Freezing

Freezing is a phase transition where a liquid turns into a solid. This change happens when the temperature drops below a specific freezing point. For most substances, the melting point and the freezing point are the same temperature. However, some materials show a difference between these two points. This phenomenon is called hysteresis. For example, the substance agar melts at 85 °C. It does not solidify until it reaches 32 to 40 °C.

Most liquids freeze through a process called crystallization. This is a first-order thermodynamic phase transition. It involves the formation of a crystalline solid from a uniform liquid. During this process, the temperature of the system stays very close to the melting point. This happens because heat is removed slowly by contact with air. Air is a poor heat conductor, which slows the process down. This delay is caused by the latent heat of fusion. The temperature will not drop further until the freezing process is finished.

Crystallization occurs in two major stages: nucleation and crystal growth. Nucleation is the first step in the sequence. In this stage, molecules begin to gather into tiny clusters on a nanometer scale. These clusters arrange themselves in a defined, periodic manner to create a crystal structure. Once these nuclei reach a critical cluster size, the second stage begins. This stage is known as crystal growth. The existing nuclei then continue to expand into larger solid structures.

Sometimes, a pure liquid can stay liquid even below its freezing point. This state is known as supercooling. This occurs because of the high activation energy required for homogeneous nucleation. To create a nucleus, an interface must form at the boundaries of the new phase. Forming this interface requires energy based on the surface energy of each phase. If a nucleus is too small, the energy released by its volume cannot cover the cost of its surface. Therefore, the nucleus cannot grow. In pure water at 1 atmosphere of pressure, supercooling can reach -48 °C. Under high pressure of 2,000 atmospheres, water can supercool to as low as -22 °C.

Supercooling can be avoided through heterogeneous nucleation. This happens when there are irregularities on a container surface. It can also happen due to gaseous or solid impurities. These impurities act as nucleators. They allow the freezing point to rise back toward the melting point. Freezing is also an exothermic process. This means that heat and pressure are released as the liquid becomes a solid. The energy released is called the enthalpy of fusion. This is the exact same amount of energy required to melt the solid. Helium is a rare exception to this rule. At temperatures below 0.3 K, Helium-3 has a negative enthalpy of fusion. Below 0.8 K, Helium-4 also shows a slightly negative enthalpy of fusion. This means heat must be added to these substances to freeze them.

Not all substances crystallize when they harden. Some materials, like glass and glycerol, undergo vitrification. These are known as amorphous solids. These materials do not have a specific freezing point. Instead, they undergo a gradual change in viscoelastic properties. This change occurs over a range of temperatures. This process is characterized by a glass transition. This transition occurs at a specific glass transition temperature. This point is often seen as the "knee" on a graph of density versus temperature. Because vitrification is a non-equilibrium process, it is not considered true freezing.

Many living organisms have evolved to survive freezing. Most organisms accumulate cryoprotectants to prevent frost damage. These include glucose, polyols, and anti-nucleating proteins. These substances protect cells from sharp ice crystals. Many plants can safely reach temperatures between -4 °C and -12 °C through a process called hardening. Some bacteria, such as Pseudomonas syringae, actually use ice nucleators. They force ice to form on plants at about -2 °C to access nutrients. Other bacteria, like Chryseobacterium greenlandensis, have been revived after thousands of years in ice. Animals also show amazing resilience. The nematode Haemonchus contortus can survive 44 weeks at liquid nitrogen temperatures. Humans use cryopreservation to freeze gametes and embryos for up to 10 years.

Freezing is also a vital tool for food preservation. It is a widely used method to keep food fresh. Freezing slows down the growth of micro-organisms. It also slows the rate of food decay. This happens because lower temperatures reduce reaction rates. Additionally, freezing makes water less available for bacteria to use. This process generally preserves the smell, flavor, and nutritional content of the food.