Lesson topic: “Specific heat of fusion. Melting graphs and
solidification of crystalline bodies."
Lesson objectives:
Develop the ability to plot a graph of the temperature of a crystalline body depending on the heating time;
Introduce the concept of specific heat of fusion;
Enter a formula to calculate the amount of heat required to melt a crystalline body of mass m, taken at the melting temperature.
Develop the ability to compare, contrast, and generalize material.
Accuracy in drawing up schedules, hard work, the ability to complete the work started.
Epigraph for the lesson:
“Without a doubt, all our knowledge begins with experience.”
Kant (German philosopher 1724 - 1804)
“It’s not a shame not to know, it’s a shame not to learn”
(Russian folk proverb)
During the classes:
I. Organizing time. Setting the topic and goals of the lesson.
II. The main part of the lesson.
1. Updating knowledge:
There are 2 people at the board:
Fill in the missing words in the definition.
“The molecules in crystals are located..., they move..., held in certain places by the forces of molecular attraction. When bodies are heated, the average speed of movement of molecules ..., and the vibrations of molecules ..., the forces that hold them, ..., the substance passes from a solid to a liquid state, this process is called ... ".
“The molecules in a molten substance are located..., they move... and... are held in certain places by forces of molecular attraction. When a body cools, the average speed of movement of molecules ..., the range of vibrations ..., and the forces holding them ..., the substance passes from a liquid state to a solid, this process is called ... ".
The rest of the class works on mini-test cards ()
Using table values in the collection of Lukashik problems.
Option #1
1. Lead melts at a temperature of 327 0C. What can you say about the solidification temperature of lead?
A) It is equal to 327 0C.
B) It is higher than the temperature
melting.
2. At what temperature does mercury acquire a crystalline structure?
A) 4200C; B) - 390C;
3. In the ground at a depth of 100 km, the temperature is about 10,000C. Which metal: Zinc, tin or iron is there in an unmolten state.
A) zinc. B) Tin. B) Iron
4. The gas coming out of the nozzle of a jet aircraft has a temperature of 500 - 7000C. Can the nozzle be made from?
Can i. B) It is impossible.
Melting and solidification of crystalline bodies.
Option No. 2
1. When a crystalline substance melts, its temperature ...
B) decreases.
2. At what temperature can zinc be in a solid and liquid state?
A) 4200C; B) - 390C;
B) 1300 - 15000С; D) 00C; D) 3270C.
3. Which metal: zinc, tin or iron will melt at the melting temperature of copper?
A) zinc. B) Tin. B) Iron
4. The temperature of the outer surface of the rocket during flight rises to 1500 - 20000C. What metals are suitable for making the outer skin of rockets?
A) Steel. B). Osmium. B) Tungsten
D) Silver. D) Copper.
Melting and solidification of crystalline bodies.
Option #3
1. Aluminum hardens at a temperature of 6600C. What can you say about the melting point of aluminum?
A) It is equal to 660 0C.
B) It is below the melting point.
B) It is higher than the temperature
melting.
2. At what temperature does the crystalline structure of steel collapse?
A) 4200C; B) - 390C;
B) 1300 - 15000С; D) 00C; D) 3270C.
3. On the surface of the Moon at night the temperature drops to -1700C. Is it possible to measure this temperature with mercury and alcohol thermometers?
A) It’s impossible.
B) You can use an alcohol thermometer.
C) You can use a mercury thermometer.
D) You can use both mercury and alcohol thermometers.
4. Which metal, when in a molten state, can freeze water?
A) Steel. B) zinc. B) Tungsten.
D) Silver. D) Mercury.
Melting and solidification of crystalline bodies.
Option No. 4
1. During crystallization (solidification) of a molten substance, its temperature ...
A) will not change. B) increases.
B) decreases.
2. The lowest air temperature -88.30C was recorded in 1960 in Antarctica at the Vostok scientific station. What thermometer can be used in this place on Earth?
A) Mercury. B) Alcohol
C) You can use both mercury and alcohol thermometers.
D) Neither mercury nor alcohol thermometers should be used.
3. Is it possible to melt copper in an aluminum pan?
Can i. B) It is impossible.
4. Which metal has a crystal lattice that is destroyed at the highest temperature?
A) In steel. B) In copper. B) In tungsten.
D) Platinum D) Osmium.
