Melting point (solidification) depends on the type of substance and ambient pressure.
At atmospheric pressure (760 mm Hg), the melting point of water ice is 0°C. The amount of heat required to convert 1 kg of ice into water (or vice versa) is called latent or specific heat of fusion r. For water ice r=335 kJ/kg.
The amount of heat required to convert ice of mass M into water is determined by the formula: Q=Mr.
It follows from the foregoing that one of the methods of artificial cooling is the removal of heat due to the melting of a substance in a solid state at a low temperature.
In practice, this method has been widely used for a long time, carrying out cooling with the help of water ice harvested in winter using natural cold or with the help of water frozen in ice makers using refrigeration machines.
When pure water ice melts, the temperature of the cooled substance can be lowered to 0°C. To achieve lower temperatures use. In this case, the temperature and latent heat of fusion depend on the type of salt and its content in the mixture. When the mixture contains 22.4% sodium chloride, the melting point of the ice-salt mixture is -21.2°C, and the latent heat of fusion is 236.1 kJ/kg.
Using calcium chloride (29.9%) in the mixture, it is possible to lower the melting point of the mixture to -55°C, in this case r = = 214 kJ/kg.
Sublimation- the transition of a substance from a solid to a gaseous state, bypassing the liquid phase, with the absorption of heat. For cooling and freezing food products, as well as their storage and transportation in a frozen state, dry ice sublimation(solid carbon dioxide). At atmospheric pressure, dry ice, absorbing heat from the environment, passes from a solid to a gaseous state at a temperature of -78.9°C. The specific heat of sublimation is r-571 kJ/kg.
Sublimation of frozen water at atmospheric pressure occurs when drying clothes in winter. This process underlies the industrial drying of food, (). To intensify freeze-drying in apparatuses (sublimators): the pressure is maintained using vacuum pumps below atmospheric pressure.
Evaporation- the process of vaporization occurring from the free surface of the liquid. Its physical nature is explained by the escape of molecules with high speed and kinetic energy of thermal motion from the surface layer. The liquid is then cooled down. In refrigeration, this effect is used in cooling towers and in evaporative condensers to transfer the heat of condensation to the air. At atmospheric pressure and temperature 0°C, latent heat is r=2509 kJ/kg, at 100°C r=2257 kJ/kg.Boiling- the process of intense vaporization on the heating surface due to the absorption of heat. Boiling, liquids at low temperature is one of the main processes in vapor compression refrigeration machines. A boiling liquid is called a refrigerant (abbreviated coolant), and the apparatus where it boils, taking heat from the cooled substance, - evaporator(the name does not quite accurately reflect the essence of the process taking place in the apparatus). The amount of heat Q supplied to the boiling liquid is determined by the formula: Q=Mr,
where M is the mass of the liquid turned into vapor. Boiling of a homogeneous ("pure") substance occurs at a constant temperature, depending on pressure. As the pressure changes, the boiling point also changes. The dependence of the boiling point on the boiling pressure (pressure of phase equilibrium) is depicted by a curve called the saturated vapor pressure curve.
Refrigerant R12, having a significantly lower latent heat of vaporization, ensures the operation of the refrigerating machine at lower (compared to operation at) condensing pressures, which can be crucial for specific conditions.
2. Throttling (Joule-Thompson effect).
Another of the main processes in vapor compression refrigeration machines, which consists in a drop in pressure and a decrease in the temperature of the refrigerant as it flows through a narrowed section under the influence of a pressure difference without performing external work and heat exchange with the environment.In a narrow section, the flow velocity increases, the kinetic energy is spent on internal friction between molecules. This results in a portion of the liquid and a decrease in the temperature of the entire stream. The process takes place in control valve or other throttle body () refrigeration machine.
3. Expansion with the performance of external work.
The process is used in gas refrigeration machines.If an expansion machine is placed in the path of a flow moving under the influence of a pressure difference (an expansion machine in which the flow rotates a wheel or pushes a piston), then the energy of the flow will perform external useful work. In this case, after the expander, simultaneously with a decrease in pressure, the temperature of the refrigerant will also decrease.
4. Vortex effect (Ranque-Hilsch effect).
It is created using a special device - a vortex tube. It is based on the separation of warm and cold air in a swirling flow inside the pipe.5. Thermoelectric effect (Peltier effect).
It is used in thermoelectric cooling devices. It is based on lowering the temperature of the junctions of semiconductors when a direct electric current passes through them.Everyone knows that water can be found in nature in three states of aggregation- solid, liquid and gaseous. On melting, the transformation solid ice into a liquid, and upon further heating, the liquid evaporates, forming water vapor. What are the conditions for melting, crystallization, evaporation and condensation of water? At what temperature does ice melt or steam form? We will talk about this in this article.
It cannot be said that water vapor and ice are rare in Everyday life. However, the most common is the liquid state - ordinary water. Experts have found that our planet is more than 1 billion cubic kilometers of water. However, no more than 3 million km 3 of water belong to fresh water bodies. Enough a large number of fresh water "rests" in glaciers (about 30 million cubic kilometers). However, melting the ice of such huge blocks is far from easy. The rest of the water is salty, belonging to the seas of the oceans.
