This article explains what crystallization and melting are. Using the example of various states of aggregation of water, it is explained how much heat is required for freezing and thawing and why these values are different. The difference between poly- and single-crystals is shown, as well as the complexity of manufacturing the latter.
Transition to another aggregate state
An ordinary person rarely thinks about it, but life at the level at which it exists now would be impossible without science. Which one? The question is not easy, because many processes occur at the intersection of several disciplines. Phenomena for which it is difficult to define the field of science precisely are crystallization and melting. It would seem, well, what's so complicated here: there was water - there was ice, there was a metal ball - there was a puddle of liquid metal. However, there are no exact mechanisms for the transition from one state of aggregation to another. Physicists are getting deeper and deeper into the jungle, but it is still not possible to predict exactly at what point the melting and crystallization of bodies will begin.turns out.
What we know
Something humanity still knows. The melting and crystallization temperatures are quite easily determined empirically. But even here everything is not so simple. Everyone knows that water melts and freezes at zero degrees Celsius. However, water is usually not just some theoretical construct, but a specific volume. Do not forget that the process of melting and crystallization is not instantaneous. The ice cube begins to melt a little before reaching exactly zero degrees, the water in the glass is covered with the first ice crystals at a temperature that is slightly above this mark on the scale.
Emission and absorption of heat during the transition to another state of aggregation
Crystallization and melting of solids are accompanied by certain thermal effects. In the liquid state, molecules (or sometimes atoms) are not very tightly bound together. Because of this, they have the property of "fluidity". When the body begins to lose heat, atoms and molecules begin to combine into the structure that is most convenient for them. This is how crystallization occurs. Often it depends on external conditions whether graphite, diamond or fullerene will be obtained from the same carbon. So not only temperature, but also pressure affects how crystallization and melting will proceed. However, to break the bonds of a rigid crystalline structure, it takes a little more energy, and hence the amount of heat, than to create them. Thus,the substance will freeze faster than melt, under the same process conditions. This phenomenon is called latent heat and reflects the difference described above. Recall that latent heat has nothing to do with heat as such and reflects the amount of heat required for crystallization and melting to occur.
Change in volume upon transition to another state of aggregation
As already mentioned, the quantity and quality of bonds in the liquid and solid state are different. The liquid state requires more energy, hence the atoms move faster, constantly jumping from one place to another and creating temporary bonds. Since the amplitude of particle oscillations is greater, the liquid also occupies a larger volume. Whereas in a solid body the bonds are rigid, each atom oscillates around one equilibrium position, it is unable to leave its position. This structure takes up less space. So the melting and crystallization of substances are accompanied by a change in volume.
Features of crystallization and melting of water
Such a common and important liquid for our planet as water, perhaps it is no coincidence that it plays a big role in the life of almost all living beings. The difference between the amount of heat that is required for crystallization and melting to occur, as well as the change in volume when changing the state of aggregation, has been described above. Some exception to both rules is water. Hydrogen of different molecules, even in the liquid state, combines for a short time, forming a weak, but still notzero hydrogen bond. This explains the incredibly high heat capacity of this universal liquid. It should be noted that these bonds do not interfere with the flow of water. But their role during freezing (in other words, crystallization) remains unclear to the end. However, it should be recognized that ice of the same mass occupies more volume than liquid water. This fact causes a lot of damage to public utilities and causes a lot of problems for people serving them.
Such messages appear in the news more than once or twice. In winter, an accident occurred at the boiler house of some remote settlement. Due to snowstorms, ice or severe frosts, we did not have time to deliver fuel. The water supplied to the radiators and taps stopped heating. If it is not drained in time, leaving the system at least partially empty, and preferably completely dry, it begins to acquire ambient temperature. Most often, unfortunately, at this time there are severe frosts. And the ice breaks the pipes, leaving people without a chance for a comfortable life in the coming months. Then, of course, the accident is eliminated, the valiant employees of the Ministry of Emergency Situations, breaking through the blizzard, throw several tons of coveted coal there by helicopter, and the unfortunate plumbers change pipes around the clock in the bitter cold.
Snow and snowflakes
When we think of ice, we most often think of cold cubes in a glass of juice or vast expanses of frozen Antarctica. Snow is perceived by people as a special phenomenon, which seems to benot related to water. But in fact it is the same ice, only frozen in a certain order that determines the shape. They say that there are no two identical snowflakes in the whole wide world. A scientist from the USA got down to business seriously and determined the conditions for obtaining these hexagonal beauties of the desired shape. His lab can even provide a snowflake blizzard of a customer-sponsored skin. By the way, hail, like snow, is the result of a very curious process of crystallization - from steam, not from water. The reverse transformation of a solid body immediately into a gaseous aggregate is called sublimation.
Single crystals and polycrystals
Everyone saw ice patterns on the glass in the bus in winter. They are formed because inside the transport the temperature is above zero Celsius. And besides, many people, exhaling along with the air from light vapors, provide increased humidity. But glass (most often thin single) has an ambient temperature, that is, negative. Water vapor, touching its surface, very quickly loses heat and turns into a solid state. One crystal sticks to another, each successive shape is slightly different from the previous one, and beautiful asymmetric patterns grow rapidly. This is an example of polycrystals. "Poly" is from the Latin "many". In this case, a number of microparts are combined into a single whole. Any metal product is also most often a polycrystal. But the perfect form of the natural prism of quartz is a single crystal. In its structure, no one will find flaws and gaps, while in polycrystalline volumes of the directionparts are arranged randomly and do not agree with each other.
Smartphone and binoculars
But in modern technology, absolutely pure single crystals are often required. For example, almost any smartphone contains a silicon memory element in its bowels. Not a single atom in this entire volume should be moved from its ideal location. Everyone must take their place. Otherwise, instead of a photo, you will get sounds at the output, and, most likely, unpleasant ones.
In binoculars, night vision devices also need sufficiently voluminous monocrystals that convert infrared radiation into visible. There are several ways to grow them, but each requires special care and verified calculations. How single crystals are obtained, scientists understand from phase diagrams of state, that is, they look at the graph of melting and crystallization of a substance. Drawing such a picture is difficult, which is why materials scientists especially appreciate scientists who decide to find out all the details of such a graph.