2. Checking what is written at the board. Error correction.
3. Studying new material.
a) Film demonstration. "Melting and crystallization of a solid"
b) Constructing a graph of changes in the physical state of the body. (2 slide)
c) detailed analysis of the graph with analysis of each segment of the graph; study of all physical processes occurring in a particular interval of the graph. (3 slide)
melting?
A) 50 0С B) 1000С C) 6000С D) 12000С
0 3 6 9 min.
D) 16 min. D) 7 min.
Option No. 2 0C
segment AB? 1000
D) Hardening. B C
segment BV?
A) Heating. B) Cooling. B) Melting. 500
D) Hardening D
3. At what temperature did the process begin?
hardening?
A) 80 0C. B) 350 0С C) 3200С
D) 450 0С D) 1000 0С
4. How long did it take for the body to harden? 0 5 10 min.
A) 8 min. B) 4 min. B) 12 min.
D) 16 min. D) 7 min.
A) Increased. B) Decreased. B) Has not changed.
6. What process on the graph characterizes the VG segment?
A) Heating. B) Cooling. B) Melting. D) Hardening.
Graph of melting and solidification of crystalline solids.
Option No. 3 0C
1.Which process on the graph characterizes 600 G
segment AB?
A) Heating. B) Cooling. B) Melting.
D) Hardening. B C
2. What process on the graph characterizes
segment BV?
A) Heating. B) Cooling. B) Melting. 300
D) Hardening.
3. At what temperature did the process begin?
melting?
A) 80 0С B) 3500С C) 3200С D) 4500С
4. How long did it take for the body to melt? A
A) 8 min. B) 4 min. B) 12 min. 0 6 12 18 min.
D) 16 min. D) 7 min.
5. Did the temperature change during melting?
A) Increased. B) Decreased. B) Has not changed.
6. What process on the graph characterizes the VG segment?
A) Heating. B) Cooling. B) Melting. D) Hardening.
Graph of melting and solidification of crystalline solids.
Option No. 4 0C
1. Which process on the graph characterizes A
segment AB? 400
A) Heating. B) Cooling. B) Melting.
D) Hardening. B C
2. . What process on the graph characterizes
segment BV?
A) Heating. B) Cooling. B) Melting. 200
D) Hardening
3. At what temperature did the process begin?
hardening?
A) 80 0C. B) 350 0С C) 3200С D
D) 450 0С D) 1000 0С
4. How long did it take for the body to harden? 0 10 20 min.
A) 8 min. B) 4 min. B) 12 min.
D) 16 min. D) 7 min.
5. Did the temperature change during curing?
A) Increased. B) Decreased. B) Has not changed.
6. What process on the graph characterizes the VG segment?
A) Heating. B) Cooling. B) Melting. D) Hardening.
III. Lesson summary.
IV. Homework (Differentiated) 5 slide
V. Grading for the lesson.
To effectively plan all construction work, you need to know how long it takes for concrete to harden. And here there are a number of subtleties that largely determine the quality of the constructed structure. Below we will describe in detail how the solution is dried and what you need to pay attention to when organizing related operations.
Theory of polymerization of cement mortar
To manage the process, it is very important to understand exactly how it happens. That is why it is worth studying in advance what hardening of cement is ().
In fact, this process is multi-stage. It includes both strength building and drying itself.
Let's look at these stages in more detail:
- Hardening of concrete and other cement-based mortars begins with the so-called setting. In this case, the substance in the formwork enters into a primary reaction with water, due to which it begins to acquire a certain structure and mechanical strength.
- Setting time depends on many factors. If we take the air temperature of 20 0 C as a standard, then for the M200 solution the process starts approximately two hours after pouring and lasts about an hour and a half.
- After setting, the concrete hardens. Here the bulk of the cement granules react with water (for this reason the process is sometimes called cement hydration). Optimal conditions for hydration are air humidity of about 75% and temperature from 15 to 20 0 C.
- At temperatures below 10 0 C, there is a risk that the material will not reach its design strength, which is why special anti-frost additives must be used for work in winter.
- The strength of the finished structure and the rate of hardening of the solution are interrelated. If the composition loses water too quickly, then not all the cement will have time to react, and pockets of low density will form inside the structure, which can become a source of cracks and other defects.
Note! Cutting reinforced concrete with diamond wheels after polymerization often clearly demonstrates the heterogeneous structure of slabs poured and dried in violation of technology.
