The melting point of a substance called the temperature at which its solid phase is in equilibrium with its own melt.

Melting temperature is the most important characteristic of a compound. By the value of the melting temperature, it is possible to identify the compound, since this constant is always given in reference books on the properties of compounds, for example, /2, 4/.

To identify substances, the so-called. "mixed melting test". To do this, carefully mix equal amounts of the substance that is identified, and knowingly known substance. If the melting point of the mixture remains unchanged, then a conclusion is made that both substances are identical. If the melting point of the sample is lower than the melting point of the initial substances, then, therefore, these substances are different. This method is based on the established fact that pure substances have a clearly defined ("sharp") melting point (with an accuracy of 0.01 ° C). The presence of impurities generally lowers the melting point. In addition, substances containing any impurities melt into temperature range, i.e., they do not have a clearly defined melting point. Thus, the determination of the melting point can provide qualitative information on the degree of purity of a substance.

Determining the melting point also makes it possible to draw indirect conclusions about the possible molecular structure of a substance. For example, it has been found that isomers with symmetrical molecules melt at more high temperature than less symmetrical substances. The melting temperature also increases with increasing degree of molecular association (for example, due to intermolecular hydrogen bonds).

Approximately estimate The melting point of a substance can be measured using an ordinary laboratory thermometer. Several crystals of the test compound are carefully placed directly on the mercury bulb of the thermometer. Next, the thermometer with crystals is carefully placed over the surface of a preheated hotplate with a closed coil. By adjusting the height of the thermometer above the heated surface, the rate of temperature rise is roughly set. Carefully observing alternately the state of the crystals and the temperature value, note Start melting of the substance (the appearance of the first droplets of the liquid phase). This process can be repeated several times, achieving the most accurate determination of the beginning of the melting process. Of course, this method gives only an approximate idea of ​​the melting temperature, but it allows you to significantly simplify the further experiment to accurately determine this constant.

To accurately determine the melting point, there are several structurally different devices of varying degrees of complexity and ease of use, but the principle of their operation is the same. The test compound is placed in a glass capillary (diameter 1 mm, length 40–50 mm) sealed at one end. Previously, the substance is ground in a mortar into a fine powder. To fill the capillary, its open end is immersed in powder, while some of the substance enters the upper part of the capillary. Further (to move the substance to the lower part of the capillary and compact the layer), the capillary is thrown with the sealed end down into a long, narrow, vertically placed glass tube (diameter
10 mm, length 40 - 50 cm). By repeating this technique several times, a dense layer of the substance is obtained in the capillary 3-5 mm high.

Direct determination of the melting temperature is carried out in a special glass device (Figure 5), consisting of a round-bottom flask (1) with a high-boiling coolant, a test tube (2) and a thermometer (3). The capillary (4) with the test substance is attached to the thermometer with a rubber tube ring (5) so that the column of the substance is at the level of the middle of the mercury ball. The device is heated in an air bath (mantle heater, electric stove) at first quickly, and the last 15-20 ° below the expected melting point, the temperature is increased at a rate of no more than 2 deg min -1. The melting point is the temperature at the moment of complete melting of the substance.

Usually, a substance melts in a temperature range, and this interval is the smaller, the purer the substance. The beginning of melting is considered the moment of the appearance of the first drop in the capillary, and the end is the disappearance of the last crystals of the substance.

Melting point ( T pl) solid crystalline substance is the temperature at which it begins to change into liquid state at atmospheric pressure. An absolutely pure individual substance has a strictly defined T sq. However, in common practice, it is rarely possible to bring a substance to a purity close to 100%; therefore, the complete transformation of a solid sample into a liquid occurs in a certain temperature range D T pl = T to - T n, where T to and T n - respectively, the temperature of the beginning and end of melting. These temperatures are usually indicated when characterizing the purity of the substance obtained (including quite often in reference books; for example, in the Chemist's Handbook, vol. II, for P-aminoacetanilide T pl 161 - 162 ° C, for vanillin 81 - 83 ° C, etc.). The purer the substance, the less D T pl. An almost pure substance has D T pl not more than 0.5 ° C. The difference between the beginning and end of melting in 1 ° C indicates good quality received product. It is wrong to take the average value as the melting point ( T n + T j)/2.

The admixture of any other substance that is fully or partially miscible with the test compound lowers its melting point and, as a rule, expands the temperature range D T sq. D value T pl turns out to be overestimated also due to incorrect, too fast, heating of the sample.

Since the substance is completely dry, the beaker can be slightly heated before placing the sample on it and hold the crushed substance on a warm surface for some time (~ 10 minutes). The open end of the capillary is touched to the “hill” of the crushed substance and the crystals that have got inside are pushed down the capillary, throwing it several times with the sealed end down into a tube 60–70 cm long and about 1 cm in diameter, placed vertically on a metal, glass or ceramic surface. The compaction of the sample in the capillary occurs when it hits a solid surface. In this case, due to the elastic deformation of the glass, the capillary jumps several times inside the tube. The height of the substance column in the capillary should be 4-5 mm (no more). The better the substance is compacted in the capillary, the more accurately the melting point can be determined.

