The changes of state between a solid and a liquid and between a liquid and a gas are reversible:

Solid–liquid reversible change

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Liquid–gas reversible change

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What do we call the temperature at which a substance freezes?

The temperature at which a substance freezes is its melting point.

To find more detailed coverage of melting and boiling, subscribe to the Analytical chemistry course. Ways of explaining these changes are explored in the Developing and using models course.