- Ideally, the solution requires 28 days before complete hardening.. However, if the structure does not have too strict requirements for load-bearing capacity, then you can begin to operate it within three to four days after pouring.
Factors affecting hardening
When planning construction or repair work, it is important to correctly assess all the factors that will influence the rate of dehydration of the solution ().
Experts highlight the following points:
- Firstly, environmental conditions play a crucial role. Depending on the temperature and humidity, the poured foundation can either dry out in just a few days (and then will not reach its design strength), or remain wet for more than a month.
- Secondly – packing density. The denser the material, the slower it loses moisture, which means that the hydration of cement occurs more efficiently. Vibration treatment is most often used for compaction, but when doing the work yourself, you can get by with bayoneting.
Advice! The denser the material, the more difficult it is to process it after hardening. This is why structures that were constructed using vibration compaction most often require diamond drilling of holes in concrete: conventional drills wear out too quickly.
- The composition of the material also affects the speed of the process. Mainly, the rate of dehydration depends on the porosity of the filler: expanded clay and slag accumulate microscopic particles of moisture and release them much more slowly than sand or gravel.
- Also, to slow down drying and more effectively gain strength, moisture-retaining additives (bentonite, soap solutions, etc.) are widely used. Of course, the price of the structure increases, but you don’t need to worry about premature drying out.
- In addition to all of the above, the instructions recommend paying attention to the formwork material. The porous walls of unedged boards draw a significant amount of liquid from the edge areas. Therefore, to ensure strength, it is better to use formwork made of metal panels or lay polyethylene film inside the wooden box.
Self-pouring of concrete foundations and floors must be carried out according to a certain algorithm.
To retain moisture in the thickness of the material and promote maximum strength gain, you need to act like this:
- To begin with, we perform high-quality waterproofing of the formwork. To do this, we cover the wooden walls with polyethylene or use special plastic collapsible panels.
- We introduce modifiers into the solution, the effect of which is aimed at reducing the rate of evaporation of the liquid. You can also use additives that allow the material to gain strength faster, but they are quite expensive, which is why they are used mainly in multi-story construction.
- Then pour the concrete, compacting it thoroughly. For this purpose, it is best to use a special vibrating tool. If there is no such device, we process the poured mass with a shovel or metal rod, removing air bubbles.
- After setting, cover the surface of the solution with plastic wrap. This is done in order to reduce moisture loss in the first few days after installation.
Note! In autumn, polyethylene also protects cement located in the open air from precipitation, which erodes the surface layer.
- After about 7-10 days, the formwork can be dismantled. After dismantling, we carefully inspect the walls of the structure: if they are wet, then you can leave them open, but if they are dry, it is better to also cover them with polyethylene.
- After this, every two to three days we remove the film and inspect the concrete surface. If a large amount of dust, cracks or peeling of the material appears, we moisten the frozen solution with a hose and cover it again with polyethylene.
- On the twentieth day, remove the film and continue drying naturally.
- After 28 days have passed since the filling, the next stage of work can begin. At the same time, if we did everything correctly, the structure can be loaded “to its fullest” - its strength will be maximum!
Conclusion
Knowing how long it takes for a concrete foundation to harden, we will be able to properly organize all other construction work. However, this process cannot be accelerated, since cement acquires the necessary performance characteristics only when it hardens for a sufficient time ().
More detailed information on this issue is presented in the video in this article.
Back forward
Attention! Slide previews are for informational purposes only and may not represent all the features of the presentation. If you are interested in this work, please download the full version.
Lesson type: combined.
Lesson type: traditional.
Lesson objectives: find out what happens to a substance when it melts and solidifies.
Tasks:
- Educational:
- consolidate existing knowledge on the topic “Structure of Matter”.
- become familiar with the concepts of melting and solidification.
- continue to develop the ability to explain processes from the point of view of the structure of matter.
- explain the concepts of melting and solidification in terms of changes in internal energy
- Educational:
- formation of communicative qualities, communication culture
- developing interest in the subject being studied
- stimulating curiosity and activity in the classroom
- development of performance
- Developmental:
- development of cognitive interest
- development of intellectual abilities
- development of skills to highlight the main thing in the material being studied
- development of skills to generalize studied facts and concepts
Forms of work: frontal, work in small groups, individual.
