Quantity of heat. Specific heat. Melting. Crystallization. Topic: “Melting and crystallization. Specific heat of fusion and crystallization
Melting is the transition of a body from a crystalline solid to a liquid state. Melting occurs with the absorption of the specific heat of fusion and is a first-order phase transition.
The ability to melt refers to the physical properties of a substance.
At normal pressure, tungsten has the highest melting point among metals (3422 ° C), simple substances in general - carbon (according to various sources 3500 - 4500 ° C) and among arbitrary substances - hafnium carbide HfC (3890 ° C). We can assume that helium has the lowest melting point: at normal pressure, it remains liquid at arbitrarily low temperatures.
Many substances at normal pressure do not have a liquid phase. When heated, they immediately pass into the gaseous state by sublimation.
Figure 9 - Melting ice
Crystallization is the process of a phase transition of a substance from a liquid state to a solid crystalline state with the formation of crystals.
A phase is a homogeneous part of a thermodynamic system separated from other parts of the system (other phases) by an interface, when passing through which the chemical composition, structure and properties of a substance change abruptly.
Figure 10 - Crystallization of water with the formation of ice
Crystallization is the process of separating a solid phase in the form of crystals from solutions or melts; in the chemical industry, the crystallization process is used to obtain substances in a pure form.
Crystallization begins when a certain limiting condition is reached, for example, supercooling of a liquid or supersaturation of a vapor, when many small crystals appear almost instantly - centers of crystallization. Crystals grow by attaching atoms or molecules from a liquid or vapor. The growth of crystal faces occurs layer by layer, the edges of incomplete atomic layers (steps) move along the face during growth. The dependence of the growth rate on crystallization conditions leads to a variety of growth forms and crystal structures (polyhedral, lamellar, acicular, skeletal, dendritic and other forms, pencil structures, etc.). In the process of crystallization, various defects inevitably arise.
The number of crystallization centers and the growth rate are significantly affected by the degree of supercooling.
The degree of supercooling is the level of cooling of a liquid metal below the temperature of its transition into a crystalline (solid) modification. It is necessary to compensate for the energy of the latent heat of crystallization. Primary crystallization is the formation of crystals in metals (and alloys) during the transition from a liquid to a solid state.
Specific heat of fusion (also: enthalpy of fusion; there is also an equivalent concept of specific heat of crystallization) is the amount of heat that must be imparted to one unit of mass of a crystalline substance in an equilibrium isobaric-isothermal process in order to transfer it from a solid (crystalline) state to a liquid (that the same amount of heat is released during the crystallization of a substance).
The amount of heat during melting or crystallization: Q=ml
Evaporation and boiling. Specific heat of vaporization
Evaporation is the process of transition of a substance from a liquid state to a gaseous state (steam). The evaporation process is the reverse of the condensation process (transition from a vapor to a liquid state. Evaporation (vaporization), the transition of a substance from a condensed (solid or liquid) phase to a gaseous (steam); first-order phase transition.
There is a more detailed concept of evaporation in higher physics
Evaporation is a process in which particles (molecules, atoms) fly out (tear off) from the surface of a liquid or solid, while Ek > Ep.
Figure 11 - Evaporation over a mug of tea
The specific heat of vaporization (vaporization) (L) is a physical quantity showing the amount of heat that must be reported to 1 kg of a substance taken at the boiling point in order to transfer it from a liquid state to a gaseous state. The specific heat of vaporization is measured in J/kg.
Boiling is the process of vaporization in a liquid (the transition of a substance from a liquid to a gaseous state), with the appearance of phase separation boundaries. The boiling point at atmospheric pressure is usually given as one of the main physicochemical characteristics of a chemically pure substance.
Boiling is a first-order phase transition. Boiling occurs much more intensively than evaporation from the surface, due to the formation of foci of vaporization, due to both the boiling point reached and the presence of impurities.
The process of bubble formation can be influenced by pressure, sound waves, ionization. In particular, it is on the principle of boiling up of liquid microvolumes from ionization during the passage of charged particles that the bubble chamber operates.
Figure 12 - Boiling water
The amount of heat during boiling, liquid evaporation and vapor condensation: Q=mL
Specific heat of fusion(also: enthalpy of fusion; there is also an equivalent concept specific heat of crystallization) is a physical quantity showing how much heat must be imparted to one unit of mass of a crystalline substance in an equilibrium isobaric-isothermal process in order to transfer it from a solid (crystalline) state to a liquid at the melting point (the same amount of heat is released during the crystallization of a substance).
The heat of fusion is a special case of the heat of a first-order phase transition.
