To effectively plan all construction work, you need to know how long concrete hardens. And here there are a number of subtleties that largely determine the quality of the erected 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 cement mortar polymerization

To manage the process, it is very important to understand exactly how it happens. That is why it is worthwhile to study in advance what constitutes the solidification of cement ().

In fact, this process is multi-stage. It includes both a set of strength and the actual drying.

Let's look at these stages in more detail:

• The hardening of concrete and other cement-based mortars begins with the so-called setting. At the same time, 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 С 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 curing, the concrete hardens. Here, the bulk of the cement granules react with water (for this reason, the process is sometimes called cement hydration). The 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 gain design strength, which is why special anti-frost additives must be used to work in the winter.

• The strength of the finished structure and the rate of curing of the solution are interrelated. If the composition loses water too quickly, then not all the cement will have time to react, and low density pockets 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 inhomogeneous structure of slabs poured and dried in violation of technology.

• Ideally, the mortar needs 28 days to fully cure.. However, if too strict requirements for bearing capacity are not put forward to the structure, then it can be started to operate already three to four days after pouring.

Factors affecting freezing

When planning construction or repair work, it is important to correctly evaluate all the factors that will affect the rate of dehydration of the solution ().

Experts highlight the following points:

• First, environmental conditions play an important role. Depending on the temperature and humidity, the poured foundation can either dry out in just a few days (and then it will not gain design strength), or remain wet for more than a month.
• Secondly, the packing density. The denser the material, the slower it loses moisture, which means that the cement is hydrated more efficiently. For compaction, vibration processing is most often used, but when doing work with your own hands, you can get by with bayoneting.

Advice! The denser the material, the more difficult it is to process after hardening. That is why for structures, during the construction of which vibration compaction was used, diamond drilling of holes in concrete is most often required: conventional drills wear out too quickly.

• The composition of the material also affects the speed of the process. The rate of dehydration mainly depends on the porosity of the filler: expanded clay and slag accumulate microscopic moisture particles and release them much more slowly than sand or gravel.
• Also, water-retaining additives (bentonite, soap solutions, etc.) are widely used to slow down drying and more effective curing. Of course, the price of the structure increases, but there is no need to worry about premature drying.

• In addition to all of the above, the instruction recommends paying attention to the formwork material. Porous walls made of unedged boards draw a significant amount of liquid from the edge sections. Therefore, to ensure strength, it is better to use formwork made of metal shields or to lay a plastic film inside a wooden box.

Self-pouring of concrete foundations and floors should be carried out according to a certain algorithm.

To keep moisture in the thickness of the material and contribute to the maximum set of strength, you need to act like this:

• To begin with, we carry out high-quality waterproofing of the formwork. To do this, we cover the wooden walls with polyethylene or use special plastic collapsible shields.
• We introduce modifiers into the composition of the solution, the action 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, and therefore they are used mainly in multi-storey construction.
• Then we pour concrete, carefully compacting it. For this purpose, it is best to use a special vibration tool. If there is no such device, we process the poured mass with a shovel or a metal rod, removing air bubbles.

• The surface of the solution after setting is covered with a plastic film. This is done in order to reduce moisture loss in the first few days after laying.

Note! In autumn, the polyethylene also protects the outdoor cement from precipitation that erodes the surface layer.

• After about 7-10 days, the formwork can be dismantled. After dismantling, we carefully examine the walls of the structure: if they are wet, then you can leave them open, but it is better to cover the dry ones with polyethylene too.
• After that, every two or three days we remove the film and inspect the surface of the concrete. If a large amount of dust, cracks or delamination of the material appears, we moisten the hardened solution from the hose and cover it again with polyethylene.
• On the twentieth day, remove the film and continue drying in natural mode.
• After 28 days have passed from the moment of pouring, the next stage of work can begin. At the same time, if we did everything correctly, you can load the structure “to the fullest” - its strength will be maximum!

Conclusion

Knowing how long the concrete foundation hardens, we can 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 ().

