Freezing: the transition of liquids to solids — process, causes, and uses
Freezing is the phase change in which a liquid becomes a solid. This article explains the physical mechanism, factors that affect freezing, common examples and applications, and notable distinctions such as supercooling.
Overview: Freezing is the physical transition in which a liquid becomes a solid. It is a type of phase change driven by loss of thermal energy: as heat leaves the system, the molecules move more slowly and arrange into a more ordered structure, often forming bonds with neighbors (bonding). The most familiar example is water turning into ice, but freezing applies to many substances and materials. Freezing is the converse of melting and is closely related to other phase changes such as evaporation.
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Physical mechanism
At the microscopic level, freezing generally involves nucleation (the initial formation of small solid clusters) followed by crystal growth. During this change a material releases the latent heat of fusion: removal of that energy is necessary for the transition to proceed. The temperature at which a pure substance freezes is called its freezing point; for water under standard pressure the nominal value is 0 °C, but many factors can alter the observed temperature.
Factors that affect freezing
Freezing behaviour depends on pressure, purity, and presence of solutes or surfaces. Dissolved salts and other impurities typically lower the freezing point (freezing point depression), while increasing pressure can raise or lower the freezing point depending on the substance. Surfaces and particles can promote nucleation, whereas in clean, still conditions liquids can become supercooled and remain liquid below their normal freezing point.
Examples, uses and applications
- Natural: formation of frost, sea ice, and permafrost; seasonal freezing of lakes and rivers.
- Industrial: metal casting and solidification processes often rely on controlled freezing to form desired microstructures; see casting.
- Food and medicine: freezing preserves perishables and is used in cryopreservation and cold-chain logistics.
- Technologies: freeze-drying, ice-templated materials, and refrigeration cycles exploit freezing and thawing steps.
Effects, hazards and notable distinctions
Freeze–thaw cycles can damage infrastructure and biological tissues by expanding water as it solidifies. Some materials form crystalline solids on freezing, while others become amorphous or glassy if cooled too rapidly (vitrification). Controlled freezing is important in manufacturing and biology; uncontrolled freezing may cause structural damage or performance loss.
For further introductory reading and definitions you can follow links on related concepts: liquid, solid, heat, molecules, bonds, ice, evaporation or applications such as casting.
General
Liquids can turn into solids because of various reasons and in various ways. Especially often solidification can be observed at constant pressure by cooling. If it is a pure substance, then the liquid begins to solidify when it reaches the solidification temperature and does not cool further until solidification is complete. The solidification temperature is almost always identical to the melting temperature. A change in pressure at constant temperature can also cause solidification. For most substances, this requires an increase in pressure, while for water and some other substances, a decrease in pressure can also lead to solidification. (See also anomaly of water). Boiling and condensation can also be caused by pressure changes, but for solidification much larger pressure changes are necessary. The relationship between temperature and pressure can be seen in phase diagrams. There you can also see that the solidification line, which separates the liquid area from the solid area in the diagram, is steeper than the melting line between liquid and gaseous.
For many mixtures (and thus also alloys) there is a temperature range, the solidification interval, in which the substance is both solid and liquid. Solidification begins at the liquidus temperature and ends at the solidus temperature. Both depend on the mixing ratio or the proportion of alloying elements and can also be taken from phase diagrams.
Solidification can also occur through chemical reactions. This is the case, for example, when a liquid at room temperature transforms into a substance with a solidification temperature higher than room temperature. The phenomenon also occurs in metallurgy: Liquid metals react with oxygen to form oxides. Liquid aluminum, for example, solidifies at 660 °C, while aluminum oxide is still solid at over 2000 °C.
During solidification, energy is released, the solidification heat. The same amount of energy is needed to melt the material again (heat of fusion).
Property Changes
During solidification, numerous physical properties change abruptly. Almost all physical properties change with the temperature of a body. As long as the body does not change its state of aggregation, these properties usually do not change abruptly, but continuously and very slowly. Changes in volume, density and solubility are of particular importance.
Volume and density
Most substances reduce their volume during solidification and thus increase their density (mass per volume) because of the constant mass. This phenomenon is called solidification shrinkage. Water, on the other hand, expands during solidification. This is why ice floats on liquid water, while most other substances sink in the melt. The expansion when water freezes can cause glass bottles filled with water to burst. In winter, water seeps into small cracks in pavement and rock, expanding and thus enlarging these cracks, which is called frost heave. Thermal expansion, on the other hand, is related to temperature and not to the state of aggregation.
In the foundry, the solidification shrinkage can cause damage to the castings. Especially with complex shaped workpieces, the reduction of the casting is blocked by the mould, which can lead to cracks. These are called hot cracks and can also occur during welding. In order to keep shrinkage as low as possible, silicon is often added to cast alloys, as it expands during solidification and can thus partially compensate for shrinkage.
Solubility
Solubility also changes greatly with solidification. In general, a substance can dissolve less and less of another substance as it cools. During solidification, however, the solubility drops sharply. If impurities are dissolved in a substance, they can be removed by recrystallization or the electroslag remelting process, since the impurities remain primarily in the area that has not yet solidified. In the foundry, gases are often also dissolved in the melt. When the melt cools rapidly, these gases do not have sufficient time to escape from the melt and then remain in the casting where they form bubbles and pores, which reduces the strength of the castings. When these castings are subsequently welded, the gases in the pores expand greatly and can damage the workpiece.
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AlegsaOnline.com Freezing: the transition of liquids to solids — process, causes, and uses Leandro Alegsa
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