Distinguish specific heat of fusion (J / kg) and molar (J / mol).
The specific heat of fusion is denoted by the letter (greek letter lambda). The formula for calculating the specific heat of fusion: , where - specific heat of fusion, - the amount of heat received by the substance during melting (or released during crystallization), - the mass of the melting (crystallizing) substance.
| Substance | Specific heat of fusion (kJ/kg) |
|---|---|
| Aluminum | |
| Iron | |
| Gold | |
| Ice | |
| Copper | |
| Naphthalene | |
| Tin | |
| Platinum | |
| Lead | |
| Silver | |
| Zinc | |
| Cast iron (white) | |
| Cast iron (grey) |
See also
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Literature
- Enohovich A. S. Brief reference book on physics. - M .: "Higher School", 1976. - S. 114. - 288 p.
An excerpt characterizing the specific heat of fusion
The night was dark, warm, autumnal. It has been raining for the fourth day. Having changed horses twice and galloping thirty miles along a muddy, viscous road in an hour and a half, Bolkhovitinov was at Letashevka at two o'clock in the morning. Climbing down at the hut, on the wattle fence of which there was a sign: "General Staff", and leaving the horse, he entered the dark passage.- The general on duty soon! Very important! he said to someone who was getting up and snuffling in the darkness of the passage.
“From the evening they were very unwell, they didn’t sleep for the third night,” whispered the orderly voice intercessively. “Wake up the captain first.
“Very important, from General Dokhturov,” said Bolkhovitinov, entering the open door he felt for. The orderly went ahead of him and began to wake someone:
“Your honor, your honor is a courier.
- I'm sorry, what? from whom? said a sleepy voice.
- From Dokhturov and from Alexei Petrovich. Napoleon is in Fominsky,” said Bolkhovitinov, not seeing in the darkness the one who asked him, but from the sound of his voice, assuming that it was not Konovnitsyn.
The awakened man yawned and stretched.
“I don’t want to wake him up,” he said, feeling something. - Sick! Maybe so, rumors.
“Here is the report,” said Bolkhovitinov, “it was ordered to immediately hand it over to the general on duty.
- Wait, I'll light the fire. Where the hell are you always going to put it? - Turning to the batman, said the stretching man. It was Shcherbinin, Konovnitsyn's adjutant. “I found it, I found it,” he added.
The orderly cut down the fire, Shcherbinin felt the candlestick.
“Oh, the nasty ones,” he said in disgust.
By the light of the sparks, Bolkhovitinov saw the young face of Shcherbinin with a candle and in the front corner of a still sleeping man. It was Konovnitsyn.
ESSAY
"Melting bodies"
Performed:
Prisyazhnyuk Olga 9-A
Checked:
Nevzorova Tatyana Igorevna
Introduction
1) Calculation of the amount of heat
2) Melting
3) Specific heat of fusion
4) Melting of metals
5) Melting point and boiling point of water
6) Melts
7) Interesting about melting
Conclusion (conclusions)
List of used literature
Introduction
Aggregate state - a state of matter characterized by certain qualitative properties: the ability or inability to maintain volume and shape, the presence or absence of long-range and short-range order, and others. A change in the state of aggregation may be accompanied by a jump-like change in free energy, entropy, density, and other basic physical properties.
There are three main states of aggregation: solid, liquid and gas. Sometimes it is not quite correct to classify plasma as a state of aggregation. There are other states of aggregation, for example, liquid crystals or Bose-Einstein condensate.
Changes in the state of aggregation are thermodynamic processes called phase transitions. The following varieties are distinguished: from solid to liquid - melting; from liquid to gaseous - evaporation and boiling; from solid to gaseous - sublimation; from gaseous to liquid or solid - condensation. A distinctive feature is the absence of a sharp boundary of the transition to the plasma state.
To describe various states in physics, a broader concept of a thermodynamic phase is used. Phenomena that describe transitions from one phase to another are called critical phenomena.
Solid: A state characterized by the ability to maintain volume and shape. Atoms of a solid body make only small vibrations around the state of equilibrium. There is both long-range and short-range order.
Liquid: The state of a substance in which it has low compressibility, that is, it retains volume well, but is unable to retain its shape. The liquid easily takes the shape of the vessel in which it is placed. Atoms or molecules of a liquid vibrate near the equilibrium state, locked by other atoms, and often jump to other free places. There is only short-range order.
Gas: A state characterized by good compressibility, lacking the ability to retain both volume and shape. Gas tends to occupy the entire volume provided to it. Atoms or molecules of a gas behave relatively freely, the distances between them are much greater than their size.
