Matter usually exist in three phases: solid, liquid and gas. When heat or thermal energy is absorbed or given off by a substance, the substance may undergo a change of phase, like in water. At room temperature, water is liquid. When water is heated, it can change into steam. When steam is cooled, it changes back into water. When water is cooled, it change into ice. When ice is heated, it change into water.
Phase Change or Phase Transition is a transition from one phase to another phase. For a given pressure a phase change takes place at a definite temperature, usually accompanied by absorption or emission of heat and a change of volume and density.
Here are the different Phase Changes that may occur around us:
Evaporation is the change of phase from liquid to gaseous. It is a type of vaporization of a liquid that occurs only on the surface of a liquid.
[APPLICATION]
When clothes are hung on a laundry line, even though the ambient temperature is below the boiling point of water, water evaporates. This is accelerated by factors such as low humidity, heat (from the sun), and wind. In a clothes dryer hot air is blown through the clothes, allowing water to evaporate very rapidly.
Sublimation is the change of phase from solid to gaseous without passing the liquid phase. It is an endothermic phase transition that occurs at temperatures and pressures below a substance's triple point in its phase diagram.
[EXAMPLES]
~Carbon dioxide
Solid carbon dioxide (dry ice) sublimates readily at atmospheric pressure at -78.5°C (197.5 °K), while liquid CO2 can be obtained at pressures and temperatures above the triple point (5.2 atm, -56.4°C).
~Water
Snow and ice sublimate, although more slowly, below the melting point temperature. This phenomenon is used in freeze drying, by hanging wet cloth outdoors in freezing weather to be retrieved dry at a later time. The loss of snow from a snowfield during a cold spell is often caused by sunshine acting directly on the outer layers of the snow. Ablation is a process which includes sublimation and erosive wear of glacier ice.
~Naphthalene, a common ingredient in mothballs, also sublimates easily.
~Arsenic can also sublimate at high temperatures. Sublimation requires additional energy and is an endothermic change. The enthalpy of sublimation (also called heat of sublimation) can be calculated as the enthalpy of fusion plus the enthalpy of vaporization.
Condensation is the change of phase from gaseous to liquid. Upon the slowing-down of the atoms/ molecules of the species, the overall attraction forces between these prevail and bring them together at distances comparable to their sizes.
[APPLICATION]
Condensation is a crucial component of distillation, an important laboratory and industrial chemistry application.
Because condensation is a naturally occurring phenomenon, it can often be used to generate water in large quantities for human use. Many structures are made solely for the purpose of collecting water from condensation, such as air wells and fog fences. Such systems can often be used to retain soil moisture in areas where active desertification is occurring—so much so that some organizations educate people living in affected areas about water condensers to help them deal effectively with the situation.
It is also a crucial process in forming particle tracks in a Cloud Chamber. In this case, ions produced by an incident particle act as nucleation centres for the vapour to condense on.
Boiling is a rapid evaporation that takes place within a liquid as well as its surface. This occurs when a liquid is heated to its boiling point, the temperature at which the vapor pressure of the liquid is equal to the pressure exerted on the liquid by the surrounding environmental pressure. While below the boiling point a liquid evaporates from its surface, at the boiling point vapor bubbles come from the bulk of the liquid. For this to be possible, the vapor pressure must be sufficiently high to win the atmospheric pressure, so that the bubbles can be "inflated". Thus, the difference between evaporation and boiling is "mechanical", rather than thermodynamical. The boiling point is lowered when the pressure of the surrounding atmosphere is reduced, for example by the use of a vacuum pump or at high altitudes. Boiling occurs in three characteristic stages, which are nucleate, transition and film boiling. These stages generally take place from low to high heating surface temperatures, respectively.
[APPLICATION]
~Distillation
In distillation, boiling is used to separate mixtures. This is possible because the vapor rising from a boiling fluid generally has a ratio of components different from that in the liquid.
~Boiling for water sterilization
Boiling can be used as a method of water disinfection but is only advocated as an emergency water treatment method, or as a method of portable water purification in rural or wilderness settings without access to a potable water infrastructure. Although bringing water to the boil is effective in killing or inactivating most bacteria, viruses and pathogens, some are resistant and can survive several minutes boiling especially at high altitude where the temperature at which water boils is reduced.