For more information on this issue, see the video in this article.

Many novice builders are familiar with the inevitable appearance of defects on the surface of concrete: small cracks, chips, quick failure of the coating. The reason is not only in non-compliance with the rules of concreting, or in the creation of a cement mortar with the wrong ratio of components, more often the problem lies in the lack of care for concrete during the curing stage.

The setting time of the cement mortar depends on numerous factors: temperature, humidity, wind, exposure to direct sunlight, etc. It is important to moisten the concrete during the curing stage, this will maximize the strength and integrity of the coating.

The setting time of a cement slurry depends on numerous factors.

General information

Depending on the temperature at which the cement hardens, the hardening period also differs. The best temperature is 20°C. Under ideal conditions, the process takes 28 days. In hot regions or during cold periods of the year, it is difficult or impossible to maintain this temperature.

In winter, concreting is required for a number of reasons:

• laying the foundation for a building that is located on crumbling soils. During the warm period of the year it is impossible to carry out construction;
• in winter, manufacturers make discounts on cement. Sometimes you can save on material really well, but storage before the onset of heat is an undesirable solution, because the quality of the cement will decrease. Pouring concrete on the interior surfaces of buildings and even outdoor work in winter is quite appropriate in the presence of discounts;
• private concrete work;
• In winter, more free time and easier to take a vacation.

The disadvantage of working in cold weather is the difficulty of digging a trench and the need to equip a place for heating for workers. Taking into account additional costs, savings do not always occur.

Features of pouring concrete at low temperatures

The curing time of the cement mortar depends on the temperature. At low temperatures, the time increases significantly. In the construction industry, it is customary to call the weather cold when the thermometer level drops to an average of 4 ° C. To successfully use cement in cold weather, it is important to take protective measures to prevent the mortar from freezing.

Features of pouring concrete at low temperatures

The setting of concrete at low temperatures proceeds somewhat differently, the water temperature has the greatest effect on the final result. The warmer the liquid, the faster the process. Ideally, for winter, it is worthwhile to ensure that the thermometer is at the level of 7-15 °. Even under warm water conditions, ambient cold slows down the hydration rate of the cement slurry. The acquisition of strength and setting takes longer.

To calculate how much cement hardens, it is important to take into account the regularity that a temperature drop of 10 ° leads to a 2-fold decrease in the hardening rate. It is important to carry out calculations, since premature removal of the formwork or operation of concrete can lead to the destruction of the material. If the ambient temperature drops to -4°C and there are no additives, heaters or heating, the solution will crystallize and the cement hydration process will stop. The final product will lose 50% strength. The hardening time will increase by 6-8 times.

Although it is necessary to determine how long the concrete hardens, and it is necessary to control the curing process, there is a downside - the possibility of improving the quality of the result. Lowering the temperature increases the strength of concrete, but only to a critical level of -4°C, although the procedure takes more time.

Factors affecting freezing

At the stage of planning work with cement, an important factor influencing the final result is the rate of concrete dehydration. Numerous factors influence the hydration process, it is possible to more accurately determine how much the cement mortar hardens, taking into account the following factors:

• Environment. Air humidity and temperature are taken into account. With high dryness and heat, concrete will harden in just 2-3 days, but it will not have time to acquire the expected strength. Otherwise, it will remain wet for 40 days or more;

Factors Affecting Concrete Curing

• fill density. As the cement compacts, the rate of moisture release decreases, which improves the hydration procedure, but somewhat reduces the rate. It is better to compact the material with a vibrating plate, but manually piercing the solution is also suitable. If the composition is dense, it will be difficult to handle after solidification. At the stage of finishing or laying communications in compacted concrete, it is necessary to use diamond drilling, since the drill bits quickly wear out;
• solution composition. The factor is quite important, because the level of porosity of the filler affects the rate of dehydration. The solution with expanded clay and slag solidifies more slowly, moisture accumulates in the filler, and it is released slowly. With gravel or sand, the composition dries faster;
• the presence of additives. Special additives with water-retaining properties help to reduce or accelerate the stages of hardening of the solution: soap solution, bentonite, antifreeze additives. The acquisition of such components increases the amount of work, but many additives simplify the work with the composition and increase the quality of the result;
• formwork material. The curing time of cement depends on the tendency to absorb or retain moisture formwork. Porous walls influence the hardening rate: unsanded boards, plastic with through holes or loose installation. The best way to complete construction work on time and with the preservation of the technical characteristics of concrete is to use metal shields or install a plastic film over the plank formwork.