Other states: Upon deep cooling, some (by far not all) substances pass into a superconducting or superfluid state. These states, of course, are separate thermodynamic phases, but they hardly deserve to be called new aggregate states of matter due to their non-universality. Inhomogeneous substances such as pastes, gels, suspensions, aerosols, etc., which under certain conditions exhibit the properties of both solids and liquids and even gases, are usually classified as dispersed materials, and not to any specific aggregate states of matter .
Melting
Rice. 1. State of pure matter (diagram)
Rice. 2. Melting temperature of a crystalline body
Rice. 3. Melting point of alkali metals
Melting - the transition of a substance from a crystalline (solid) state to a liquid; occurs with the absorption of heat (phase transition of the first order). The main characteristics of P. of pure substances are the melting point (Tmelt) and the heat that is necessary for the implementation of the P. process (heat of melting Qmelt).
P.'s temperature depends on the external pressure p; on the state diagram of a pure substance, this dependence is depicted by the melting curve (the curve of the coexistence of solid and liquid phases, AD or AD "in Fig. 1). The melting of alloys and solid solutions occurs, as a rule, in the temperature range (with the exception of eutectics with a constant Tmelt) The dependence of the temperature of the beginning and end of the P. of an alloy on its composition at a given pressure is depicted on state diagrams by special lines (liquidus and solidus curves, see Binary systems). from a solid crystalline state to an isotropic liquid occurs in stages (in a certain temperature range), each stage characterizes a certain stage in the destruction of the crystalline structure.
The presence of a certain temperature P. is an important sign of the correct crystalline structure of solids. On this basis, they are easy to distinguish from amorphous solids, which do not have a fixed Tm. Amorphous solids pass into the liquid state gradually, softening with increasing temperature (see Amorphous state). Tungsten has the highest temperature among pure metals (3410°C), and mercury has the lowest temperature (-38.9°C). Particularly refractory compounds include: TiN (3200 °C), HfN (3580 °C), ZrC (3805 °C), TaC (4070 °C), HfC (4160 °C), etc. As a rule, for substances with high Tm is characterized by higher values of Qm. Impurities present in crystalline substances reduce their Tm. This is used in practice to obtain alloys with low Tmelt (see, for example, Wood's alloy with Tmelt = 68 °C) and cooling mixtures.
P. begins when the crystalline substance reaches Tpl. From the beginning of P. until its completion, the temperature of the substance remains constant and equal to Tmelt, despite the transfer of heat to the substance (Fig. 2). It is not possible under normal conditions to heat a crystal to T > Tmelt (see Overheating), while during crystallization a significant supercooling of the melt is achieved relatively easily.
The nature of the dependence of Tm on pressure p is determined by the direction of volumetric changes (DVm) at P. (see Clapeyron-Clausius equation). In most cases, P. of a substance is accompanied by an increase in their volume (usually by several percent). If this is the case, then an increase in pressure leads to an increase in Tm (Fig. 3). However, in some substances (water, a number of metals and metallides, see Fig. 1), during P., a decrease in volume occurs. The temperature of P. of these substances decreases with increasing pressure.
P. is accompanied by a change in the physical properties of the substance: an increase in entropy, which reflects the disorder of the crystalline structure of the substance; an increase in heat capacity, electrical resistance [with the exception of some semimetals (Bi, Sb) and semiconductors (Ge), which have a higher electrical conductivity in the liquid state]. During P., the shear resistance drops to almost zero (transverse elastic waves cannot propagate in the melt, see Liquid), the speed of propagation of sound (longitudinal waves), etc. decreases.
According to molecular and kinetic representations, P. is carried out as follows. When heat is applied to a crystalline body, the energy of vibrations (oscillation amplitude) of its atoms increases, which leads to an increase in body temperature and contributes to the formation of various kinds of defects in the crystal (unfilled nodes of the crystal lattice - vacancies; violations of the periodicity of the lattice by atoms embedded between its nodes, etc. ., see Defects in crystals). In molecular crystals, partial disordering of the mutual orientation of the axes of molecules can occur if the molecules do not have a spherical shape. A gradual increase in the number of defects and their association characterize the premelting stage. When Tmelt is reached, a critical concentration of defects is created in the crystal, and crystallization begins; the crystal lattice breaks up into easily mobile submicroscopic regions. The heat supplied during P. is used not to heat the body, but to break interatomic bonds and destroy long-range order in crystals (see Long-range order and short-range order). In the submicroscopic regions themselves, on the other hand, the short-range order in the arrangement of atoms does not change significantly at melting point (the coordination number of the melt at Tmelt in most cases remains the same as that of the crystal). This explains the lower values of the heats of fusion Qm compared to the heats of vaporization and the relatively small change in a number of physical properties of substances during their P.
