AP® Chemistry · Unit 4 · Topic 4.4
Physical and Chemical Changes
A physical change rearranges how particles are packed or mixed without making a new substance, while a chemical change breaks and remakes bonds inside molecules to create something new.
The test isn't whether bonds break. Bonds break in both physical and chemical changes. The real question is whether a new substance forms with different properties and a different formula. If you can reverse the change and get back the exact same substance you started with, it was physical, even if real bonds had to break to get there.
Compare three cases. Ice melting breaks hydrogen bonds between water molecules, loosening the rigid lattice, but every molecule is still H_2O. NaCl dissolving breaks the ionic bonds holding Na+ and Cl- together in the crystal, but the ions float off intact and unchanged, surrounded by water. Boil that water away and you get your NaCl back. Methane burning is different: CH4 and O2 molecules are torn apart completely, and the atoms recombine into CO2 and H_2O, substances with entirely different properties. That rearrangement of atoms into new molecules is what makes combustion chemical.
This distinction sets up the rest of the unit. A balanced chemical equation is only meaningful for genuine chemical changes, where reactant bonds break and product bonds form in fixed ratios you can track with moles and grams. Physical changes, like dissolving or melting, don't get balanced equations with new products, because nothing new is made. Knowing which kind of change you're looking at tells you whether stoichiometry even applies.
Key terms
- Physical change
- A change in form, phase, or mixing that does not produce a new substance and can usually be reversed.
- Chemical change
- A change where bonds within molecules break and reform, producing a new substance with different properties.
- Intermolecular forces
- Attractions between separate molecules, such as hydrogen bonds, that are weaker than bonds within a molecule.
- Covalent bond
- A bond formed by atoms sharing electrons, holding atoms together within a single molecule.
- Ionic bond
- The electrostatic attraction holding oppositely charged ions together in a crystal lattice.
Watch it explained
- How it worksHow does the bond-breaking involved separate a physical process from a chemical one?
- How it worksCompare what breaks when ice melts, when NaCl dissolves, and when methane burns.
- Common mistakeWhy isn't boiling water an example of splitting water into hydrogen and oxygen?
- Going furtherWhat changes about the classification when a covalent solid like sugar dissolves instead?
Check yourself
End of topic.