The type of base also affects how much the cement mortar hardens. Dry soil quickly absorbs moisture. When concrete hardens in the sun, the hardening time increases many times, in order to prevent low strength of the material, the surface should be constantly moistened and the area should be shaded.

Artificial increase in the speed of solidification

The hardening time of the cement mortar in cold weather increases greatly, but the time is still limited. To speed up the process, various techniques have been developed.

BITUMAST Antifreeze admixture for concrete

In modern construction, drying time can be accelerated with:

• introduction of additives;
• electric heating;
• increasing the required proportions of cement.

Using modifiers

The easiest way to complete the work on time, even in winter, is to apply modifiers. When a certain proportion is introduced, the hydration period is reduced; when using some additives, hardening occurs even at -30 ° C.

Conventionally, additives that affect the rate of hardening are divided into several groups:

• type C - drying accelerators;
• type E - water-substituting additives with accelerated setting.

The foundation solidification calculator and reviews show maximum efficiency when potassium chloride is added to the solution. The material diverges economically, since its mass fraction is up to 2%.

If type C concrete curing mixtures are used, it is worth taking care of heating, as they do not protect against freezing.

Plasticizers and additives for concrete

It is recommended to take care of laying communications in the foundation or screed in advance, otherwise drilling holes will be required. Making communication holes after solidification will lead to the need for a special tool and. The procedure is quite laborious and reduces the strength of the structure.

Concrete heating

Mostly, a special cable is used to heat the composition, which converts electric current into heat. The technique provides the most natural way of solidification. An important factor is the need to follow the instructions for installing the wire. The method protects against liquid crystallization, there are also tools (hair dryer, welding machine) and thermal insulation to protect against freezing.

Increasing the dosage of cement

Increasing the cement concentration is used only with a slight decrease in temperature. It is important to increase the dosage in a small amount, otherwise the quality and durability will be significantly reduced.

Concrete is a multifunctional composition from which any structure can be built. In modern construction, a variety of cement compositions and methods of its processing are used:

• The first step in the construction of a building is drawing up a diagram and calculating the load. Durability also depends on various characteristics. It is important to follow all the masonry rules to obtain the calculated strength;

• common in private construction. They improve thermal insulation properties, reduce the load on the foundation, make it easy and quick to lay walls. You can make them yourself. are formed according to a similar algorithm with blocks;
• in damp rooms there is a need for additional protection of concrete. A special one is used, since standard mixtures do not completely cover the concrete wall;
• one of the most popular and frequent procedures for working with a solution is a screed. The proportions of cement and sand for screed differ depending on the task.

Conclusion

Concreting in hot or cold conditions requires special precautions. If you create ideal conditions for the hydration of concrete, it will acquire high strength, be able to withstand significant bearing loads and acquire resistance to destruction. The main task of the builder is to prevent freezing or premature drying of the solution.

The vast majority of amateur builders believe, for reasons that are not entirely clear, that after the completion of laying in the formwork or the completion of work on leveling the screed, the concreting process is completed. Meanwhile, the setting time of concrete is much longer than the time for its laying. A concrete mixture is a living organism in which, after the completion of laying work, complex and time-consuming physical and chemical processes occur, associated with the transformation of the solution into a reliable basis for building structures.

Before demoulding and enjoying the results of the efforts made, it is necessary to create the most comfortable conditions for the maturation and optimal hydration of concrete, without which it is impossible to achieve the required grade strength of the monolith. Building codes and regulations contain verified data, which are given in the concrete setting time tables.

