In the previous paragraph, we considered the graph of melting and solidification of ice. The graph shows that while the ice is melting, its temperature does not change (see Fig. 18). And only after all the ice has melted, the temperature of the resulting liquid begins to rise. But after all, even during the melting process, the ice receives energy from the fuel burning in the heater. And from the law of conservation of energy it follows that it cannot disappear. What is the energy consumption of the fuel during melting?
We know that in crystals the molecules (or atoms) are arranged in a strict order. However, even in crystals they are in thermal motion (oscillate). When the body is heated, the average speed of the molecules increases. Consequently, their average kinetic energy and temperature also increase. On the graph, this is section AB (see Fig. 18). As a result, the range of vibrations of molecules (or atoms) increases. When the body is heated to the melting temperature, the order in the arrangement of particles in crystals will be violated. Crystals lose their shape. A substance melts, changing from a solid state to a liquid state.
Consequently, all the energy that a crystalline body receives after it has already been heated to the melting point is spent on the destruction of the crystal. In this regard, the body temperature ceases to rise. On the graph (see Fig. 18) this is the BC section.
Experiments show that for the transformation of various crystalline substances of the same mass into a liquid at a melting point, a different amount of heat is required.
The physical quantity showing how much heat must be imparted to a crystalline body weighing 1 kg in order to completely transfer it to a liquid state at the melting point is called the specific heat of fusion.
The specific heat of fusion is denoted by λ (Greek letter "lambda"). Its unit is 1 J/kg.
Determine the specific heat of fusion in the experiment. Thus, it was found that the specific heat of melting of ice is 3.4 10 5 - . This means that for the transformation of a piece of ice weighing 1 kg, taken at 0 ° C, into water of the same temperature, 3.4 10 5 J of energy is required. And in order to melt a bar of lead weighing 1 kg, taken at its melting point, it will take 2.5 10 4 J of energy.
Therefore, at the melting point, the internal energy of a substance in the liquid state is greater than the internal energy of the same mass of substance in the solid state.
To calculate the amount of heat Q required to melt a crystalline body of mass m, taken at its melting temperature and normal atmospheric pressure, the specific heat of fusion λ must be multiplied by the mass of the body m:
From this formula, it can be determined that
λ = Q / m, m = Q / λ
Experiments show that during the solidification of a crystalline substance exactly the same amount of heat is released that is absorbed during its melting. So, during the solidification of water weighing 1 kg at a temperature of 0 ° C, an amount of heat equal to 3.4 10 5 J is released. Exactly the same amount of heat is required for the melting of ice weighing 1 kg at a temperature of 0 ° C.
When a substance solidifies, everything happens in the reverse order. The speed, and hence the average kinetic energy of molecules in a cooled molten substance, decrease. Attractive forces can now keep slowly moving molecules close to each other. As a result, the arrangement of particles becomes ordered - a crystal is formed. The energy released during crystallization is used to maintain a constant temperature. On the graph, this is the EF section (see Fig. 18).
Crystallization is facilitated if any foreign particles, such as dust particles, are present in the liquid from the very beginning. They become centers of crystallization. Under normal conditions, there are many centers of crystallization in a liquid, near which the formation of crystals occurs.
Table 4
Specific heat of fusion of certain substances (at normal atmospheric pressure)
During crystallization, energy is released and transferred to surrounding bodies.
The amount of heat released during the crystallization of a body of mass m is also determined by the formula
In this case, the internal energy of the body decreases.
Example. To prepare tea, the tourist put ice weighing 2 kg and having a temperature of 0 ° C into the pot. How much heat is needed to turn this ice into boiling water at 100°C? The energy spent on heating the kettle is not taken into account.
What amount of heat would be needed if, instead of ice, a tourist took water of the same mass at the same temperature from the hole?
Let's write down the condition of the problem and solve it.
Questions
- How to explain the process of body melting on the basis of the doctrine of the structure of matter?
- What is the fuel energy spent on during the melting of a crystalline body heated to the melting point?
- What is the specific heat of fusion?
- How to explain the process of hardening on the basis of the doctrine of the structure of matter?
- How is the amount of heat required to melt a crystalline body taken at the melting point calculated?
- How to calculate the amount of heat released during the crystallization of a body that has a melting point?
Exercise 12
Exercise
- Place two identical cans on the stove. Pour water weighing 0.5 kg into one, put several ice cubes of the same mass into the other. Note how long it takes for the water in both jars to boil. Write a short account of your experience and explain the results.