~Boiling in Cookery
In cookery, boiling is the method of cooking food in boiling water, or other water-based liquid such as stock or milk. Simmering is gentle boiling, while in poaching the cooking liquid moves but scarcely bubbles.
Boiling is a very harsh technique of cooking. Delicate foods such as fish cannot be cooked in this fashion because the bubbles can damage the food. Foods such as red meat, chicken, and root vegetables can be cooked with this technique because of their tough texture.
Regelation is the process of melting under pressure and the subsequent refreezing when the pressure is removed. Many textbooks and reference books as well as websites often claim that regelation can be demonstrated by looping a fine wire around a block of ice , with a heavy weight attached to it. The pressure exerted on the ice slowly melts it locally, permitting the wire to pass through the entire block. The wire's track will refill as soon as pressure is relieved, so the ice block will remain solid even after wire passes completely through. This experiment is possible for ice at –10 °C or cooler, and while essentially valid, the details of the process by which the wire passes through the ice are complex. It has been suggested that the heating of the wire under pressure and tension may also play a role.
Regelation was discovered by Michael Faraday. Regelation occurs only for substances, such as ice, that have the property of expanding upon freezing, for the melting points of those substances decrease with increasing external pressure. The melting point of ice falls by 0.0072 °C for each additional atm of pressure applied. For example, a pressure of 500 atmospheres is needed for ice to melt at –4 °C
[EXAMPLE]
A glacier can exert a sufficient amount of pressure on its lower surface to lower the melting point of its ice. The melting of the ice at the glacier's base allows it to move from a higher elevation to a lower elevation. Liquid water may flow from the base of a glacier at lower elevations when the temperature of the air is above the freezing point of water.
Melting is a physical process that results in the phase change of a substance from a solid to a liquid. The internal energy of a substance is increased, typically by the application of heat or pressure, resulting in a rise of its temperature to the melting point, at which the rigid ordering of molecular entities in the solid breaks down to a less-ordered state and the solid liquefies. An object that has melted completely is molten.
Melting point
Under a standard set of conditions, the melting point of a substance is a characteristic property. The melting point is often equal to the freezing point. However, under carefully created conditions, supercooling or superheating past the melting or freezing point can occur. Water on a very clean glass surface will often supercool several degrees below the freezing point without freezing. Fine emulsions of pure water have been cooled to -38 degrees Celsius without nucleation to form ice. Nucleation occurs due to fluctuations in the properties of the material. If the material is kept still there is often nothing (such a physical vibration) to trigger this change, and supercooling (or superheating) may occur. Thermodynamically, the supercooled liquid is in the metastable state with respect to the crystalline phase, and it is likely to crystallize suddenly.
Freezing is a phase change in which a liquid turns into a solid when its temperature is lowered below its freezing point. The reverse process is melting.
All known liquids, except liquid helium, freeze when the temperature is lowered enough. Liquid helium remains liquid at atmospheric pressure even 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 displays a hysteresis in its melting and freezing temperatures. It melts at 85 °C (185 °F) and solidifies from 31 °C to 40 °C (89.6 °F to 104 °F).
[TRIVIAS]
~Bacteria
Three species of bacteria, Carnobacterium pleistocenium, as well as Chryseobacterium greenlandensis and Herminiimonas glaciei, have reportedly been revived after surviving for thousands of years frozen in ice.
~Plants
Many plants undergo a process called hardening which allows them to survive temperatures below 0 °C for weeks to months.
~Animals
The nematode Haemonchus contortus can survive 44 weeks frozen at liquid nitrogen temperatures. Other nematodes that survive at temperatures below 0 °C include Trichostrongylus colubriformis and Panagrolaimus davidi. Many species of reptiles and amphibians survive freezing.
~Human gametes and 2-, 4- and 8-cell embryos can survive freezing and are viable for up to 10 years, a process known as cryopreservation.
~Experimental attempts to freeze human beings for later revival are known as cryonics.
