The specific heat of fusion of iron is equal to. Abstract: Melting of bodies

Melting

Melting is the process of converting a substance from a solid to a liquid.

Observations show that if crushed ice, having, for example, a temperature of 10 ° C, is left in a warm room, its temperature will increase. At 0 °C, the ice will begin to melt, and the temperature will not change until all the ice turns into liquid. After this, the temperature of the water formed from the ice will increase.

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 molecular motion. 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 kinetic, but due to the potential energy of interaction of molecules. The energy received from outside is spent on the destruction of the crystal lattice. Any crystalline body melts in a similar way.

Amorphous bodies do not have a specific melting point. As the temperature increases, they gradually soften until they turn into liquid.

Crystallization

Crystallization is the process of transition of a substance from a liquid state to a solid state. As the liquid cools, it will release some 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 crystallization process will begin, during this process the temperature of the substance will not change until the entire substance turns 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 temperature of a solid crystalline substance versus time during its heating from room temperature to the melting point, melting, heating of the substance in the liquid state, cooling of 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 temperature, it is necessary to impart a different amount of heat to it.

Specific heat of fusion- this 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 the melting point. Experience shows that the specific heat of fusion is equal to specific heat of crystallization .

The specific heat of fusion is indicated 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 fusion of aluminum is 3.9*10 5 J/kg. This means that to melt 1 kg of aluminum at the melting temperature, it is necessary to expend an amount of heat of 3.9 * 10 5 J. The same value is equal to the increase in internal energy of 1 kg of aluminum.

To calculate the amount of heat Q required to melt a substance of mass m, taken at the melting temperature, follows the specific heat of fusion λ multiplied by the mass of the substance: Q = λm.

The same formula is used to calculate the amount of heat released during crystallization of a liquid.

During melting, the spatial lattice of the crystalline body is destroyed. This process requires a certain amount of energy from some external source. As a result, the internal energy of the body increases during the melting process.

The amount of heat required for a body to change from solid to liquid at the melting point is called the heat of fusion.

In the process of solidification of the body, on the contrary, the internal energy of the body decreases. The body gives off heat to surrounding bodies. According to the law of conservation of energy, the amount of heat absorbed by a body during melting (at the melting temperature) is equal to the amount of heat given off by this body during solidification (at the solidification temperature).

Specific heat of fusion

The heat of fusion depends on the mass of the melting substance and its properties. The dependence of the heat of fusion on the type of substance is characterized by the specific heat of fusion of this substance.

The specific heat of fusion of a substance is the ratio of the heat of fusion of a body of this substance to the mass of the body.

Let us denote the heat of fusion by Q pl , body weight letter T and specific heat of fusion by the letter λ . Then

Thus, in order to melt a crystalline body weighing m taken at the melting point, an amount of heat is required equal to

(8.8.2)

Heat of crystallization

According to the law of conservation of energy, the amount of heat released during crystallization of a body (at the crystallization temperature) is equal to

(8.8.3)

From formula (8.8.1) it follows that the specific heat of fusion in SI is expressed in joules per kilogram.

The specific heat of fusion of ice is quite high: 333.7 kJ/kg. The specific heat of fusion of lead is only 23 kJ/kg, and that of gold is 65.7 kJ/kg.

Formulas (8.8.2) and (8.8.3) are used when solving problems of drawing up heat balance equations in cases where we are dealing with the melting and solidification of crystalline bodies.

The role of the heat of melting of ice and crystallization of water in nature

The absorption of heat when ice melts and its release when water freezes have a significant impact on changes in air temperature, especially near bodies of water. You have all probably noticed that during heavy snowfalls it usually becomes warmer.

The high specific heat of fusion of ice is very important. Back at the end of the 18th century. Scottish scientist D. Black (1728-1799), who discovered the existence of the heat of melting and crystallization, wrote: “If ice did not have a significant heat of fusion, then in the spring the entire mass of ice would have to melt in a few minutes or seconds, since the heat from air is continuously transferred to the ice. But then the consequences of this would be terrible: after all, even in the current situation, large floods and strong flows of water arise when large masses of ice and snow melt.”

