Ships At Winter

In spite the second law thermodynamics, crystallization pure liquids usually begins at lower temperature than the melting point, due to high activation energy homogeneous nucleation. The creation a nucleus implies the formation an interface at the boundaries the new phase. Some energy is expended to form this interface, based on the surface energy each phase. If a hypothetical nucleus is too small, the energy that would be released by forming its volume is not enough to create its surface, and nucleation does not proceed. Freezing does not start until the temperature is low enough to provide enough energy to form stable nuclei. In presence irregularities on the surface the containing vessel, solid or gaseous impurities, pre-formed solid crystals, or other nucleators, heterogeneous nucleation may occur, where some energy is released by the partial destruction the previous interface, raising the supercooling point to be near or equal to the melting point. The melting point water at 1 atmosphere pressure is very close to 0 °C (32 °F, 273.15 K), and in the presence nucleating substances the freezing point water is close to the melting point, but in the absence nucleators water can super cool to −40 °C (−40 °F, 233 K) before freezing. Under high pressure (2,000 atmospheres) water will super cool to as low as −70 °C (−94 °F, 203 K) before freezing.
Exothermicity
Freezing is an exothermic process, meaning that as liquid changes into solid, heat is released. This is ten seen as counter-intuitive, since the temperature the material does not rise during freezing, except if the liquid was supercooled, 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 enthalpy fusion and is exactly the same as the energy required to melt the same amount the solid.