Concrete temperature, С	Curing time of concrete, days
	1	2	3	4	5	6	7	14	28
	Strength of concrete, %
0	20	26	31	35	39	43	46	61	77
10	27	35	42	48	51	55	59	75	91
15	30	39	45	52	55	60	64	81	100
20	34	43	50	56	60	65	69	87	-
30	39	51	57	64	68	73	76	95	-
40	48	57	64	70	75	80	85	-	-
50	49	62	70	78	84	90	95	-	-
60	54	68	78	86	92	98	-	-	-
70	60	73	84	96	-	-	-	-	-
80	65	80	92	-	-	-	-	-	-

Concrete care after pouring: main goals and methods

The processes associated with the activities that precede stripping contain several technological methods. The purpose of such activities is the same - the creation of a reinforced concrete structure that, in terms of its physical and technical properties, is as close as possible to the parameters that are included in the project. The fundamental measure, of course, is the care of the laid concrete mixture.

Care consists in the implementation of a set of measures that are designed to create conditions that optimally correspond to the physical and chemical transformations taking place in the mixture during the curing of concrete. Strict adherence to the requirements prescribed by the care technology allows you to:

• reduce shrinkage phenomena in the concrete composition of plastic origin to the minimum values;
• ensure the strength and temporal values ​​of the concrete structure in the parameters provided for by the project;
• protect the concrete mixture from temperature dysfunctions;
• prevent preliminary hardening of the laid concrete mix;
• protect the structure from various origins of impacts of mechanical or chemical origin.

Care procedures for a freshly constructed reinforced concrete structure should be started immediately after the completion of the mixture laying and continue until it reaches 70% of the strength provided for by the project. This is provided for by the requirements set out in paragraph 2.66 of SNiP 3.03.01. Stripping can be carried out at an earlier date, if this is justified by the prevailing parametric circumstances.

After the concrete mix has been placed, the formwork structure should be inspected. The purpose of such an inspection is to determine the preservation of geometric parameters, to detect leakage of the liquid component of the mixture and mechanical damage to the formwork elements. Taking into account how long the concrete hardens, or rather, taking into account the setting time, the defects that have appeared must be eliminated. The average time for which a freshly laid concrete mixture can grab is about 2 hours, depending on the temperature parameters and the brand of Portland cement. The structure must be protected from any mechanical impact in the form of shocks, vibrations, vibration manifestations for as long as the concrete dries.

Strengthening stages of a concrete structure

A concrete mixture of any composition tends to set and obtain the necessary strength characteristics when passing through two stages. Compliance with the optimal ratio of time, temperature parameters and reduced humidity values ​​is of decisive importance for obtaining a monolithic structure with planned properties.

The stage characteristics of the process are:

• setting of the concrete composition. The pre-setting time is not long and is approximately 24 hours at an average temperature of +20 Co. The initial setting processes occur within the first two hours after mixing the mixture with water. The final setting occurs, as a rule, within 3-4 hours. The use of specialized polymer additives makes it possible, under certain conditions, to reduce the period of initial setting of the mixture to several tens of minutes, but the expediency of such an extreme method is justified for the most part in the in-line production of reinforced concrete elements of industrial structures;
• hardening of concrete. Concrete gains strength when the process of hydration takes place in its mass, in other words, the removal of water from the concrete mixture. Part of the water during the passage of this process is removed during its evaporation, the other part is associated at the molecular level with the chemical compounds that make up the mixture. Hydration can occur with strict observance of the temperature and humidity conditions of hardening. Violation of the conditions leads to failures in the passage of physical and chemical processes of hydration and, accordingly, to a deterioration in the quality of the reinforced concrete structure.