Water surrounds modern man everywhere, during most daily procedures. Many believe that water resources are inexhaustible, and humanity will always be able to use the resources of the Earth's hydrosphere. However, this is not the case. The water resources of our planet are gradually depleted, and in a few hundred years, fresh water on Earth may not remain at all. Therefore, absolutely every person needs to carefully treat fresh water and save it. After all, even in our time there are states in which water supplies are catastrophically small.
Water properties
Before talking about the melting temperature of ice, it is worth considering the main properties of this unique liquid.
So, water has the following properties:
- Lack of color.
- Lack of smell.
- Lack of taste (however, high-quality drinking water tastes good).
- Transparency.
- Fluidity.
- The ability to dissolve various substances (for example, salts, alkalis, etc.).
- Water does not have its own permanent shape and is able to take the shape of the vessel into which it enters.
- The ability to be purified by filtration.
- Water expands when heated and contracts when cooled.
- Water can evaporate to become steam and freeze to form crystalline ice.
This list presents the main properties of water. Now let's figure out what are the features of the solid state of aggregation of this substance, and at what temperature ice melts.
Ice is a solid crystalline substance that has a rather unstable structure. It, like water, is transparent, colorless and odorless. Ice also has properties such as brittleness and slipperiness; it is cold to the touch.
Snow is also frozen water, but it has a loose structure and has White color. It snows every year in most countries of the world.
Both snow and ice are extremely unstable substances. It doesn't take much effort to melt the ice. When does it start melting?
In nature, solid ice exists only at temperatures of 0 °C and below. If the ambient temperature rises and becomes more than 0 °C, the ice begins to melt.
At the melting temperature of ice, at 0 ° C, another process occurs - freezing, or crystallization, of liquid water.
This process can be observed by all inhabitants of the temperate continental climate. In winter, when the temperature outside drops below 0 °C, it often snows and does not melt. And the liquid water that was on the streets freezes, turning into solid snow or ice. In the spring, you can see the reverse process. The ambient temperature rises, so the ice and snow melt, forming numerous puddles and mud, which can be considered the only disadvantage of spring warming.
Thus, we can conclude that at what temperature the ice begins to melt, at the same temperature the process of water freezing begins.
Quantity of heat
In a science such as physics, the concept of the amount of heat is often used. This value shows the amount of energy required for heating, melting, crystallization, boiling, evaporation or condensation of various substances. Moreover, each of these processes has its own characteristics. Let's talk about how much heat is required to heat ice under normal conditions.
To heat the ice, you must first melt it. This requires the amount of heat needed to melt the solid. Heat equals the product of the mass of ice and the specific heat of its melting (330-345 thousand Joules / kg) and is expressed in Joules. Suppose we are given 2 kg of solid ice. Thus, in order to melt it, we need: 2 kg * 340 kJ / kg = 680 kJ.
After that, we need to heat the resulting water. The amount of heat for this process will be a little more difficult to calculate. To do this, you need to know the initial and final temperature of the heated water.
So, let's say that we need to heat the water resulting from the melting of ice by 50 ° C. That is, the difference between the initial and final temperatures = 50 °C (initial water temperature - 0 °C). Then you should multiply the temperature difference by the mass of water and its specific heat capacity, which is equal to 4,200 J * kg / ° C. That is, the amount of heat required to heat water = 2 kg * 50 °C * 4,200 J*kg/°C = 420 kJ.
Then we get that for the melting of ice and the subsequent heating of the resulting water, we need: 680,000 J + 420,000 J = 1,100,000 Joules, or 1.1 Megajoules.
Knowing at what temperature ice melts, you can solve many difficult problems in physics or chemistry.
Finally
So, in this article, we learned some facts about water and its two states of aggregation - solid and liquid. Water vapor, however, is an equally interesting object to study. For example, our atmosphere contains approximately 25*10 16 cubic meters of water vapor. In addition, unlike freezing, the evaporation of water occurs at any temperature and is accelerated when it is heated or in the presence of wind.
We learned at what temperature ice melts and liquid water freezes. Such facts will always be useful to us in everyday life, since water surrounds us everywhere. It is important to always remember that water, especially fresh water, is a finite resource of the Earth and needs to be treated with care.
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 increase in the volume of water when it freezes is of great importance in nature. Due to the lower density of ice compared to the density of water (at 0 ° C, the density of ice is 900 kg / m 3, and water 1000 kg / m 3), ice floats on water. Possessing poor thermal conductivity, the layer of ice protects the water under it from cooling and freezing. Therefore, fish and other living creatures in the water do not die during frosts. If the ice sank, then not very deep reservoirs would freeze through during the winter.
When freezing water expands in a closed vessel, huge forces arise that can break a thick-walled cast-iron ball. Such an experiment is easy to carry out with a bottle filled with water up to the neck and exposed to frost. An ice plug forms on the surface of the water, clogging the bottle, and when the freezing water expands, the bottle will break.
