In everyday life, we all now and then encounter phenomena that accompany the processes of transition of substances from one state of aggregation to another. And most often we have to observe such phenomena on the example of one of the most common chemical compounds - well-known and familiar water. From the article you will learn how the transformation of liquid water into solid ice occurs - a process called water crystallization - and what features characterize this transition.
What is a phase transition?
Everyone knows that in nature there are three main aggregate states (phases) of matter: solid, liquid and gaseous. Often a fourth state is added to them - plasma (due to the features that distinguish it from gases). However, when passing from gas to plasma, there is no characteristic sharp boundary, and its properties are determined not so muchthe relationship between the particles of matter (molecules and atoms), how much the state of the atoms themselves.
All substances, passing from one state to another, under normal conditions abruptly change their properties (with the exception of some supercritical states, but we will not touch on them here). Such a transformation is a phase transition, or rather, one of its varieties. It occurs at a certain combination of physical parameters (temperature and pressure), called the phase transition point.
The transformation of a liquid into a gas is evaporation, the reverse phenomenon is condensation. The transition of a substance from a solid to a liquid state is melting, but if the process goes in the opposite direction, then it is called crystallization. A solid body can immediately turn into a gas and vice versa - in these cases they speak of sublimation and desublimation.
During crystallization, water turns into ice and clearly demonstrates how much its physical properties change. Let's dwell on some important details of this phenomenon.
The concept of crystallization
When a liquid solidifies during cooling, the nature of the interaction and arrangement of the particles of the substance changes. The kinetic energy of the random thermal motion of its constituent particles decreases, and they begin to form stable bonds with each other. When molecules (or atoms) line up in a regular, orderly fashion through these bonds, the crystal structure of a solid is formed.
Crystallization does not simultaneously cover the entire volume of the cooled liquid, but begins with the formation of small crystals. These are the so-called centers of crystallization. They grow in layers, stepwise, by adding more and more molecules or atoms of matter along the growing layer.
Crystallization conditions
Crystallization requires cooling the liquid to a certain temperature (it is also the melting point). Thus, the crystallization temperature of water under normal conditions is 0 °C.
For each substance, crystallization is characterized by the amount of latent heat. This is the amount of energy released during this process (and in the opposite case, respectively, the energy absorbed). The specific heat of crystallization of water is the latent heat released by one kilogram of water at 0 °C. Of all the substances near water, it is one of the highest and is about 330 kJ / kg. Such a large value is due to the structural features that determine the parameters of water crystallization. We will use the formula for calculating latent heat below, after considering these features.
To compensate for the latent heat, it is necessary to supercool the liquid in order to start crystal growth. The degree of supercooling has a significant effect on the number of crystallization centers and on the rate of their growth. While the process is proceeding, further cooling of the temperature of the substance does not change.
Water molecule
To better understand how water crystallizes, you need to know how the molecule of this chemical compound is arranged, becausethe structure of a molecule determines the characteristics of the bonds it forms.
One oxygen atom and two hydrogen atoms are combined in a water molecule. They form an obtuse isosceles triangle in which the oxygen atom is located at the apex of an obtuse angle of 104.45°. In this case, oxygen strongly pulls the electron clouds in its direction, so that the molecule is an electric dipole. The charges in it are distributed over the vertices of an imaginary tetrahedral pyramid - a tetrahedron with internal angles of approximately 109 °. As a result, the molecule can form four hydrogen (proton) bonds, which, of course, affects the properties of water.
Features of the structure of liquid water and ice
The ability of a water molecule to form proton bonds is manifested in both liquid and solid states. When water is a liquid, these bonds are quite unstable, easily destroyed, but also constantly formed again. Due to their presence, water molecules are more strongly bonded to each other than particles of other liquids. Associating, they form special structures - clusters. For this reason, the phase points of water are shifted towards higher temperatures, because the destruction of such additional associates also requires energy. Moreover, the energy is quite significant: if there were no hydrogen bonds and clusters, the temperature of water crystallization (as well as its melting) would be –100 °C, and boiling +80 °C.
The structure of clusters is identical to the structure of crystalline ice. Connecting each with four neighbors, water molecules build an openwork crystalline structure with a base in the shape of a hexagon. Unlike liquid water, where microcrystals - clusters - are unstable and mobile due to the thermal movement of molecules, when ice forms, they rearrange themselves in a stable and regular manner. Hydrogen bonds fix the mutual arrangement of the crystal lattice sites, and as a result, the distance between the molecules becomes somewhat larger than in the liquid phase. This circumstance explains the jump in the density of water during its crystallization - the density drops from almost 1 g/cm3 to about 0.92 g/cm3.
About latent heat
Features of the molecular structure of water are very seriously reflected in its properties. This can be seen, in particular, from the high specific heat of crystallization of water. It is due precisely to the presence of proton bonds, which distinguishes water from other compounds that form molecular crystals. It has been established that the hydrogen bond energy in water is about 20 kJ per mole, that is, for 18 g. A significant part of these bonds are established “en masse” when water freezes - this is where such a large return of energy comes from.