The dependence of the curing time on the brand of concrete mix

It is logically clear that the use of different grades of Portland cement for the preparation of concrete compositions leads to a change in the hardening time of concrete. The higher the grade of Portland cement, the less time it takes for the mixture to set strength. But when using any brand, be it brand 300 or 400, significant mechanical loads should not be applied to the reinforced concrete structure earlier than after 28 days. Although the concrete setting time according to the tables given in the building regulations may be less. This is especially true for concretes prepared using Portland cement grade 400.

Grade of cement	Hardening time of various grades of concrete
	for 14 days		for 28 days
	100	150	100	150	200	250	300	400
300	0.65	0.6	0.75	0.65	0.55	0.5	0.4	-
400	0.75	0.65	0.85	0.75	0.63	0.56	0.5	0.4
500	0.85	0.75	-	0.85	0.71	0.64	0.6	0.46
600	0.9	0.8	-	0.95	0.75	0.68	0.63	0.5

The design, construction and final arrangement of any buildings using reinforced concrete components requires careful attention to all stages of construction. But the durability and reliability of the entire structure largely depends on the thoroughness of the manufacture of concrete components, especially foundations. Compliance with the deadlines for which concrete mixes and compositions set can be safely called the basis of success in any construction process.

Goals and objectives of the lesson: improving the skills of graphical problem solving, repetition of basic physical concepts on this topic; development of oral and written speech, logical thinking; activation of cognitive activity through the content and degree of complexity of tasks; generating interest in the topic.

Lesson plan.

During the classes

Necessary equipment and materials: computer, projector, screen, whiteboard, Ms Power Point program, for each student : laboratory thermometer, test tube with paraffin, test tube holder, glass with cold and hot water, calorimeter.

Control:

Start presentation "F5 key", stop - "Esc key".

Changes of all slides are organized by clicking the left mouse button (or by pressing the right arrow key).

Return to the previous slide "left arrow".

I. Repetition of the studied material.

1. What aggregate states of matter do you know? (Slide 1)

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

3. Give examples of finding a substance in various states of aggregation in nature. (Slide 3)

4. What is the practical significance of the phenomena of the transition of matter from one state of aggregation to another? (Slide 4)

5. What process corresponds to the transition of a substance from a liquid state to a solid state? (Slide 5)

6. What process corresponds to the transition of a substance from a solid state to a liquid? (Slide 6)

7. What is sublimation? Give examples. (Slide 7)

8. How does the speed of the molecules of a substance change during the transition from a liquid to a solid state?

II. Learning new material

In the lesson, we will study the process of melting and crystallization of a crystalline substance - paraffin, and plot these processes.

In the course of performing a physical experiment, we will find out how the temperature of paraffin changes during heating and cooling.

You will perform the experiment according to the descriptions for the work.

Before starting work, I will remind you of the safety rules:

When performing laboratory work, be careful and careful.

Safety engineering.

1. Calorimeters contain water 60? C, be careful.

2. Use caution when handling glassware.

3. If the device is accidentally broken, then inform the teacher, do not remove the fragments yourself.

III. Frontal physical experiment.

On the tables of the students there are sheets with a description of the work (Appendix 2), according to which they perform the experiment, build a graph of the process and draw conclusions. (Slides 5).

IV. Consolidation of the studied material.

Summing up the results of the frontal experiment.

Findings:

When heated paraffin in the solid state to a temperature of 50? C, the temperature increases.

During melting, the temperature remains constant.

When all the paraffin has melted, the temperature increases with further heating.

When liquid paraffin is cooled, the temperature decreases.

During crystallization, the temperature remains constant.

When all of the paraffin has solidified, the temperature decreases with further cooling.

Structural diagram: "Melting and solidification of crystalline bodies"

(Slide 12) Work according to the scheme.

Phenomena	Scientific facts	Hypothesis	Ideal object	Quantities	The laws	Application
	When a crystalline body melts, the temperature does not change. When a crystalline solid solidifies, the temperature does not change.	When a crystalline body melts, the kinetic energy of atoms increases, the crystal lattice is destroyed. During solidification, the kinetic energy decreases and the crystal lattice is built.	A solid body is a body whose atoms are material points arranged in an orderly manner (crystal lattice), interacting with each other by forces of mutual attraction and repulsion.	Q is the amount of heat Specific heat of fusion	Q = m - absorbed Q = m - stands out	1. To calculate the amount of heat 2. For use in engineering, metallurgy. 3. thermal processes in nature (melting of glaciers, freezing of rivers in winter, etc.) 4. Write your examples.