Freezing of water in the cracks of rocks leads to their destruction.
The ability of water to expand during hardening must be taken into account when laying water and sewer pipes, as well as water heating. To avoid rupture when water freezes, underground pipes must be laid at such a depth that the temperature does not fall below 0 ° C. The outer parts of the pipes must be covered with heat-insulating materials for the winter.
Melting point versus pressure
If the melting of a substance is accompanied by an increase in its volume, then with an increase in external pressure, the melting point of the substance rises. This can be explained as follows. Compression of a substance (with an increase in external pressure) prevents an increase in the distance between molecules and, consequently, an increase in the potential energy of interaction of molecules, which is required for the transition to a liquid state. Therefore, it is necessary to heat the body to a higher temperature until the potential energy of the molecules reaches the required value.
If the melting of a substance is accompanied by a decrease in its volume, then with an increase in external pressure, the melting point of the substance decreases.
So, for example, ice at a pressure of 6 10 7 Pa melts at a temperature of -5 ° C, and at a pressure of 2.2 10 8 Pa, the melting point of ice is -22 ° C.
The decrease in the melting point of ice with increasing pressure is well illustrated by experience (Fig. 8.34). The nylon thread passes through the ice without breaking it. The fact is that due to the significant pressure of the thread on the ice, it melts under it. Water, flowing out from under the thread, immediately freezes again.
triple point
A liquid can be in equilibrium with its vapor (saturated vapor). Figure 6.5 (see § 6.3) shows the saturation vapor pressure as a function of temperature (curve AB), obtained experimentally. Since the boiling of a liquid occurs at a pressure equal to the pressure of its saturated vapors, the same curve gives the dependence of the boiling point on pressure. Area below the curve AB, corresponds to the gas state, and above - to the liquid state.
Crystalline bodies melt at a certain temperature, at which the solid phase is in equilibrium with the liquid. The melting point depends on the pressure. This dependence can be shown in the same figure, which shows the dependence of the boiling point on pressure.
In Figure 8.35, the curve TC characterizes the dependence of boiling point on pressure. It ends at the point TO, corresponding to the critical temperature, since above this temperature the liquid cannot exist. To the left of the curve TC the experimental points plotted the curve TS the dependence of the melting point on pressure (to the left, since the solid phase corresponds to lower temperatures than the liquid phase). Both curves intersect at point T.
What will happen to the substance at a temperature below the temperature t t p , corresponding point T? The liquid phase at this temperature can no longer exist. The substance will either be in a solid or gaseous state. Curve FROM(see Fig. 8.35) corresponds to the equilibrium states of a solid - gas, arising from the sublimation of solids.
three curves CT, TS And FROM divide the phase plane into three regions in which the substance can be in one of the three phases. The curves themselves describe the equilibrium states of liquid - vapor, liquid - solid and solid - vapor. There is only one point T, in which all three phases are in equilibrium. This is the triple point.
The triple point corresponds to the only values of temperature and pressure. It can be accurately reproduced, and it serves as one of the most important reference points in the construction of an absolute temperature scale. For water, the absolute temperature of the triple point is assumed to be Ttr = 273.16 K, or t t p = 0.01°C.
Figure 8.35 shows the phase diagram of water, in which the melting point decreases with increasing pressure. For ordinary substances, the curve TS inclined in the opposite direction with respect to the vertical passing through the point T.
For example, the phase diagram of carbon monoxide CO 2 will have this form. CO 2 triple point temperature t tr \u003d -56.6 ° С, and pressure p tr \u003d 5.1 atm. Therefore, at normal atmospheric pressure and temperature close to room temperature, carbon dioxide cannot be in a liquid state. The solid phase of CO 2 is usually called dry ice. It has a very low temperature and does not melt, but immediately evaporates (sublimation).
The change in volume during melting and solidification is directly related to the dependence of the melting temperature on pressure. For the vast majority of substances, the melting point increases with pressure. In water and some other substances, on the contrary, it decreases. For the inhabitants of the Earth on high geographical latitudes this is a great boon.
There is a single point on the diagram p-T (triple point), at which all three phases of matter are in equilibrium.
In conclusion, we note the great importance of solid state physics for the development of technology and civilization in general.
Mankind has always used and will continue to use solid bodies. But if earlier solid state physics did not keep pace with the development of technology based on direct experience, now the situation has changed. Theoretical research is beginning to lead to the creation of solids, the properties of which are completely unusual and which would be impossible to obtain by the "trial and error" method. The invention of the transistor, which will be discussed later, is a prime example of how understanding the structure of solids led to a revolution in all radio engineering.
The creation of materials with specified mechanical, magnetic and other properties is one of the main areas of solid state physics. Approximately half of the world's physicists are now working in the field of solid state physics.
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 A 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 A 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: an ordered spatial arrangement its molecules are replaced by disordered ones, the distance between 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, rocket and space technology, nuclear energy.
We emphasize once again that during melting, the substance absorbs energy. During crystallization, on the contrary, it gives it to 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. How more weight 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.