The pyrolysis process plays an important role in nature (the pyrolysis of snow and ice on the earth's surface, the mineralization of minerals in its depths, and so on) and in technology (the production of metals and alloys, mold casting, etc.).
Specific heat of fusion
Specific heat of fusion (also: enthalpy of fusion; there is also an equivalent concept of specific heat of crystallization) - the amount of heat that must be imparted to one unit of mass of a crystalline substance in an equilibrium isobaric-isothermal process in order to transfer it from a solid (crystalline) state to a liquid (same the amount of heat released during the crystallization of a substance). The heat of fusion is a special case of the heat of a first-order phase transition. Distinguish specific heat of fusion (J/kg) and molar (J/mol).
The specific heat of fusion is indicated by the letter (Greek letter lambda) The formula for calculating the specific heat of fusion is:
where is the specific heat of fusion, is the amount of heat received by the substance during melting (or released during crystallization), is the mass of the melting (crystallizing) substance.
Melting of metals
When melting metals, certain rules must be observed. Suppose that they are going to melt lead and zinc. Lead will melt quickly, having a melting point of 327°; zinc, on the other hand, will remain solid for a long time, since its melting point is above 419 °. What will happen to lead with such overheating? It will begin to be covered with a film of iridescent color, and then its surface will be hidden under a layer of non-melting powder. Lead burned out from overheating, oxidized by combining with oxygen in the air. This process, as you know, occurs at ordinary temperature, but when heated, it goes much faster. Thus, by the time the zinc begins to melt, there will be very little metallic lead left. The alloy will turn out to be completely different composition, as expected, and a large amount of lead will be lost in the form of waste. It is clear that we must first melt the more refractory zinc and then put lead into it. The same thing will happen if zinc is alloyed with copper or brass, first heating the zinc. Zinc will burn by the time the copper melts. This means that you must always first melt the metal with a higher melting point.
But this one can not avoid the frenzy. If a properly heated alloy is kept on fire for a long time, a film is again formed on the surface of the liquid metal as a result of fumes. It is clear that the more fusible metal will again turn into oxide and the composition of the plug will change; This means that the metal cannot be overheated for a long time unnecessarily. Therefore, they try in every possible way to reduce the waste of the metal, laying it in a compact mass; small pieces, sawdust, shavings are first “packaged”, pieces of more or less the same size are melted, they are heated at a sufficient temperature, and the metal surface is protected from contact with air. For this purpose, the master can take a borax or simply cover the surface of the metal with a layer of ash, which will always float on top (due to its lower specific gravity) and will not interfere when pouring the metal. When the metal solidifies, another phenomenon occurs, probably also familiar to young craftsmen. The metal, solidifying, decreases in volume, and this decrease occurs due to internal, not yet solidified metal particles. A more or less significant funnel-shaped depression, the so-called shrinkage cavity, is formed on the surface of the casting or inside it. Usually, the mold is made in such a way that shrinkage holes are formed in those places of the casting, which are subsequently removed, trying to protect the product itself as much as possible. It is clear that shrinkage holes spoil the casting and can sometimes make it unusable. After melting, the metal is slightly superheated so that it is thinner and hotter and therefore better fills the details of the mold and does not freeze prematurely from contact with a colder mold.
Since the melting point of alloys is usually lower than the melting point of the most refractory of the metals that make up the alloy, it is sometimes beneficial to do the opposite: first melt the more fusible metal, and then the more refractory. However, this is permissible only for metals that are not strongly oxidized, or provided that these metals are protected from excessive oxidation. It is necessary to take more metal than is required for the thing itself, so that it fills not only the mold, but also the sprue channel. It is clear that you must first calculate the required amount of metal.
Melting and boiling point of water
The most surprising and blissful property of water for living nature is its ability to be a liquid under "normal" conditions. Molecules of compounds very similar to water (for example, H2S or H2Se molecules) are much heavier, but form a gas under the same conditions. Thus, water seems to contradict the laws of the periodic table, which, as you know, predicts when, where and what properties of substances will be close. In our case, it follows from the table that the properties of hydrogen compounds of elements (called hydrides) located in the same vertical columns should change monotonically with increasing mass of atoms. Oxygen is an element of the sixth group of this table. In the same group are sulfur S (with an atomic weight of 32), selenium Se (with an atomic weight of 79), tellurium Te (with an atomic weight of 128) and pollonium Po (with an atomic weight of 209). Consequently, the properties of the hydrides of these elements should change monotonously when passing from heavy elements to lighter ones, i.e. in the sequence H2Po → H2Te → H2Se → H2S → H2O. Which is what happens, but only with the first four hydrides. For example, the boiling and melting points rise as the atomic weight of the elements increases. In the figure, the crosses mark the boiling points of these hydrides, and the circles mark the melting points.