Space rocket nozzle

Let us give an interesting technical example of the practical use of the heat of fusion and vaporization. When making a nozzle for a space rocket, it should be taken into account that the stream of gases emerging from the rocket nozzle has a temperature of about 4000 °C. There are practically no materials in nature that, in their pure form, could withstand such temperatures. Therefore, you have to resort to all sorts of tricks to cool the nozzle material during fuel combustion.

The nozzle is made by powder metallurgy. Refractory metal powder (tungsten) is placed into the mold cavity. It is then subjected to compression. The powder is sintered, resulting in a porous structure like pumice. Then this “pumice” is impregnated with copper (its melting point is only 1083 ° C).

The resulting material is called pseudoalloy. Figure 8.31 shows a photograph of the microstructure of a pseudoalloy. Copper inclusions of irregular shape are visible against the white background of the tungsten frame. This alloy can, incredibly, work for a short time even at the temperature of the gases formed during fuel combustion, i.e. above 4000°C.

This happens as follows. Initially, the temperature of the alloy increases until it reaches the melting point of copper t 1 (Fig. 8.32). After this, the nozzle temperature will not change until all the copper has melted (time interval from τ 1 to τ 2 ). Subsequently, the temperature increases again until the copper boils. This occurs at temperature t 2 = 2595 °C, lower than the melting point of tungsten (3380 °C). Until all the copper boils away, the nozzle temperature will not change again, since the evaporating copper takes heat from tungsten (time interval from τ 3 to τ 4 ). Of course, the nozzle will not work for any length of time. After the copper evaporates, the tungsten will begin to heat up again. However, the rocket engine only runs for a few minutes, and during this time the nozzle will not have time to overheat and melt.

The energy that a body gains or loses during heat transfer is called heat quantity. It is designated by the letter Q and measured in joules (J).

The amount of heat required to heat a body (or released by it when cooling)
depends on the type of substance of which it consists, on the mass of this body and on changes in its temperature.

To calculate the amount of heat required to heat a body or released by it during cooling, the specific heat capacity of the substance must be multiplied by the mass of the body and by the difference between its higher and lower temperatures.

Where c is the specific heat capacity of a given substance, m is its mass, t 1 is the initial temperature of the body, t 2 is its final temperature.

A physical quantity that shows how much heat is required to change the temperature of a body of a given substance weighing 1 kg by 1 °C is called specific heat capacity. It is measured in J/(kg·ºС).

As a rule, metals have a low specific heat capacity, so they heat up quickly and cool down just as quickly.

The transition of a substance from a solid to a liquid state is called melting. The temperature at which a substance melts is called the melting point of the substance. The transition of a substance from a liquid to a solid state is called solidification or crystallization. The temperature at which a substance hardens (crystallizes) is called the solidification or crystallization temperature. Substances solidify at the same temperature at which they melt. The melting and crystallization temperatures depend on atmospheric pressure: the higher the pressure, the higher the melting point. Therefore, in the table the melting point values ​​are presented at normal atmospheric pressure.

A physical quantity showing how much heat must be imparted to a crystalline body weighing 1 kg in order to completely transform it into a liquid state at the melting point is called the specific heat of fusion. It is designated by the letter λ and measured in J/kg.

The amount of heat required to melt a substance of mass m, taken at the melting temperature, is calculated by the formula: Q=λ·m.

To calculate the amount of heat in these processes, the specific values ​​are given in the tables.

molecular interaction.

In a heated vessel, both ice and water are simultaneously present - two states of aggregation of the same substance, until all the ice melts. Next, the resulting water is heated. Since the specific heat capacity of water is greater than the specific heat capacity of ice, the water heats up more slowly and the angle of inclination of the line is smaller.

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 its melting point in order for it to pass from a solid to a liquid state (or vice versa).

We will study the formula for finding the amount of heat that is necessary to melt (or is released during crystallization) of a substance.