- Read the paragraph “Amorphous bodies. Melting of amorphous bodies". Prepare a report on it.
In this lesson, we will study the concept of "specific heat of fusion". This value characterizes the amount of heat that must be imparted to 1 kg of a substance at the melting point in order for it to pass from a solid state to a liquid state (or vice versa).
We will study the formula for finding the amount of heat required to melt (or release during crystallization) a substance.
Topic: Aggregate states of matter
Lesson: Specific heat of fusion
This lesson is devoted to the main characteristic of the melting (crystallization) of a substance - the specific heat of fusion.
In the last lesson, we touched on the question: how does the internal energy of a body change during melting?
We found that when heat is supplied, the internal energy of the body increases. At the same time, we know that the internal energy of a body can be characterized by such a concept as temperature. As we already know, during melting, the temperature does not change. Therefore, a suspicion may arise that we are dealing with a paradox: the internal energy increases, but the temperature does not change.
The explanation for this fact is quite simple: all the energy is spent on the destruction of the crystal lattice. Similarly, in the reverse process: during crystallization, the molecules of a substance are combined into a single system, while excess energy is given off and absorbed by the external environment.
As a result of various experiments, it was possible to establish that for the same substance a different amount of heat is required to transfer it from a solid to a liquid state.
Then it was decided to compare these quantities of heat with the same mass of matter. This led to the emergence of such a characteristic as the specific heat of fusion.
Definition
Specific heat of fusion- the amount of heat that must be imparted to 1 kg of a substance heated to the melting point in order to transfer it from a solid to a liquid state.
The same value is released during the crystallization of 1 kg of a substance.
The specific heat of fusion is indicated (Greek letter, read as "lambda" or "lambda").
Units: . In this case, there is no temperature in the dimension, since the temperature does not change during melting (crystallization).
To calculate the amount of heat required to melt a substance, the formula is used:
The amount of heat (J);
Specific heat of fusion (which is searched for in the table;
The mass of the substance.
When the body crystallizes, it is written with a “-” sign, since heat is released.
An example is the specific heat of melting of ice:
. Or the specific heat of fusion of iron:
.
That the specific heat of melting of ice turned out to be greater than the specific heat of melting of iron should not be surprising. The amount of heat that a particular substance needs to melt depends on the characteristics of the substance, in particular, on the energy of bonds between the particles of this substance.
In this lesson, we looked at the concept of specific heat of fusion.
In the next lesson, we will learn how to solve problems for heating and melting crystalline bodies.
Bibliography
- Gendenshtein L.E., Kaidalov A.B., Kozhevnikov V.B. Physics 8 / Ed. Orlova V.A., Roizena I.I. - M.: Mnemosyne.
- Peryshkin A. V. Physics 8. - M .: Bustard, 2010.
- Fadeeva A. A., Zasov A. V., Kiselev D. F. Physics 8. - M .: Education.
- Physics, mechanics, etc. ().
- Cool physics ().
- Internet portal Kaf-fiz-1586.narod.ru ().
Homework
In order to melt any substance in the solid state, it is necessary to heat it. And when any body is heated, one curious feature is noted
The peculiarity is this: the temperature of the body rises up to the melting point, and then stops until the entire body passes into a liquid state. After melting, the temperature begins to rise again, if, of course, heating is continued. That is, there is a period of time during which we heat the body, but it does not heat up. Where does the heat energy that we use go? To answer this question, we must look inside the body.
In a solid, the molecules are arranged in a certain order in the form of crystals. They practically do not move, only slightly oscillating in place. In order for a substance to pass into a liquid state, the molecules must be given additional energy so that they can escape from the attraction of neighboring molecules in the crystals. By heating the body, we give the molecules this necessary energy. And until all the molecules receive enough energy and all the crystals are destroyed, the body temperature does not rise. Experiments show that different substances of the same mass require different amounts of heat to completely melt it.
That is, there is a certain value on which depends, how much heat must be absorbed by a substance to melt. And this value is different for different substances. This value in physics is called the specific heat of fusion of a substance. Again, as a result of experiments, the values \u200b\u200bof the specific heat of fusion for various substances were established and collected in special tables from which this information can be gleaned. The specific heat of fusion is denoted by the Greek letter λ (lambda), and the unit of measurement is 1 J / kg.
Specific heat of fusion formula
The specific heat of fusion is found by the formula:
where Q is the amount of heat required to melt a body of mass m.