The capillary is attached to the thermometer, as mentioned above, and the device begins to heat up.

If the melting point is to be measured to determine the purity of a known product, the apparatus is first heated rapidly to a temperature approximately 10°C lower than known from the handbook. T m of pure substance. After that, the burner is set aside for a short time, but the thermometer still continues to rise due to thermal inertia. Then, carefully dosing the heat supply by placing the burner flame under the grid, very slowly raise the temperature (1 - 2 ° C in 1 minute). The slower the mercury column rises in the thermometer, the more accurately the melting point can be measured.

During heating, the state of the substance in the capillary is monitored. The temperature at which the substance column begins to collapse as a result of the appearance of the liquid phase, decreasing in volume (“shrinks”), is taken as the beginning of melting. At this point, note the thermometer reading ( T n). The rate of heating is slowed down even more and they wait until the moment when the substance in the capillary completely turns into a liquid. This is the end of melting. It corresponds to the thermometer T to.

If it is necessary to determine the melting point of an unknown substance, then, first of all, one should make sure that it is generally capable of melting at a temperature that lies within the usual range of values T pl organic compounds (<300° C). Это можно сделать, нагревая небольшое количество продукта на стеклянной палочке над пламенем горелки. Только убедившись в том, что неизвестное вещество плавится на нагретой стеклянной палочке, можно приступить к определению его температуры плавления в капилляре. В этом случае обычно проводят не менее двух испытаний. В первом опыте T pl is determined approximately at a relatively fast rate of heating. For the second experiment, you should use a newly filled capillary and determine T pl more carefully with a slow increase in temperature, as described above.

In the device shown in Fig. 3 filled with concentrated sulfuric acid, it is forbidden to determine the melting points of substances melting above 200 ° C.

Other precautions to be taken when using a melting point instrument filled with concentrated sulfuric acid should also be noted. Definition of high T pl (180 - 200 ° C) should be carried out with goggles or observe the melting through a protective screen. The outlet (5) of the vessel (Fig. 3) during heating must be turned in the direction where there are no people. If the capillary has fallen to the bottom of the inner vessel, do not try to remove it with a glass rod and in no case turn the device upside down! Do not force-cool a hot appliance with cold water; before re-determining the melting point, the apparatus must be allowed to gradually cool in air.

test questions

1. What are the differences in the properties of a substance and its impurities based on the method of purification of a solid substance by recrystallization?

2. How does the solubility of organic substances usually change with temperature?

3. What properties must a solvent have in order to be suitable for the recrystallization of a substance?

4. How is a solvent suitable for recrystallization of a substance selected in practice?

5. How to properly prepare a hot saturated solution of a substance: a) in water; b) in a highly volatile flammable solvent?

6. How is the removal of impurities of resin products that give the substances a brown-yellow color?

7. Why and how is “hot” filtration carried out?

8. What precautions should be taken when adding activated charcoal to the solution?

9. How and why is the melting point of a substance determined?

The melting and crystallization temperature is the temperature at which a solid crystalline body makes a transition to a liquid state and vice versa. At the melting point, a substance can be in both a liquid and a solid state. When additional heat is added, the substance will go into a liquid state, and the temperature will not change until all substance does not melt in the system under consideration. When removing excess heat (cooling) substance will go into a solid state (harden) and, until it hardens completely, temperature Will not change.

Melting/solidification temperature and boiling/condensing temperature considered important physical properties substances. The solidification temperature coincides with the melting point only for a pure substance. Based on this property, special calibrators thermometers for high temperatures. Since the pour point of a pure substance, such as tin, is stable, it is enough to melt and wait until the melt begins to crystallize. At this time, under the condition of good thermal insulation, the temperature of the solidifying ingot does not change and exactly coincides with reference temperature indicated in the manuals.

Mixtures of substances do not have a melting / solidification temperature at all and make a transition in a certain temperature range (the temperature at which the liquid phase appears is called the point solidus, the temperature of complete melting - point liquidus). Since it is impossible to accurately measure the melting point of such substances, special methods are used ( GOST 20287 and ASTM D 97). But some mixtures eutectic composition) have a certain melting point, as pure substances.

Amorphous (non-crystalline) substances, as a rule, do not have a clear melting point. As the temperature rises viscosity of such substances is reduced and the material becomes more liquid.

There is a common misconception that window panes slowly "slide" down under the influence of gravity. Medieval stained-glass windows are given as an example, the glass of which was thicker at the bottom. The true reason for the uneven thickness is associated with the old technology for the manufacture of window glass. The glassblower would blow a large bubble out of softened glass and flatten it. Narrow glass for stained-glass windows was cut from the resulting disk from the center to the edges. Such glasses were thicker on the side corresponding to the edge of the disc, and when installed in the binding, they were placed with the thick part down.

where r s (\displaystyle r_(s)) is the average elementary cell radius, θ (\displaystyle \theta ) - Debye temperature, and the parameter x m (\displaystyle x_(m)) for most materials varies in the range of 0.15-0.3.

Melting points of some substances