Means of education:
- Textbook “Physics 8” A.V. Peryshkin § 12, 13, 14.
- Collection of problems in physics for grades 7-9, A.V. Peryshkin, 610 - 618.
- Handouts (tables, cards).
- Presentation.
- Computer.
- Illustrations on the topic.
Lesson plan:
- Organizing time.
- Repetition of learned material. Filling the table: solid, liquid, gaseous.
- Determining the topic of the lesson.
- Transition from solid to liquid state of aggregation and vice versa.
- Write down the topic of the lesson in your notebook.
- Learning a new topic:
- Determination of the melting point of a substance.
- Working with the textbook table “Melting Point”.
- The solution of the problem.
- View the melting and solidification animation.
- Working with the Melting and Solidification graph.
- Filling out the table: melting, solidification.
- Consolidation of the studied material.
- Summarizing.
- Homework.
| Stage number | Teacher's work. | Student work. | Notebook entries. | What is used. | Time | |
Organizing time. Greetings. |
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In 7th grade we became acquainted with various states of matter. What states of matter do you know? Examples? |
Solid, liquid, gaseous states of matter. For example, water, ice, water vapor. |
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Let's remember what properties substances have in a particular state of aggregation and why. We will remember by filling out the table. ( Annex 1). The teacher records the order in which the groups raise their hands and stops work after 2 minutes. |
The class is divided into groups of 3-4 people. Each group receives a sheet with a blank table and cards with answers. In 2 minutes they must place the cards in the appropriate cells of the table. When ready, group members raise their hands. After 2 minutes the groups report on their work. One group explains which card they placed in which cell, why, and members of the remaining groups either agree or correct the answer. As a result, the table for each group is filled out correctly. The first group to complete the tasks correctly receives one point. |
Slide 2 handout |
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So, what is common and what is different in the properties of solids and liquids? |
Both solids and liquids retain volume, but only solids retain shape. |
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Today in class we will talk about how a solid can turn into a liquid and vice versa. Let us find out what conditions are necessary for these transitions. |
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Remember what the transition of a substance from a solid to a liquid state of aggregation is called? |
As a rule, students remember the name of the process - melting. |
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What is the reverse process called: the transition of a substance from a liquid to a solid state of aggregation? What is the internal structure of solids called? |
If students do not answer a question right away, you can help them a little, but usually the students themselves give the answer. The process of transition of a substance from a liquid to a solid state is called solidification. The molecules of solids form a crystal lattice, so the process can be called crystallization. |
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So, the topic of today's lesson is: “Melting and solidification of crystalline bodies.” |
Write down the topic of the lesson in your notebook. |
Melting and solidification of crystalline bodies |
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Let us remember once again what we already know about the states of matter and the transition of matter from one state of matter to another. |
Students answer questions. For each correct answer (in this case and in the future), the student receives 1 point. |
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Why do bodies retain their shape only in a solid state of aggregation? How does the internal structure of solids differ from the internal structure of liquids and gases? |
In solids, particles are arranged in a certain order (forming a crystal lattice) and cannot move far away from each other. |
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What changes in the internal structure of the substance? |
When melting, the order of the molecules is disrupted, i.e. the crystal lattice is destroyed. |
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What needs to be done to melt the body? Destroy the crystal lattice? |
The body must be heated, that is, a certain amount of heat must be imparted to it, energy must be transferred. |
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To what temperature should the body be heated? Examples? |
In order for the ice to melt, you need to heat it to 0 0C. In order for iron to melt, it must be heated to a higher temperature. |
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So, to melt a solid, you need to heat it to a certain temperature. This temperature is called the melting point. |
Write down the determination of the melting point in your notebook. |
Melting point is the temperature at which a solid melts. |
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Each substance has its own melting point. At temperatures above the melting point, the substance is in a liquid state, below - in a solid state. Consider the textbook table on page 32. |
Open textbooks on the specified page. |
Slide 5 table 3 textbooks |
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|
|
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At what temperature does water solidify? Iron? Oxygen? |
At 0°C, 1539°C, -219°C. |
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Substances solidify at the same temperature at which they melt. |
The crystallization temperature of a substance is equal to its melting point. |
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Let's return to the question: What happens to the internal structure of a substance when it melts? Crystallization? |
