Crystallization and melting: graph of the change in the aggregate state of matter

This article tells what crystallization and melting are. The example of various aggregate states of water explains how much heat is required for freezing and thawing and why these values are different. The difference between poly and monocrystals is shown, as well as the complexity of manufacturing the latter.

Transition to another aggregate state

An ordinary person thinks about it rarely, but life at the level at which it exists now would be impossible without science. Which one? The question is not simple, because many processes occur at the junction of several disciplines. Phenomena, for which it is difficult to pinpoint the field of science, are crystallization and melting. It would seem, well, it's complicated here: there was water - there was ice, there was a metal ball - there was a puddle of liquid metal. However, there are no precise mechanisms for the transition from one aggregate state to another. Physicists get deeper into the jungle, but it's still impossible to predict exactly at what point the melting and crystallization of the bodies will begin.

What we know

Somehow, humanity still knows. The melting and crystallization temperature is easily determined empirically. But here everything is not so simple. Everyone knows that the water melts and freezes at zero degrees Celsius. However, water is usually not just a 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 slightly earlier than it reaches exactly zero degrees, the water in the glass is covered with the first ice crystals at a temperature that slightly exceeds this mark on the scale.

Isolation and absorption of heat upon transition to another aggregate state

Crystallization and melting of solids are accompanied by certain thermal effects. In a liquid state, molecules (or sometimes atoms) are not very tightly connected. It is because of this that they have the property of "fluidity". When the body begins to lose heat, atoms and molecules begin to unite in the most convenient structure. Thus, crystallization occurs. Often, depending on external conditions, it turns out that the same carbon is graphite, diamond or fullerene. So, not only the temperature, but also the pressure, affects how crystallization and melting will proceed. However, in order to break the bonds of a rigid crystal structure, it takes a little more energy, and hence more heat, than to create them. Thus, the substance will freeze faster than melting, under the same process conditions. This phenomenon is called latent heat and reflects the difference described above. Recall that latent heat is not related to heat as such and reflects the amount of heat necessary for crystallization and melting to occur.

Volume change upon transition to another aggregate state

As already mentioned, the quantity and quality of bonds in the liquid and solid state are different. The liquid state requires a lot of energy, therefore, the atoms move faster, constantly jumping from one place to another and creating temporary connections. Since the amplitude of the oscillations of the particles is larger, the liquid also occupies a larger volume. Whereas in the solid body of communication rigid, each atom oscillates about one equilibrium position, it is not able to leave its position. This structure takes up less space. So 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, like water, probably plays a big role in the life of almost all living beings. The above is the difference between the amount of heat that is required for crystallization and melting to occur, as well as a change in volume when the aggregate state changes. Some exception to both rules is water. The hydrogen of different molecules, even in the liquid state, combines for a short time, forming a weak, but still non-zero 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 fluidity of water. But their role in freezing (in other words, crystallization) to the end remains unclear. However, it should be recognized: ice of the same mass occupies more volume than liquid water. This fact causes a lot of damage to utility networks and causes many problems to the people serving them.

Not once or twice in the news such messages flash. In winter, an accident occurred in the boiler room of some remote settlement. Due to snowstorms, ice or severe frosts, they did not manage to deliver fuel. The water supplied to the heating batteries and the taps ceased to heat up. If it is not drained in time, leaving the system at least partially empty, or even generally dry, it begins to acquire the temperature of the environment. Most often, as luck would have it, at this time there are severe frosts. And the ice tears 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 Measures, breaking through the snowstorm, throw there a few tons of desired coal on the helicopter, and the unfortunate plumbers change the pipes around the clock round the clock.

Snow and snowflakes

When imagining ice, we often think of cold cubes in a glass with juice or huge spaces of frozen Antarctica. Snow is perceived by people as a special phenomenon, which seems to be not connected with water. But in fact it is the same ice, only frozen in a certain order, which determines the shape. They say that there are no two identical snowflakes in the whole world. The scientist from the US seriously took up the matter and determined the conditions for obtaining these hexagonal beauties of the desired form. His laboratory can even provide a blizzard of snowflakes paid by the client's appearance. 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 into a gaseous aggregate is called sublimation.

Single crystals and polycrystals

Everyone saw the icy patterns on the glass on the bus in winter. They are formed because the temperature inside the transport is above zero Celsius. Moreover, many people, exhaling with air from light vapor, provide increased humidity. But the glass (most often a thin single) has an ambient temperature, that is, negative. Water vapor, touching its surface, very quickly loses heat and passes into a solid state. One crystalline adheres to another, each of the next form slightly differs from the previous one, and beautiful asymmetric patterns grow rapidly. This is an example of polycrystals. "Poly" - from the Latin "a lot." In this case, a number of micro-parts are combined into a single whole. Any metal product is also often a polycrystal. But the perfect shape of the natural quartz prism is a single crystal. In its structure, no one will find flaws and breaks, whereas in polycrystalline volumes the direction of the parts is chaotic and does not agree with each other.

Smartphone and binoculars

But modern technology often requires absolutely pure monocrystals. For example, almost any smartphone contains in its bowels a silicon memory element. No atom in this whole volume should be moved relative to the ideal location. Everyone should take his place. Otherwise, instead of a photo, you will get sounds at the output, and, most likely, unpleasant.

In binoculars, night vision devices, too, large enough monocrystals are needed that convert infrared radiation into visible radiation. There are several ways of growing them, but each requires special thoroughness and verified calculations. How single crystals are obtained, scientists understand from the phase diagrams of the state, that is, they look at the graph of melting and crystallization of matter. It is difficult to compose such a picture, therefore, materials scientists especially appreciate scientists who have decided to find out all the details of this schedule.