Topic: Aggregate states of matter

Lesson: Specific Heat of Melting

This lesson is devoted to the main characteristic of 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 out that when heat is added, 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, the temperature does not change during melting. 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 destroying the crystal lattice. The reverse process is similar: 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 the same substance requires different amounts of heat to convert it from a solid to a liquid state.

Then it was decided to compare these amounts of heat with the same mass of substance. This led to the appearance 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 amount is released during the crystallization of 1 kg of substance.

It is denoted by the specific heat of fusion (Greek letter, read as “lambda” or “lambda”).

Units: . In this case, there is no temperature in the dimension, since during melting (crystallization) the temperature does not change.

To calculate the amount of heat required to melt a substance, the formula is used:

Amount of heat (J);

Specific heat of fusion (, which is looked for in the table;

Mass of the substance.

When a body crystallizes, it is written with a “-” sign, since heat is released.

An example is the specific heat of fusion of ice:

. Or the specific heat of fusion of iron:

.

The fact that the specific heat of fusion of ice turned out to be greater than the specific heat of fusion of iron should not be surprising. The amount of heat that a particular substance requires for melting 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 involving heating and melting crystalline bodies.

Bibliography

1. Gendenshtein L. E., Kaidalov A. B., Kozhevnikov V. B. Physics 8 / Ed. Orlova V. A., Roizena I. I. - M.: Mnemosyne.
2. Peryshkin A.V. Physics 8. - M.: Bustard, 2010.
3. Fadeeva A. A., Zasov A. V., Kiselev D. F. Physics 8. - M.: Education.

1. Physics, mechanics, etc. ().
2. Cool physics ().
3. Internet portal Kaf-fiz-1586.narod.ru ().

Homework

Topic: “Melting and crystallization.

Specific heat of fusion and crystallization"

Lesson objectives:

As a result of the work in the lesson, students must learn the definition of the concepts “melting”, “crystallization”, “melting temperature”, “specific heat of melting and crystallization”; be able to explain the invariability of temperature and energy transformations in the processes of melting and crystallization; analyze the graph of the dependence of body temperature on the time of its heating and the graph of cooling of the heated liquid; know the formula for calculating the amount of heat required for melting (crystallization) of a body.

During the classes.

Frontal survey.

1. In what states of aggregation can the same substance exist?

2. What determines this or that state of aggregation of a substance?

3. What are the features of the molecular structure of gases, liquids and solids?

4. Transitions are possible: from solid to liquid, from liquid to gaseous, from gaseous to solid and reverse transitions: from solid to gaseous, from gaseous to liquid, from liquid to solid. Establish a correspondence between transitions and the phenomena corresponding to them. (The teacher names the phenomenon, students determine which transition this phenomenon corresponds to).

T → F: melting ice, melting metal;

F → G: formation of steam when water boils; evaporation of water;

T → G: smell of mothballs, evaporation of dry ice;

F → T: freezing of water;

G → F: dew, fog formation;

G → T: formation of patterns on windows in winter.

In nature there is a water cycle. Evaporation of water, the formation of fog, clouds, snow, dew... In order to understand the processes occurring in nature and be able to control them, you need to know the conditions under which the transformation of one aggregate state of matter into another occurs.

Introduction to the topic of the lesson.

Today in the lesson we will get acquainted in more detail with the transitions of a substance from a solid state to a liquid state, from a liquid state to a solid state, i.e., with the process of melting crystalline bodies and its reverse process - the process of crystallization.

Learning new material. (20 minutes)
Experimental study

Students identify the problem, goal, and hypothesis of the study.

Research problem: to determine how the temperature of ice will change when it is heated and melted.

Purpose of the study: to study the change in temperature during various processes - heating and melting of ice, to construct a graph of the dependence of ice temperature on time.

We assume that when ice is heated, its temperature will increase to the melting temperature, at which the ice will melt without changing temperature.

Rationale for the hypothesis: the melting point of ice is 0 °C, so the ice will first heat up to the melting temperature. Since melting is a process that takes place at a constant temperature, the temperature of the ice will not increase until all the ice turns into water.

Equipment:

Calorimeter. Crushed ice. Thermometer. Watch.