During melting, the crystal lattice is destroyed, and during crystallization it is restored. |
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Let's take a piece of ice at a temperature of -10 °C and impart energy to it. What happens to a piece of ice? |
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Problem: What amount of heat must be imparted to 2 kg of ice in order to heat it by 10 °C? |
Using the table on page 21, solve the problem. (orally). It will take 2100 2 10 = 42000 J = 42 kJ |
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What is heat used for in this case? |
To increase the kinetic energy of molecules. The ice temperature rises. |
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Let us consider how the temperature of ice changes when a certain amount of heat is uniformly imparted to it, what happens to the internal structure of ice (water) in the above processes. |
They look at the proposed presentation, note what happens to a substance when it is heated, melted, cooled, or solidified. |
Slides 7 - 10 |
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Schedule. What process does the section AB, BC correspond to? Will the temperature of ice increase when it begins to melt? Airplane schedule. |
Section AB corresponds to the process of heating ice. IC – ice melting. When melting begins, the temperature of the ice stops increasing. |
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Does the ice continue to receive energy? What is it spent on? |
The ice continues to receive energy. It is spent on the destruction of the crystal lattice. |
During the melting process, the temperature of the substance does not change; energy is spent on the destruction of the crystal lattice. |
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In what state of aggregation is the substance at point B? at point C? At what temperature? |
B – ice at 0 °C. C – water at 0 °C. |
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Which has more internal energy: ice at 0 °C or water at 0 °C? |
Water has greater internal energy, since the substance gained energy during the melting process. |
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Why does the temperature begin to rise in section CD? |
At point C, the destruction of the lattice ends and further energy is spent on increasing the kinetic energy of water molecules. |
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Fill the table ( Appendix 2) using the graph and the proposed animation. Time limit: 2 minutes. The teacher monitors the process of filling out the table, records who has completed the task, and stops the work after 2 minutes. |
Fill out the table. After completing the table, students raise their hand. After 2 minutes, students read their notes and explain them: 1 student - 1 line, 2 student - 2 lines, etc. If the answerer makes a mistake, other students correct it. Students who complete the task correctly and completely within 2 minutes receive 1 point. |
Handout |
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So, energy is consumed by a substance during melting and heating, and released during crystallization and cooling, and no temperature change occurs during melting and crystallization. Try to apply this knowledge when completing the following tasks. |
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Iron taken at a temperature of 20 °C was completely melted. What schedule corresponds to this process? |
Select a graph on the slide that corresponds to the specified process, raise your hands, indicating the number of the selected graph with the number of fingers. One of the students (at the teacher's choice) explains his choice. |
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Water taken at a temperature of 0 °C was turned into ice at -10 °C. What schedule corresponds to this process? |
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Solid mercury, taken at a temperature of -39 °C, was heated to a temperature of 20 0C. What schedule corresponds to this process? |
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Will ice taken at 0°C melt in a room with a temperature of 0°C? |
No, energy is needed to destroy the crystal lattice, and heat transfer is only possible from a body with a higher temperature to a body with a lower temperature, therefore, in this case, heat transfer will not take place. |
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Lesson summary. Students who score 5 or more points in a lesson receive positive grades. |
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Homework. |
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Used Books:
- Peryshkin A.V. textbook "Physics 7"
- Peryshkin A.V. “Collection of problems in physics grades 7 – 9”, Moscow, “Exam”, 2006.
- V.A. Orlov “Thematic tests in physics grades 7 – 8”, Moscow, “Verbum - M”, 2001.
- G.N. Stepanova, A.P. Stepanov “Collection of questions and problems in physics grades 5 – 9”, St. Petersburg, “Valeria SPD”, 2001.
- http://kak-i-pochemu.ru
We present to your attention a video lesson on the topic “Melting and solidification of crystalline bodies. Melting and solidification schedule." Here we begin the study of a new broad topic: “Aggregative states of matter.” Here we will define the concept of a state of aggregation and consider examples of such bodies. And let’s look at what the processes in which substances pass from one state of aggregation to another are called and what they are. Let us dwell in more detail on the processes of melting and crystallization of solids and draw up a temperature graph of such processes.
Topic: Aggregate states of matter
Lesson: Melting and solidification of crystalline bodies. Melting and solidification schedule
Amorphous bodies- bodies in which atoms and molecules are ordered in a certain way only near the area under consideration. This type of arrangement of particles is called short-range order.
