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How does melting differ from freezing?

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Diagram showing four physical states of matter: solid with tightly packed molecules, liquid with loosely packed molecules, gas with widely spaced molecules, and plasma with free electrons and nuclei, each labeled with descriptions and an arrow indicating increasing energy. — phase States of matter.

Melting and freezing are inverse phase transitions between a solid and a liquid state, distinguished primarily by the direction of heat transfer.12

Melting: This process occurs when heat is applied to a solid, causing it to transition into a liquid. For a pure crystalline solid, this happens at a specific melting point.1
Freezing: This is the reverse process, where a liquid becomes a solid. To achieve this, the heat of fusion—the same quantity of energy required to melt the substance—must be removed from the liquid.21

While the melting and freezing points are often the same temperature for pure substances, they can differ in mixtures or certain organic compounds. Additionally, some liquids can be supercooled below their freezing point without immediately forming solid crystals until a trigger, such as a seed crystal, is introduced.2