As can be seen, as the atomic weight decreases, the temperatures decrease quite linearly. The area of existence of the liquid phase of hydrides becomes more and more "cold", and if the oxygen hydride H2O were a normal compound, similar to its neighbors in the sixth group, then liquid water would exist in the range from -80 ° C to -95 ° C. At more At high temperatures, H2O would always be a gas. Fortunately for us and all life on Earth, water is anomalous, it does not recognize a periodic pattern, but follows its own laws.
This is explained quite simply - most of the water molecules are connected by hydrogen bonds. It is these bonds that distinguish water from liquid hydrides H2S, H2Se, and H2Te. If they were not, then the water would boil already at minus 95 ° C. The energy of hydrogen bonds is quite high, and they can be broken only at a much higher temperature. Even in the gaseous state, a large number of H2O molecules retain their hydrogen bonds, combining to form (H2O)2 dimers. Fully hydrogen bonds disappear only at a water vapor temperature of 600 °C.
Recall that boiling consists in the fact that vapor bubbles form inside a boiling liquid. At normal pressure, pure water boils at 100 "C. If heat is supplied through the free surface, the process of surface evaporation will be accelerated, but volumetric evaporation characteristic of boiling does not occur. Boiling can also be carried out by lowering the external pressure, since in this case the pressure vapor equal to the external pressure is achieved at a lower temperature.At the top of a very high mountain, the pressure and, accordingly, the boiling point are so low that the water becomes unsuitable for cooking - the required temperature of the water is not reached.With a sufficiently high pressure, water can be heated so much that in it can melt lead (327°C) and yet it will not boil.
In addition to super-large melting boiling points (and the latter process requires too much heat of fusion for such a simple liquid), the very range of existence of water is anomalous - one hundred degrees by which these temperatures differ - a rather large range for such a low molecular weight liquid as water. The limits of allowable values of hypothermia and overheating of water are unusually large - with careful heating or cooling, water remains liquid from -40 ° C to +200 ° C. This extends the temperature range in which water can remain liquid to 240 °C.
When ice is heated, its temperature first rises, but from the moment the mixture of water and ice is formed, the temperature will remain unchanged until all the ice has melted. This is explained by the fact that the heat supplied to the melting ice is primarily spent only on the destruction of crystals. The temperature of melting ice remains unchanged until all crystals are destroyed (see latent heat of fusion).
melts
Melts are a liquid molten state of substances at temperatures within certain limits remote from the critical melting point and located closer to the melting point. The nature of melts is inherently determined by the type of chemical bonds of elements in the molten substance.
Melts are widely used in metallurgy, glass-making and other fields of technology. Melts usually have a complex composition and contain various interacting components (see phase diagram).
Melts are
1. Metallic (Metals (the name comes from the Latin metallum - mine, mine) - a group of elements with characteristic metallic properties, such as high thermal and electrical conductivity, positive temperature coefficient of resistance, high ductility and metallic luster);
2. Ionic (Ion (ancient Greek ἰόν - going) - a monatomic or polyatomic electrically charged particle formed as a result of the loss or addition of one or more electrons to an atom or molecule. Ionization (the process of formation of ions) can occur at high temperatures, under by the influence of an electric field);
3. Semiconductor with covalent bonds between atoms (Semiconductors - materials that, in terms of their conductivity, occupy an intermediate place between conductors and dielectrics and differ from conductors in a strong dependence of conductivity on impurity concentration, temperature and various types of radiation. The main property of these materials is an increase in electrical conductivity with increasing temperature);
4. Organic melts with van der Waals bonds;
5. High polymer (Polymers (Greek πολύ- - many; μέρος - part) - inorganic and organic, amorphous and crystalline substances obtained by repeated repetition of various groups of atoms, called "monomeric units", connected into long macromolecules by chemical or coordination bonds)
Melts according to the type of chemical compounds are:
1. Salt;
2.Oxide;
3. Oxide-silicate (slag), etc.
Melts with special properties:
1.Eutectic
Interesting about melting
Ice grains and stars.
Take a piece of pure ice into a warm room and watch it melt. It will quickly become clear that the ice, which seemed monolithic and homogeneous, breaks up into many small grains - individual crystals. In the volume of ice, they are located randomly. An equally interesting picture can be seen when the ice melts from the surface.