Progress of the study:

Place crushed ice in the calorimeter. Measure the temperature of the ice. Continue taking measurements at regular intervals. Enter the measurement results into the table.

Table 1. Experimental data for the study

The temperature of the ice rose until it reached 0 °C, and so the heating process took place, the temperature of the ice increased. As soon as the temperature became equal to 0, the ice began to melt and did not change for a long time (until the ice melted). And as soon as all the ice melted, the temperature began to increase again. Thus, we can say that the heating process occurs with increasing temperature, and the melting process occurs at a constant temperature.

We have established that the temperature of the ice first rises, and then, having reached 0°C (the ice begins to melt), remains unchanged until all the ice has melted.

The transition of a substance from solid to liquid is called melting.

The temperature at which the transition from solid to liquid occurs is called the melting point. The melting point of various substances is a tabular value.

Remember

For each substance there is a temperature above which it cannot be in a solid state under given conditions. The melting process requires energy. The temperature of a substance does not change during melting.
View the process of solidification of liquids via video.

The process of transition of a substance from a liquid to a solid state is called crystallization.

When a substance melts, it gains energy. During crystallization, on the contrary, it releases it into the environment.

Remember:

For each substance, there is a temperature at which the substance changes from a liquid to a solid state (crystallization temperature). The hardening process is accompanied by the release of energy. The temperature remains constant during crystallization.

Conclusions: Melting and crystallization are two opposite processes. In the first case, the substance absorbs energy from the outside, and in the second, it releases it into the environment.

PHYSICAL MINUTE

Consider a graph of ice melting and crystallization.

Analysis of the melting and crystallization graph and its explanation based on knowledge of the molecular structure of the substance. Each substance has its own melting point and this temperature determines the areas of application of solids in everyday life and technology. Refractory metals are used to make heat-resistant structures in aircraft and rockets, nuclear reactors, etc.
Specific heat of fusion and crystallization.

A physical quantity numerically equal to the amount of heat that a solid body weighing 1 kg absorbs at the melting point to transform into a liquid state is called the specific heat of fusion.

l – specific heat of melting and crystallization.

A physical quantity showing how much heat is needed to convert 1 kg of a crystalline substance taken at the melting point into a liquid is called the specific heat of fusion.

In SI, the specific heat of fusion and crystallization is measured in joules per kilogram.

IY. Solving quality problems. (5 minutes)

Y. Solving TRIZ problems (5 min)

YI. Consolidation of the studied material. (5 minutes)

Yii. Lesson summary. (2 min) Summing up the lesson. Giving grades for work.

Homework: §9, 10, exercise 8 (1-3). Creative task: find interesting facts about the lowest temperature and the highest temperature.

Routing

designing a physics lesson in

Physics teacher, State Educational Institution “Secondary School No. 42”

Lesson topic: Melting and crystallization. Specific heat of fusion and crystallization

Lesson type: lesson on studying and initially consolidating new knowledge.

The purpose of the lesson: to ensure the deepening and systematization of students’ knowledge about the structure of matter; teach students to understand the essence of thermal phenomena such as melting and crystallization; mastering the concept of “specific heat of fusion” and the formula for calculating the amount of heat required for melting; formation of skills to analyze energy transformations during melting and crystallization of matter.

Lesson objectives:

Educational: to study the features in the behavior of a substance during the transition from solid to liquid and back; explain the graph of melting and solidification, explain the processes of melting and solidification based on the molecular structure of the substance.

Developmental: continue the formation of positive motives for learning, develop independence when performing and observing experiments, teach how to apply the acquired knowledge in practice.

Educational: continue the formation of a worldview using the example of thermal processes, show cause-and-effect relationships, show the importance of knowledge and skills using the example of analyzing qualitative problems.

Demonstrations and equipment for the experiment: study of the dependence of the melting temperature of ice on time (calorimeter, thermometer, clock, crushed ice, alcohol lamp, tripod), video film about the crystallization of water, table of melting temperatures of some substances, table of specific heat of fusion of some substances, graph of melting and crystallization .