Let's give a simple calculation. Let 1650 kJ of energy be released during the crystallization of water. This is a lot: equivalent energy can be obtained, for example, from the explosion of six F-1 lemon grenades. Let us calculate the mass of water that has undergone crystallization. Formula relating the amount of latent heat Q, mass m and specific heat of crystallizationλ is very simple: Q=– λm. The minus sign simply means that heat is given off by the physical system. Substituting the known values, we get: m=1650/330=5 (kg). Only 5 liters are needed for as much as 1650 kJ of energy to be released during the crystallization of water! Of course, the energy is not given away instantly - the process lasts for a sufficiently long time, and the heat is dissipated.
Many birds, for example, are well aware of this property of water, and use it to bask near the freezing water of lakes and rivers, in such places the air temperature is several degrees higher.
Crystallization of solutions
Water is a wonderful solvent. Substances dissolved in it shift the crystallization point, as a rule, downward. The higher the concentration of the solution, the lower the temperature will freeze. A striking example is sea water, in which many different s alts are dissolved. Their concentration in ocean water is 35 ppm, and such water crystallizes at -1.9 °C. The salinity of water in different seas is very different, so the freezing point is different. Thus, the B altic water has a salinity of no more than 8 ppm, and its crystallization temperature is close to 0 °C. Mineralized groundwater also freezes at temperatures below zero. It should be borne in mind that we are always talking only about the crystallization of water: sea ice is almost always fresh, in extreme cases, slightly s alty.
Aqueous solutions of various alcohols also differ in reducedfreezing point, and their crystallization does not proceed abruptly, but with a certain temperature range. For example, 40% alcohol begins to freeze at -22.5°C and finally crystallizes at -29.5°C.
But a solution of such an alkali as caustic soda NaOH or caustic is an interesting exception: it is characterized by an increased crystallization temperature.
How does pure water freeze?
In distilled water, the cluster structure is broken due to evaporation during distillation, and the number of hydrogen bonds between the molecules of such water is very small. In addition, such water does not contain impurities such as suspended microscopic dust particles, bubbles, etc., which are additional centers of crystal formation. For this reason, the crystallization point of distilled water is lowered to -42 °C.
It is possible to supercool distilled water even down to -70 °C. In this state, supercooled water is able to crystallize almost instantly over the entire volume with the slightest shaking or the ingress of an insignificant impurity.
Paradoxical hot water
An amazing fact - hot water turns into a crystalline state faster than cold water - was called the "Mpemba effect" in honor of the Tanzanian schoolboy who discovered this paradox. More precisely, they knew about it in antiquity, however, not finding an explanation, natural philosophers and natural scientists eventually stopped paying attention to the mysterious phenomenon.
In 1963, Erasto Mpemba was surprised thatWarm ice cream mix sets faster than cold ice cream mix. And in 1969, an intriguing phenomenon was confirmed already in a physical experiment (by the way, with the participation of Mpemba himself). The effect is explained by a whole range of reasons:
- more centers of crystallization such as air bubbles;
- high heat dissipation of hot water;
- high rate of evaporation, resulting in a decrease in liquid volume.
Pressure as a crystallization factor
The relationship between pressure and temperature as key quantities that affect the process of water crystallization is clearly reflected in the phase diagram. It can be seen from it that with increasing pressure, the temperature of the phase transition of water from a liquid to a solid state decreases extremely slowly. Naturally, the converse is also true: the lower the pressure, the higher the temperature required for the formation of ice, and it grows just as slowly. To achieve conditions under which water (not distilled!) Is able to crystallize into ordinary ice Ih at the lowest possible temperature of -22 ° C, the pressure must be increased to 2085 atmospheres.
The maximum crystallization temperature corresponds to the following combination of conditions, called the triple point of water: 0.006 atmospheres and 0.01 °C. With such parameters, the points of crystallization-melting and condensation-boiling coincide, and all three states of aggregation of water coexist in equilibrium (in the absence of other substances).
Many types of ice
Currently known about 20 modificationssolid state of water - from amorphous to ice XVII. All of them, except for ordinary Ih ice, require crystallization conditions that are exotic for the Earth, and not all of them are stable. Only ice Ic is very rarely found in the upper layers of the earth's atmosphere, but its formation is not associated with the freezing of water, since it is formed from water vapor at extremely low temperatures. Ice XI was found in Antarctica, but this modification is a derivative of ordinary ice.
By crystallization of water at extremely high pressures, it is possible to obtain such ice modifications as III, V, VI, and with a simultaneous increase in temperature - ice VII. It is likely that some of them can be formed under conditions unusual for our planet on other bodies of the solar system: on Uranus, Neptune or large satellites of the giant planets. One must think that future experiments and theoretical studies of the still little studied properties of these ices, as well as the features of their crystallization processes, will clarify this issue and open up many more new things.