Bring a smooth piece of ice to the lamp and wait until it begins to melt. When melting touches the inner grains, very fine patterns will begin to appear there. With a strong magnifying glass, you can see that they have the shape of hexagonal snowflakes. In fact, these are melted depressions filled with water. The shape and direction of their rays correspond to the orientation of ice single crystals. These patterns are called "Tyndall stars" after the English physicist who discovered and described them in 1855. "Tyndall stars", similar to snowflakes, are actually depressions on the surface of melted ice, about 1.5 mm in size, filled with water. In their center, air bubbles are visible, which have arisen due to the difference in the volumes of melted ice and melt water.
DID YOU KNOW?
There is a metal, the so-called Wood's alloy, which can be easily melted even in warm water (+68 degrees Celsius). So when stirring sugar in a glass, a metal spoon made of this alloy will melt faster than sugar!
The most refractory substance, tantalum carbide TaCO-88, melts at a temperature of 3990°C.
In 1987, German researchers were able to supercool water to -700C while keeping it in a liquid state.
Sometimes, to make the snow on the sidewalks melt faster, they are sprinkled with salt. The melting of ice occurs because a solution of salt in water is formed, the freezing point of which is lower than the air temperature. The solution just flows off the sidewalk.
Interestingly, feet get colder more on wet pavement, as the temperature of the salt-water solution is lower than that of pure snow.
If you pour tea from a teapot into two mugs: with sugar and without sugar, then the tea in a mug with sugar will be colder, because. the dissolution of sugar (the destruction of its crystal lattice) also consumes energy.
In severe frosts, to restore the smoothness of the ice, the ice rink is watered with hot water. Hot water melts the thin top layer of ice, does not freeze so quickly, has time to spread, and the ice surface is very smooth.
Conclusion (conclusions)
Melting is the transition of a substance from a solid to a liquid state.
When heated, the temperature of the substance increases, and the speed of thermal motion of particles increases, while the internal energy of the body increases.
When the temperature of a solid reaches its melting point, the crystal lattice of the solid begins to break down. Thus, the main part of the energy of the heater, conducted to the solid body, is spent on reducing the bonds between the particles of the substance, i.e., on the destruction of the crystal lattice. In this case, the energy of interaction between particles increases.
The molten substance has a greater store of internal energy than in the solid state. The remaining part of the heat of fusion is spent on doing work to change the volume of the body during its melting.
During melting, the volume of most crystalline bodies increases (by 3-6%), and decreases during solidification. But, there are substances in which, when melted, the volume decreases, and when solidified, it increases. These include, for example, water and cast iron, silicon and some others. . That is why ice floats on the surface of the water, and solid cast iron - in its own melt.
Solids called amorphous (amber, resin, glass) do not have a specific melting point.
The amount of heat required to melt a substance is equal to the product of the specific heat of fusion times the mass of the substance.
The specific heat of fusion shows how much heat is needed to completely convert 1 kg of a substance from a solid to a liquid state, taken at the melting rate.
The unit of specific heat of fusion in SI is 1J/kg.
During the melting process, the temperature of the crystal remains constant. This temperature is called the melting point. Each substance has its own melting point.
The melting point for a given substance depends on atmospheric pressure.
List of used literature
1) Data from the electronic free encyclopedia "Wikipedia"
http://ru.wikipedia.org/wiki/Main_page
2) Site "Class! Physics for the curious" http://class-fizika.narod.ru/8_11.htm
3) Website "Physical properties of water"
http://all-about-water.ru/boiling-temperature.php
4) Website "Metals and structures"
http://metaloconstruction.ru/osnovy-plavleniya-metallov/
Melting
Melting It is the process of changing a substance from a solid to a liquid state.
Observations show that if crushed ice, having, for example, a temperature of 10 ° C, is left in a warm room, then its temperature will rise. At 0 °C, the ice will begin to melt, and the temperature will not change until all the ice has turned into a liquid. After that, the temperature of the water formed from the ice will rise.
This means that crystalline bodies, which include ice, melt at a certain temperature, which is called melting point. It is important that during the melting process the temperature of the crystalline substance and the liquid formed during its melting remains unchanged.
In the experiment described above, the ice received a certain amount of heat, its internal energy increased due to an increase in the average kinetic energy of the movement of molecules. Then the ice melted, its temperature did not change, although the ice received a certain amount of heat. Consequently, its internal energy increased, but not due to the kinetic, but due to the potential energy of the interaction of molecules. The energy received from the outside is spent on the destruction of the crystal lattice. Similarly, the melting of any crystalline body occurs.