Liquids- substances without an ordered structure of particle arrangement, molecules in liquids move more freely, and intermolecular forces are weaker than in solids. The most important property: they retain volume, easily change shape and, due to their fluidity properties, take the shape of the vessel in which they are located (Fig. 3).
Rice. 3. The liquid takes the shape of a flask ()
Gases- substances whose molecules interact weakly with each other and move chaotically, often colliding with each other. The most important property: they do not retain volume and shape and occupy the entire volume of the vessel in which they are located.
It is important to know and understand how transitions between states of matter occur. We depict a diagram of such transitions in Figure 4.
1 - melting;
2 - hardening (crystallization);
3 - vaporization: evaporation or boiling;
4 - condensation;
5 - sublimation (sublimation) - transition from a solid to a gaseous state, bypassing the liquid;
6 - desublimation - transition from a gaseous state to a solid state, bypassing the liquid state.
In today's lesson we will pay attention to processes such as melting and solidification of crystalline bodies. It is convenient to begin considering such processes using the example of the most common melting and crystallization of ice in nature.
If you place ice in a flask and start heating it with a burner (Fig. 5), you will notice that its temperature will begin to rise until it reaches the melting temperature (0 o C), then the melting process will begin, but at the same time the temperature of the ice will not increase, and only after the process of melting all the ice is completed, the temperature of the resulting water will begin to increase.
Rice. 5. Ice melting.
Definition.Melting- the process of transition from solid to liquid. This process occurs at a constant temperature.
The temperature at which a substance melts is called the melting point and is a measured value for many solids, and therefore a tabular value. For example, the melting point of ice is 0 o C, and the melting point of gold is 1100 o C.
The reverse process to melting - the process of crystallization - is also conveniently considered using the example of freezing water and turning it into ice. If you take a test tube with water and start cooling it, you will first observe a decrease in the temperature of the water until it reaches 0 o C, and then it freezes at a constant temperature (Fig. 6), and after complete freezing, further cooling of the formed ice.
Rice. 6. Freezing of water.
If the described processes are considered from the point of view of the internal energy of the body, then during melting all the energy received by the body is spent on destroying the crystal lattice and weakening intermolecular bonds, thus, energy is spent not on changing temperature, but on changing the structure of the substance and the interaction of its particles. During the process of crystallization, energy exchange occurs in the opposite direction: the body gives off heat to the environment, and its internal energy decreases, which leads to a decrease in the mobility of particles, an increase in the interaction between them and solidification of the body.
It is useful to be able to graphically depict the processes of melting and crystallization of a substance on a graph (Fig. 7).
The axes of the graph are: the abscissa axis is time, the ordinate axis is the temperature of the substance. As the substance under study, we will take ice at a negative temperature, i.e., ice that, upon receiving heat, will not immediately begin to melt, but will be heated to the melting temperature. Let us describe the areas on the graph that represent individual thermal processes:
Initial state - a: heating of ice to a melting point of 0 o C;
a - b: melting process at a constant temperature of 0 o C;
b - a point with a certain temperature: heating the water formed from ice to a certain temperature;
A point with a certain temperature - c: cooling of water to a freezing point of 0 o C;
c - d: the process of freezing water at a constant temperature of 0 o C;
d - final state: cooling of ice to a certain negative temperature.
Today we looked at various states of matter and paid attention to processes such as melting and crystallization. In the next lesson we will discuss the main characteristic of the process of melting and solidification of substances - the specific heat of fusion.
1. Gendenshtein L. E., Kaidalov A. B., Kozhevnikov V. B. /Ed. Orlova V. A., Roizena I. I. Physics 8. - 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. Dictionaries and encyclopedias on Academician ().
2. Course of lectures “Molecular physics and thermodynamics” ().
3. Regional collection of the Tver region ().
1. Page 31: questions No. 1-4; page 32: questions No. 1-3; page 33: exercises No. 1-5; page 34: questions No. 1-3. Peryshkin A.V. Physics 8. - M.: Bustard, 2010.
2. A piece of ice floats in a pan of water. Under what condition will it not melt?
3. During melting, the temperature of the crystalline body remains unchanged. What happens to the internal energy of the body?
4. Experienced gardeners, in the event of spring night frosts during the flowering of fruit trees, water the branches generously in the evening. Why does this significantly reduce the risk of losing future crops?