Again, it is known from experiments that, during solidification, substances emit the same amount of heat that was required to be spent on their melting. Molecules, losing energy, form crystals, being unable to resist the attraction of other molecules. And again, the temperature of the body will not decrease until the moment when the whole body solidifies, and until all the energy that was expended on its melting is released. That is, the specific heat of fusion shows how much energy must be expended to melt a body of mass m, and how much energy will be released during the solidification of this body.
For example, the specific heat of fusion of water in the solid state, that is, the specific heat of fusion of ice is 3.4 * 105 J / kg. These data allow us to calculate how much energy is required to melt ice of any mass. Knowing also the specific heat capacity of ice and water, it is possible to calculate exactly how much energy is required for a particular process, for example, to melt ice with a mass of 2 kg and a temperature of -30 ° C and bring the resulting water to a boil. Such information for various substances is very necessary in industry to calculate the real energy consumption in the production of any goods.
Everyone knows that water can be found in nature in three states of aggregation - solid, liquid and gaseous. During melting, solid ice turns 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.
This is not to say 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. A fairly large amount 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 take care of 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 has a loose structure and is white in 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 an exhausting resource of the Earth and needs to be treated with care.
Density, thermal conductivity and heat capacity of ice depending on temperature
The table shows the values of density, thermal conductivity, specific heat capacity of ice depending on the temperature in the range from 0 to -100°C.
According to the table, it can be seen that with decreasing temperature, the specific heat capacity of ice decreases, while the thermal conductivity and density of ice, on the contrary, increase. For example, at a temperature of 0 ° C, the density of ice has a value of 916.2 kg / m 3, and at a temperature of minus 100°C, its density becomes equal to 925.7 kg/m 3 .
The specific heat capacity of ice at 0°C is 2050 J/(kg deg). When the temperature of ice decreases from -5 to -100°C, its specific heat capacity decreases by 1.45 times. The heat capacity of ice is two times less.
The thermal conductivity of ice when its temperature is lowered from 0 to minus 100°C increases from 2.22 to 3.48 W/(m deg). Ice is more thermally conductive than water - it can conduct 4 times more heat under the same boundary conditions.
It should be noted that the density of ice is less, however, with decreasing temperature, the density of ice increases and, as the temperature approaches absolute zero, the density of ice becomes close to the density of water.
| Temperature, °C | Density, kg / m 3 | Thermal conductivity, W/(m deg) | Heat capacity, J/(kg deg) |
|---|---|---|---|
| 0.01 (Water) | 999,8 | 0,56 | 4212 |
| 0 | 916,2 | 2,22 | 2050 |
| -5 | 917,5 | 2,25 | 2027 |
| -10 | 918,9 | 2,30 | 2000 |
| -15 | 919,4 | 2,34 | 1972 |
| -20 | 919,4 | 2,39 | 1943 |
| -25 | 919,6 | 2,45 | 1913 |
| -30 | 920,0 | 2,50 | 1882 |
| -35 | 920,4 | 2,57 | 1851 |
| -40 | 920,8 | 2,63 | 1818 |
| -50 | 921,6 | 2,76 | 1751 |
| -60 | 922,4 | 2,90 | 1681 |
| -70 | 923,3 | 3,05 | 1609 |
| -80 | 924,1 | 3,19 | 1536 |
| -90 | 924,9 | 3,34 | 1463 |
| -100 | 925,7 | 3,48 | 1389 |
Thermophysical properties of ice and snow
The table shows the following properties of ice and snow:
- ice density, kg/m 3 ;
- thermal conductivity of ice and snow, kcal/(m h deg) and W/(m deg);
- specific mass heat capacity of ice, kcal/(kg deg) and J/kg deg);
- thermal diffusivity, m 2 /hour and m 2 /sec.
The properties of ice and snow are presented depending on the temperature in the range: for ice from 0 to -120°C; for snow from 0 to -50°С depending on compaction (density). The thermal diffusivity of ice and snow in the table is given with a factor of 10 6 . For example, the thermal diffusivity of ice at 0°C is 1.08·10 -6 m 2 /s.
Saturated vapor pressure of ice
The table shows the pressure values of the saturated steam of ice during sublimation (transition of ice into vapor, past the liquid phase) depending on the temperature in the range from 0.01 to -80°C. It can be seen from the table that as the temperature of the ice decreases, the pressure of its saturated vapor decreases.
Sources:
- Volkov. A.I., Zharsky. THEM. Big chemical reference book. - M: Soviet School, 2005. - 608 p.