Amorphous bodies do not have a specific melting point. As the temperature rises, they gradually soften until they turn into a liquid.
Crystallization
Crystallization is the process by which a substance changes from a liquid state to a solid state. Cooling, the liquid will give off a certain amount of heat to the surrounding air. In this case, its internal energy will decrease due to a decrease in the average kinetic energy of its molecules. At a certain temperature, the process of crystallization will begin, during this process the temperature of the substance will not change until the entire substance passes into a solid state. This transition is accompanied by the release of a certain amount of heat and, accordingly, a decrease in the internal energy of the substance due to a decrease in the potential energy of interaction of its molecules.
Thus, the transition of a substance from a liquid state to a solid state occurs at a certain temperature, called the crystallization temperature. This temperature remains constant throughout the melting process. It is equal to the melting point of this substance.
The figure shows a graph of the dependence of the temperature of a solid crystalline substance on time in the process of heating it from room temperature to the melting point, melting, heating the substance in the liquid state, cooling the liquid substance, crystallization and subsequent cooling of the substance in the solid state.
Specific heat of fusion
Different crystalline substances have different structures. Accordingly, in order to destroy the crystal lattice of a solid at its melting point, it is necessary to inform it of a different amount of heat.
Specific heat of fusion is the amount of heat that must be imparted to 1 kg of a crystalline substance in order to turn it into a liquid at its melting point. Experience shows that the specific heat of fusion is specific heat of crystallization .
The specific heat of fusion is denoted by the letter λ . Unit of specific heat of fusion - [λ] = 1 J/kg.
The values of the specific heat of fusion of crystalline substances are given in the table. The specific heat of melting of aluminum is 3.9 * 10 5 J / kg. This means that for the melting of 1 kg of aluminum at the melting temperature, it is necessary to spend an amount of heat of 3.9 * 10 5 J. The increase in internal energy of 1 kg of aluminum is equal to the same value.
To calculate the amount of heat Q, required to melt a substance with a mass m, taken at the melting point, follows the specific heat of fusion λ multiply by the mass of the substance: Q = λm.
The same formula is used when calculating the amount of heat released during the crystallization of a liquid.
The transition of a substance from a solid crystalline state to a liquid state is called melting. To melt a solid crystalline body, it must be heated to a certain temperature, that is, heat must be supplied.The temperature at which a substance melts is calledthe melting point of the substance.
The reverse process - the transition from a liquid to a solid state - occurs when the temperature drops, that is, heat is removed. The transition of a substance from a liquid to a solid state is calledhardening , or crystallysis . The temperature at which a substance crystallizes is calledcrystal temperaturetions .
Experience shows that any substance crystallizes and melts at the same temperature.
The figure shows a graph of the dependence of the temperature of a crystalline body (ice) on the heating time (from the point BUT to the point D) and cooling time (from point D to the point K). It shows time on the horizontal axis and temperature on the vertical axis.
It can be seen from the graph that the observation of the process began from the moment when the temperature of the ice was -40 °C, or, as they say, the temperature at the initial moment of time tearly= -40 °С (point BUT on the chart). With further heating, the temperature of the ice increases (on the graph, this is the area AB). The temperature rises to 0 °C, the melting point of ice. At 0°C, ice begins to melt and its temperature stops rising. During the entire melting time (i.e., until all the ice has melted), the temperature of the ice does not change, although the burner continues to burn and heat is therefore supplied. The melting process corresponds to the horizontal section of the graph sun . Only after all the ice has melted and turned into water does the temperature begin to rise again (section CD). After the water temperature reaches +40 ° C, the burner is extinguished and the water begins to cool, i.e. heat is removed (for this, a vessel with water can be placed in another, larger vessel with ice). The water temperature begins to drop (section DE). When the temperature reaches 0 °C, the temperature of the water stops decreasing, despite the fact that heat is still removed. This is the process of crystallization of water - the formation of ice (horizontal section EF). Until all the water turns to ice, the temperature will not change. Only after this does the temperature of the ice begin to decrease (section FK).
The view of the considered graph is explained as follows. Location on AB due to the heat input, the average kinetic energy of the ice molecules increases, and its temperature rises. Location on sun all the energy received by the contents of the flask is spent on the destruction of the crystal lattice of ice: the ordered spatial arrangement of its molecules is replaced by disordered, the distance between the molecules changes, i.e. molecules are rearranged in such a way that the substance becomes liquid. The average kinetic energy of the molecules does not change, so the temperature remains unchanged. A further increase in the temperature of molten ice-water (in the area CD) means an increase in the kinetic energy of water molecules due to the heat supplied by the burner.