The temperature at which a solid changes to a liquid state is called the melting point.

The crystallization process will also proceed at a constant temperature. It is called the crystallization temperature. In this case, the melting temperature is equal to the crystallization temperature.

Thus, melting and crystallization are two symmetrical processes. In the first case, the substance absorbs energy from the outside, and in the second - it gives it to the environment.

Different melting temperatures determine the scope of various solids in everyday life and technology. Refractory metals are used to make heat-resistant structures in aircraft and rockets, nuclear reactors and electrical engineering.

Consolidation of knowledge and preparation for independent work.

1. The figure shows a graph of heating and melting of a crystalline body. (Slide)

2. For each of the situations listed below, select a graph that most accurately reflects the processes occurring with the substance:

a) copper is heated and melted;

b) zinc is heated to 400°C;

c) melting stearin is heated to 100°C;

d) iron taken at 1539°C is heated to 1600°C;

e) tin is heated from 100 to 232°C;

f) aluminum is heated from 500 to 700°C.

Answers: 1-b; 2-a; 3-in; 4-in; 5 B; 6-d;

The graph reflects observations of the change in temperature of two

crystalline substances. Answer the questions:

(a) At what time did the observation of each substance begin? How long did it last?

b) Which substance began to melt first? Which substance melted first?

c) State the melting point of each substance. Name the substances whose heating and melting graphs are shown.

4. Is it possible to melt iron in an aluminum spoon?

5.. Is it possible to use a mercury thermometer at the Pole of Cold, where the lowest temperature was recorded - 88 degrees Celsius?

6. The combustion temperature of powder gases is about 3500 degrees Celsius. Why doesn't the barrel of a gun melt when fired?

Answers: It is impossible, since the melting point of iron is much higher than the melting point of aluminum.

5. It is impossible, since mercury will freeze at this temperature, and the thermometer will fail.

6. It takes time to heat and melt a substance, and the short duration of the combustion of gunpowder does not allow the gun barrel to heat up to the melting point.

4. Independent work. (Appendix 3).

Option 1

Figure 1a shows a graph of heating and melting of a crystalline body.

I. What was the body temperature at the first observation?

1. 300 °C; 2. 600 °C; 3. 100 °C; 4. 50 °C; 5. 550 °C.

II. Which process on the graph characterizes segment AB?

III. What process on the graph characterizes the BV segment?

1. Heating. 2. Cooling. 3. Melting. 4. Curing.

IV. At what temperature did the melting process begin?

1. 50 °C; 2. 100 °C; 3. 600 °C; 4. 1200 °C; 5. 1000 °C.

V. How long did the body melt?

1. 8 min; 2. 4 min; 3. 12 min; 4. 16 min; 5.7 min.

VI. Did body temperature change during melting?

VII. What process on the graph characterizes the segment VG?

1. Heating. 2. Cooling. 3. Melting. 4. Curing.

VIII. What temperature was the body at the last observation?

1. 50 °C; 2. 500 °C; 3. 550 °С; 4. 40 °C; 5. 1100 °C.

Option 2

Figure 101.6 shows a graph of cooling and solidification of a crystalline body.

I. What temperature was the body at the first observation?

1. 400 °C; 2. 110°C; 3. 100 °C; 4. 50 °C; 5. 440 °C.

II. Which process on the graph characterizes segment AB?

1. Heating. 2. Cooling. 3. Melting. 4. Curing.

III. What process on the graph characterizes the BV segment?

1. Heating. 2. Cooling. 3. Melting. 4. Curing.

IV. At what temperature did the curing process begin?

1. 80 °C; 2. 350 °C; 3. 320 °С; 4. 450 °C; 5. 1000 °C.

