays the same. Key terms from the CED: intermolecular forces, lattice energy, solvation/hydration, dissociation. For the AP exam, you should be able to link macroscopic signs (phase change, solubility) to these bond/intermolecular changes (CED 4.4.A.1–2). For a short guide and practice, see the Topic 4.4 study guide (https://library.fiveable.me/ap-chemistry/unit-4/physical-vs-chemical-changes/study-guide/klo7E1MlmIr8w2mRq9jA) and more unit review (https://library.fiveable.me/ap-chemistry/unit-4).
Is dissolving sugar in water physical or chemical and why?
Dissolving sugar (sucrose) in water is considered a physical process, because no covalent bonds in the sugar molecules are broken or new covalent bonds formed—only intermolecular interactions change. The sugar molecules separate from the solid lattice and become surrounded by water via hydrogen bonding (solvation/hydration). That matches the CED rule: processes that only change intermolecular forces are typically physical (4.4.A.1). Note the nuance from the CED: some dissolutions (like ionic salts) can involve breaking ionic bonds and forming ion–dipole interactions, so they can be argued either way. Sucrose is covalent, so dissolution is straightforwardly physical. For a quick AP-aligned refresher, see the Topic 4.4 study guide (https://library.fiveable.me/ap-chemistry/unit-4/physical-vs-chemical-changes/study-guide/klo7E1MlmIr8w2mRq9jA). Practice more examples at Fiveable’s AP Chem practice page (https://library.fiveable.me/practice/ap-chemistry) to get comfortable with CED language on the exam.
Why is it sometimes hard to classify a process as physical or chemical?
Because the CED defines chemical processes as those that break/form chemical bonds and physical processes as changes in intermolecular interactions, most cases are clear (phase changes = physical; reactions that change connectivity = chemical). But some situations sit in the gray area because both kinds of interactions change. For example, dissolving an ionic solid involves breaking ionic lattice bonds (bond breaking) and forming ion–dipole solvation (new interactions), so you can plausibly call it physical (dissolution) or chemical (dissociation). Likewise, mechanochemical cleavage, polymer cracking, or reactions that only rearrange electrons but not atoms can be ambiguous. To classify a process for AP, state which bonds (ionic or covalent) are broken/formed, what intermolecular forces change (hydrogen bonding, ion-dipole, dispersion), and justify with energy/lattice vs. hydration (CED 4.4.A.1–2). If you want practice explaining these distinctions like on the exam, see the Topic 4.4 study guide (https://library.fiveable.me/ap-chemistry/unit-4/physical-vs-chemical-changes/study-guide/klo7E1MlmIr8w2mRq9jA) and Unit 4 resources (https://library.fiveable.me/ap-chemistry/unit-4). For extra targeted practice, try Fiveable’s AP Chem practice set (https://library.fiveable.me/practice/ap-chemistry).
What are ion-dipole interactions and when do they form?
Ion-dipole interactions are attractive forces between a full ion (like Na+ or Cl−) and the partial charges on a polar molecule (most often solvent molecules such as H2O). They form when an ionic compound dissolves or dissociates in a polar solvent: solvent dipoles orient so their opposite partial charge faces the ion (oxygen toward Na+, hydrogen toward Cl−), stabilizing separated ions. These interactions compete with the ionic lattice energy; if the hydration (solvation) energy from ion-dipole interactions is large enough to overcome lattice energy, the salt dissolves. On the AP exam, this shows up in Topic 4.4 when you classify dissolution as a physical process that can involve ionic bond breaking and new intermolecular (ion-dipole) interactions (CED 4.4.A.2). For a quick refresher, see the Topic 4.4 study guide (https://library.fiveable.me/ap-chemistry/unit-4/physical-vs-chemical-changes/study-guide/klo7E1MlmIr8w2mRq9jA). For more practice problems, check Fiveable’s AP Chem practice set (https://library.fiveable.me/practice/ap-chemistry).
How do I remember which processes are physical vs chemical for the AP exam?
Think in terms of bonds vs. intermolecular forces. If a process breaks or forms chemical bonds (ionic or covalent) it’s usually chemical; if it only changes intermolecular interactions (phase change, mixing without new bonds) it’s usually physical—that’s the CED rule (4.4.A.1). Quick checklist you can run on test day: - Did new substances form (new bonds, new formulas)? → chemical. - Is it just melting, boiling, dissolving (no new covalent/ionic species)? → usually physical. - Macroscopic clues: color change, gas evolved, precipitate, or large energy release = likely chemical. - Watch exceptions: dissolution can involve breaking ionic lattices and forming ion–dipole interactions, so AP expects you to justify when it’s ambiguous (4.4.A.2). For AP-style questions you’ll often need to explain the bond-level reason (link macroscopic observation to bond/intermolecular changes). Review the Topic 4.4 study guide (https://library.fiveable.me/ap-chemistry/unit-4/physical-vs-chemical-changes/study-guide/klo7E1MlmIr8w2mRq9jA) and practice lots of items (https://library.fiveable.me/practice/ap-chemistry) to get the pattern down.
Can you give me examples of when physical processes break chemical bonds?
Short answer: usually breaking/forming bonds = chemical, but some physical processes can break bonds—most common AP example is dissolution. When an ionic solid like NaCl dissolves, the ionic lattice is broken (ionic bonds separate) and ions become solvated by water (ion–dipole interactions). That process can reasonably be called physical or chemical (CED 4.4.A.2 gives this example). Other examples: melting or vaporizing very strong-network solids (e.g., melting SiO2 at very high T) requires breaking covalent bonds even though we call it a phase change; and mechanical fragmentation (grinding) of a covalent polymer can break covalent chains (scission) without making new molecules. Key point for the AP: most phase changes only change intermolecular forces, but dissolution, high-T melting of network solids, and mechanical scission are exceptions where chemical bonds are broken during a physical process (see the Topic 4.4 study guide: https://library.fiveable.me/ap-chemistry/unit-4/physical-vs-chemical-changes/study-guide/klo7E1MlmIr8w2mRq9jA). For extra practice, try related problems at Fiveable (https://library.fiveable.me/practice/ap-chemistry).
I don't understand how salt dissolving can be argued as both physical and chemical - help?
Short answer: both views are defensible because salt dissolution sits on the boundary between breaking chemical bonds and changing intermolecular interactions. Why: an ionic solid (NaCl) has ionic bonds in a lattice. When it "dissolves" the lattice breaks apart (you’re separating ion pairs), which is bond breaking in a sense—so you can argue a chemical process occurred. But those separated ions aren’t forming new covalent bonds; they become solvated by water via ion–dipole (intermolecular) forces (hydration/solvation). If you classify processes by only forming/breaking chemical (covalent/ionic) bonds, dissolution looks physical because the major new interactions are intermolecular (ion–dipole). If you emphasize lattice energy vs. hydration energy and the actual breaking of ionic attractions, you can treat it as chemical. The CED explicitly allows both arguments (4.4.A.1–4.4.A.2). For AP exam framing: state which definition you’re using, justify with lattice energy vs hydration energy and show whether new chemical bonds are formed. For a quick refresher, check the Topic 4.4 study guide (https://library.fiveable.me/ap-chemistry/unit-4/physical-vs-chemical-changes/study-guide/klo7E1MlmIr8w2mRq9jA) and practice problems (https://library.fiveable.me/practice/ap-chemistry).