When cooling water (section DE) part of the energy is taken away from it, water molecules move at lower speeds, their average kinetic energy drops - the temperature decreases, the water cools. At 0°C (horizontal section EF) molecules begin to line up in a certain order, forming a crystal lattice. Until this process is completed, the temperature of the substance will not change, despite the heat removed, which means that when solidifying, the liquid (water) releases energy. This is exactly the energy that the ice absorbed, turning into a liquid (section sun). The internal energy of a liquid is greater than that of a solid. During melting (and crystallization), the internal energy of the body changes abruptly.
Metals that melt at temperatures above 1650 ºС are called refractory(titanium, chromium, molybdenum, etc.). Tungsten has the highest melting point among them - about 3400 ° C. Refractory metals and their compounds are used as heat-resistant materials in aircraft construction, rocketry and space technology, and nuclear energy.
We emphasize once again that during melting, the substance absorbs energy. During crystallization, on the contrary, it gives it to the environment. Receiving a certain amount of heat released during crystallization, the medium heats up. This is well known to many birds. No wonder they can be seen in winter in frosty weather sitting on the ice that covers rivers and lakes. Due to the release of energy during the formation of ice, the air above it turns out to be several degrees warmer than in the forest on the trees, and birds take advantage of this.
Melting of amorphous substances.
The presence of a certain melting points is an important feature of crystalline substances. It is on this basis that they can be easily distinguished from amorphous bodies, which are also classified as solids. These include, in particular, glass, very viscous resins, and plastics.
Amorphous substances(unlike crystalline) do not have a specific melting point - they do not melt, but soften. When heated, a piece of glass, for example, first becomes soft from hard, it can be easily bent or stretched; at a higher temperature, the piece begins to change its shape under the influence of its own gravity. As it heats up, the thick viscous mass takes the shape of the vessel in which it lies. This mass is at first thick, like honey, then like sour cream, and, finally, it becomes almost as low-viscosity liquid as water. However, it is impossible to indicate a specific temperature for the transition of a solid to a liquid here, since it does not exist.
The reasons for this lie in the fundamental difference between the structure of amorphous bodies and the structure of crystalline ones. Atoms in amorphous bodies are arranged randomly. Amorphous bodies in their structure resemble liquids. Already in solid glass, the atoms are arranged randomly. This means that an increase in the temperature of the glass only increases the range of vibrations of its molecules, gives them gradually more and more freedom of movement. Therefore, the glass softens gradually and does not exhibit the sharp "solid-liquid" transition characteristic of the transition from the arrangement of molecules in a strict order to a disorderly one.
Melting heat.
Melting heat- this is the amount of heat that must be imparted to a substance at constant pressure and a constant temperature equal to the melting point in order to completely transfer it from a solid crystalline state to a liquid one. The heat of fusion is equal to the amount of heat that is released during the crystallization of a substance from a liquid state. During melting, all the heat supplied to the substance goes to increase the potential energy of its molecules. The kinetic energy does not change because melting occurs at a constant temperature.
Studying experimentally the melting of various substances of the same mass, one can notice that different amounts of heat are required to turn them into a liquid. For example, in order to melt one kilogram of ice, you need to expend 332 J of energy, and in order to melt 1 kg of lead - 25 kJ.
The amount of heat released by the body is considered negative. Therefore, when calculating the amount of heat released during the crystallization of a substance with a mass m, you should use the same formula, but with a minus sign:
Heat of combustion.
Heat of combustion(or calorific value, calories) is the amount of heat released during the complete combustion of fuel.
To heat bodies, the energy released during the combustion of fuel is often used. Conventional fuel (coal, oil, gasoline) contains carbon. During combustion, carbon atoms combine with oxygen atoms in the air, resulting in the formation of carbon dioxide molecules. The kinetic energy of these molecules turns out to be greater than that of the initial particles. The increase in the kinetic energy of molecules during combustion is called the release of energy. The energy released during the complete combustion of fuel is the heat of combustion of this fuel.
The heat of combustion of fuel depends on the type of fuel and its mass. The greater the mass of the fuel, the greater the amount of heat released during its complete combustion.
The physical quantity showing how much heat is released during the complete combustion of fuel weighing 1 kg is called specific heat of combustion of fuel.The specific heat of combustion is denoted by the letterqand is measured in joules per kilogram (J/kg).
Quantity of heat Q released during combustion m kg of fuel is determined by the formula:
To find the amount of heat released during the complete combustion of a fuel of arbitrary mass, it is necessary to multiply the specific heat of combustion of this fuel by its mass.
