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Why does an amorphous solid not have a sharply defined freezing point?

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Amorphous solids, such as glass, do not have a sharply defined freezing point because they do not undergo the abrupt structural transition characteristic of crystallization.12

When a liquid cools to form a crystalline solid, it typically reaches a specific freezing point ( $T_{f}$ ) where the atoms rearrange into a regular, periodic lattice, resulting in a discontinuous change in volume. In contrast, when a material is cooled rapidly enough to bypass crystallization, it enters a glass transition range.12

Within this range, the material's molecular response time increases significantly as the temperature drops. Eventually, the liquid-like mobility of the atoms disappears, and the atomic configuration becomes "frozen" into a disordered, fixed state. Because this process is a gradual transition rather than a sudden phase change, there is no single, sharp temperature at which the material freezes. Instead, the glass transition temperature ( $T_{g}$ ) can shift depending on the cooling rate, reflecting the continuous nature of the transition from an undercooled liquid to an amorphous solid.12