Physical Change
A physical change alters the form, size, or state of a substance without changing what it is made of; melting, cutting, and dissolving are physical changes.
Definition
A physical change alters a substance's shape, size, state, or whether it is dissolved, without changing its chemical composition or forming any new substance, so the same molecules remain the same molecules throughout. Phase changes such as melting, freezing, boiling, condensing, and subliming, along with dissolving, grinding, and bending, are all physical changes governed by shifts in intermolecular arrangement or energy rather than by breaking or forming intramolecular chemical bonds, so the chemical identity is preserved. Phase transitions in particular are driven by the balance between intermolecular forces and thermal energy and involve characteristic enthalpies, like fusion and vaporization, without changing any molecular formula, and even dissolving molecular solutes and forming alloys count as physical, though ionic dissolution sits closer to the boundary.
Example
Crumpling paper, melting an ice cube, and cutting an apple are all physical changes, since the paper is still paper, the ice is still water, and the apple is still apple afterward, and many such changes, like melted ice or dissolved salt, can be reversed. When water boils, liquid $H_2O$ becomes gaseous $H_2O$ with each molecule still two hydrogens bonded to one oxygen, and melting ice needs only $6.01\text{ kJ/mol}$, the enthalpy of fusion, to loosen the hydrogen bonds between molecules while leaving the much stronger $463\text{ kJ/mol}$ $O$-$H$ covalent bonds inside each molecule completely untouched.
Key Insight
A change of state is always physical even when it looks dramatic, as when dry ice seems to vanish into a cloud but is really just carbon dioxide switching from solid to gas. Reversibility is a tempting but unreliable test for telling physical from chemical: grinding a crystal to powder is physical yet hard to undo, while some genuinely chemical changes, like the interconversion of $NO_2$ and $N_2O_4$, reverse instantly, so the real question is always whether bonds broke, not whether the change can be undone.