V. How long did the body harden?

1. 8 min; 2. 4 min; 3. 12 min;-4. 16 min; 5.7 min.

VI. Did body temperature change during hardening?

1. Increased. 2. Decreased. 3. Has not changed.

VII. What process on the graph characterizes the segment VG?

1. Heating. 2. Cooling. 3. Melting. 4. Curing.

VIII. What temperature was the body at the time of the last observation?

1. 10 °C; 2. 500 °C; 3. 350 °C; 4. 40 °C; 5. 1100 °C.

Summing up the results of independent work.

1 option

I-4, II-1, III-3, IV-5, V-2, VI-3, VII-1, VIII-5.

Option 2

I-2, II-2, III-4, IV-1, V-2, VI-3, VII-2, VIII-4.

Additional material: Watch the video: "Ice melting at t<0C?"

Student reports on the use of melting and crystallization in industry.

Homework.

14 textbooks; questions and tasks for the paragraph.

Tasks and exercises.

Collection of problems by V. I. Lukashik, E. V. Ivanova, No. 1055-1057

Bibliography:

1. Peryshkin A.V. Physics grade 8. - M.: Bustard. 2009.
2. Kabardin O. F. Kabardina S. I. Orlov V. A. Tasks for the final control of students' knowledge in physics 7-11. - M.: Enlightenment 1995.
3. Lukashik V. I. Ivanova E. V. Collection of problems in physics. 7-9. - M.: Enlightenment 2005.
4. Burov V. A. Kabanov S. F. Sviridov V. I. Frontal experimental tasks in physics.
5. Postnikov AV Checking students' knowledge in physics 6-7. - M.: Enlightenment 1986.
6. Kabardin OF, Shefer NI Determination of solidification temperature and specific heat of paraffin crystallization. Physics at school No. 5 1993.
7. Video cassette "School physical experiment"
8. Pictures from sites.

Lesson topic: “Specific heat of fusion. Melting charts and

solidification of crystalline bodies.

Lesson Objectives:

To form the ability to plot a graph of the dependence of the temperature of a crystalline body on the time of heating;

Introduce the concept of specific heat of fusion;

Enter a formula for calculating the amount of heat required to melt a crystalline body of mass m, taken at the melting temperature.

To form the ability to compare, contrast, generalize the material.

Accuracy in scheduling, diligence, the ability to bring the work started to the end.

Epigraph to the lesson:

“Undoubtedly, 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 objectives of the lesson.

II. The main part of the lesson.

1. Knowledge update:

There are 2 people at the board:

Fill in the missing words in the definition.

“Molecules in crystals are located… they move…., being 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 state to a liquid, this process is called ... ".

“The molecules in a molten substance are located ... they move ... and ... are held in certain places by the forces of molecular attraction. When the body is cooled, the average speed of the molecules ..., the range of oscillations ..., and the forces that hold them ..., the substance passes from a liquid state to a solid state, this process is called ... ".

The rest of the class works on mini-test cards ()

Using tabular values ​​in Lukaszyk's problem book.

Option number 1

1. Lead melts at a temperature of 327 0C. What can be said about the solidification temperature of lead?

A) It is equal to 327 0C.

B) it is hotter

melting.

2. At what temperature does mercury acquire a crystalline structure?

A) 4200C; B) - 390С;

3. In the earth at a depth of 100 km, the temperature is about 10,000C. Which of the metals: Zinc, tin or iron - is there in an unmelted 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) You can't.

Melting and solidification of crystalline bodies.

Option number 2

1. When a crystalline substance melts, its temperature ...

B) is decreasing.

2. At what temperature can zinc be in a solid and liquid state?

A) 4200C; B) - 390С;

C) 1300 - 15000C; D) 00С; D) 3270C.

