Topic 4.4 Physical and Chemical Changes: distinguish physical changes (affecting intermolecular forces) from chemical changes (breaking and forming chemical bonds) and classify processes accordingly.

What this topic is asking

The College Board (Topic 4.4) wants you to draw the line between a physical change and a chemical change at the level of forces and bonds. A physical change overcomes intermolecular forces (or breaks up an ionic lattice) without breaking the chemical bonds within molecules; a chemical change breaks and forms chemical bonds, rearranging atoms into new substances. This sharpens the more intuitive distinction introduced in Topic 4.1.

Forces versus bonds

This is the precise version of the distinction. Boiling water overcomes the hydrogen bonds between $\text{H}_2\text{O}$ molecules, but each molecule remains $\text{H}_2\text{O}$, with its O-H bonds intact, so it is physical. Electrolyzing water breaks those O-H bonds and forms new H-H and O-O bonds, producing $\text{H}_2$ and $\text{O}_2$, so it is chemical. The test is always whether the bonds inside the molecules change.

Why state changes are physical

This is why water vapor is still water, ice is still water, and liquid water is still water: the same molecules persist through every state change. It also explains why a state change can be reversed simply by cooling or heating, while a chemical change generally cannot be reversed so easily, because new substances have formed.

The borderline case of dissolving

Dissolving sits on the boundary and is usually treated as a physical change. When sodium chloride dissolves, the ionic lattice is broken apart and the ions are surrounded by water through ion-dipole forces. No covalent bonds are broken and no new substance forms, so it is physical: evaporating the water recovers the original $\text{NaCl}$. The College Board treats dissolving as a physical change for this reason, even though strong ion-dipole attractions form, because the components are unchanged and recoverable.

Try this

Q1. Classify condensing steam to liquid water, and justify in terms of forces or bonds. [2 points]

• Cue. Physical; intermolecular forces re-form between intact $\text{H}_2\text{O}$ molecules, with no bonds within molecules broken.

Q2. State the single question that decides whether a change is physical or chemical. [1 point]

• Cue. Are the chemical bonds within the molecules broken and re-formed (chemical), or only the forces between particles overcome (physical)?