Question

When water solidifies to ice, then heat is____________.

Answer 1

Hint: The phase transition in which a liquid changes into a solid when the temperature is lowered below a certain degree or point is called Freezing. In case of water, the temperature below which water turns into ice is called freezing point of water. Water starts freezing at \[0^\circ C\] or \[273.15K\].

Complete answer:
Freezing is always an exothermic process, which means that as liquid changes into solid, heat and pressure are released. This might be a bit confusing since the temperature of the material does not rise during freezing. But this can be understood since heat must be continually removed from the freezing liquid or the freezing process will stop. The energy released upon freezing is a latent heat and is known as the latent heat of fusion.
Hence, we can say that when water solidifies to ice, then heat is evolved.
Water turns into ice by the process of crystallization, in which formation of crystalline solids take place. Crystallization consists of two major parts, nucleation and crystal growth. Nucleation is defined as the step in which the molecules start to gather into clusters, on the nanometre scale, arranging in a defined and periodic manner that defines the crystal structure. Crystal growth is the subsequent growth of the nuclei that succeed in achieving the critical cluster size.

Note:
The melting and freezing point of water are the same thing. This means that at \[1\] atmospheric pressure, ice starts melting at \[0^\circ C\] or \[273.15K\]. Under high pressure \[(2000{\text{ atm}})\], water can be supercooled to as low as \[ - 70^\circ C{\text{ }}(203K)\] before it starts turning into ice.