In physical science, freezing or solidification is the process in which a liquid turns into a solid when cold enough. The freezing point is the temperature at which this happens. Melting, the process of turning a solid to a liquid, is almost the exact opposite of freezing. All known liquids undergo freezing when the temperature is lowered, with the sole exception of helium, which remains liquid at absolute zero and can only be solidified under pressure. For most substances, the melting and freezing points are the same temperature, however, certain substances possess differing solid-liquid transition temperatures. For example, agar melts at 85 Â°C (185 Â°F) and solidifies from 31 Â°C to 40 Â°C (89.6 Â°F to 104 Â°F); this process is known as thermal hysteresis.
Most liquids freeze by crystallization, formation of crystalline solid from the uniform liquid. This is a first-order thermodynamic phase transition, which means that as long as solid and liquid coexist, the equilibrium temperature of the system remains constant and equal to the melting point. Crystallization consists of two major events, nucleation and crystal growth. Nucleation is the step where the molecules start to gather into clusters, on the nanometer scale, arranging in a defined and periodic manner that defines the crystal structure. The crystal growth is the subsequent growth of the nuclei that succeed in achieving the critical cluster size.
In spite of the second law of thermodynamics, crystallization of pure liquids usually begins at lower temperature than the melting point, due to high activation energy of homogeneous nucleation.
Freezing does not start until the temperature is low enough to provide enough energy to form stable nuclei. In presence of irregularities on the surface of 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 of the previous interface, rising the supercooling point to be near or equal to the melting point. The melting point of water at 1 atmosphere of pressure is very close to 0 Â°C (32 Â°F, 273.15 K), and in the presence of nucleating substances the freezing point of water is close to the melting point, but in the absence of nucleators water can super cool to âˆ’42 Â°C (âˆ’43.6 Â°F, 231 K) before freezing.
Freezing is an exothermic process, meaning that as liquid changes into solid, heat is released. This is often seen as counter-intuitive, since the temperature of 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 type of latent heat known as enthalpy of fusion and is exactly the same as the energy required to melt the same amount of the solid.