3. Which of the metals: zinc, tin or iron - will melt at the melting point of copper?

A) zinc. B) Tin. B) Iron

4. The temperature of the outer surface of the rocket during the flight rises to 1500 - 20000C. What metals are suitable for the manufacture of the outer skin of rockets?

A) steel. B). Osmium. B) tungsten

D) Silver. D) Copper.

Melting and solidification of crystalline bodies.

Option number 3

1. Aluminum hardens at a temperature of 6600C. What can be said about the melting point of aluminum?

A) It is equal to 660 0C.

b) It is below the melting point.

B) it is hotter

melting.

2. At what temperature is the crystalline structure of steel destroyed?

A) 4200C; B) - 390С;

C) 1300 - 15000C; D) 00С; D) 3270C.

3. On the surface of the Moon at night the temperature drops to -1700C. Is it possible to measure such a temperature with mercury and alcohol thermometers?

A) You can't.

B) You can use an alcohol thermometer.

C) You can use a mercury thermometer.

D) You can use both mercury and alcohol thermometers.

4. What metal, being in a molten state, can freeze water?

A) steel. B) zinc. B) tungsten.

D) Silver. D) mercury.

Melting and solidification of crystalline bodies.

Option number 4

1. During crystallization (solidification) of a molten substance, its temperature ...

A) will not change. B) is increasing.

B) is decreasing.

2. The lowest air temperature -88.30С was registered 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) Do not use either mercury or alcohol thermometers.

3. Is it possible to melt copper in an aluminum pan?

Can i. B) You can't.

4. Which metal has a crystal lattice that is destroyed at the highest temperature?

A) steel. B) copper. C) tungsten.

D) For platinum E) For osmium.

2. Checking what is written at the blackboard. Error correction.

3. Learning new material.

a) Film screening. "Melting and crystallization of a solid body"

b) Construction of a graph of changes in the state of aggregation of the body. (2 slide)

c) a detailed analysis of the graph with the analysis of each segment of the graph, the study of all physical processes occurring on 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 №2 0С

segment AB? 1000

D) curing. B C

segment BV?

A) heating. B) cooling. B) melting. 500

D) Hardening D

3. At what temperature did the process start

curing?

A) 80 0С. B) 350 0C C) 3200C

D) 450 0С E) 1000 0С

4. How long did the body harden? 0 5 10 min.

A) 8 min. B) 4 min. C) 12 min.

D) 16 min. D) 7 min.

A) increased. B) decreased. B) hasn't changed.

6. What process on the graph characterizes the segment VG?

A) heating. B) cooling. B) melting. D) curing.

Graph of melting and solidification of crystalline bodies.

Option №3 0С

1. What process on the graph characterizes 600 G

segment AB?

A) heating. B) cooling. B) melting.

D) curing. B C

2. What process on the graph characterizes

segment BV?

A) heating. B) cooling. B) melting. 300

D) curing.

3. At what temperature did the process start

melting?

A) 80 0С B) 3500С C) 3200С D) 4500С

4. How long did the body melt? BUT

A) 8 min. B) 4 min. C) 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) hasn't changed.

6. What process on the graph characterizes the segment VG?

A) heating. B) cooling. B) melting. D) curing.

Graph of melting and solidification of crystalline bodies.

Option №4 0С

1. Which process on the graph characterizes A

segment AB? 400

A) heating. B) cooling. B) melting.

D) curing. B C

2. . Which process on the graph characterizes

segment BV?

A) heating. B) cooling. B) melting. 200

D) hardening

3. At what temperature did the process start

curing?

A) 80 0С. B) 350 0C C) 3200C D

D) 450 0С E) 1000 0С

4. How long did the body harden? 0 10 20 min.

A) 8 min. B) 4 min. C) 12 min.

D) 16 min. D) 7 min.

5. Did the temperature change during curing?

A) increased. B) decreased. B) hasn't changed.

6. What process on the graph characterizes the segment VG?

A) heating. B) cooling. B) melting. D) curing.

III. Summary of the lesson.

IV. Homework (Differentiated) 5 slide

V. Grading a lesson.