Properties of substances in different states of aggregation. Teaching comparison elements in chemistry lessons

>> Substances and their properties. Experimenting at home. Properties of some foods

Initial chemical concepts

Substances and their properties

The paragraph will help you:

> distinguish between substances, physical bodies and materials;
> characterize substances by physical properties.

Substance.

AT Everyday life we come across many substances. Among them are water, sand, iron, gold, sugar, salt, starch, coal... This list can be continued for a very long time. Hundreds of times more substances are used and produced scientists .

Rice. 20. Natural substances

More than 20 million substances are now known. Many of them exist in nature (Fig. 20). There are various gases in the air; in rivers, seas and oceans, except for water, - substances dissolved in it; in the solid surface layer of our planet - numerous minerals, rocks, ores, etc. An extremely large number of substances are also found in living organisms.

Rice. 21. Man-made substances

Aluminum, zinc, acetone, lime, soap, aspirin, polyethylene, and many other substances do not exist in nature. They are produced by industry (Fig. 21).

Some substances that exist in nature can also be obtained in a chemical laboratory. So, when potassium permanganate is heated, oxygen is released, and when chalk is heated, carbon dioxide is released. gas. Scientists at high temperature and pressure turn graphite into diamond, but artificial diamond crystals are very small and unsuitable for making jewelry. It is not possible to obtain the semi-precious stone malachite with the help of chemical experiments.

Mass is an integral feature of matter. Light rays, a magnetic field have no mass and are not related to substances.

Matter is what the physical body is made of.

They call everything that has mass and volume. Physical bodies are, for example, a drop of water, a crystal of a mineral, a piece of glass, a piece of plastic, a grain of wheat, an apple, a nut, and also any object made by man - a watch, a toy, a book, a piece of jewelry, etc.

Name the substances that make up such physical bodies: an ice floe, a nail, a pencil.

Substances used for the manufacture of objects, equipment, as well as in construction and other industries, are called materials (Fig. 22).

The first in the history of mankind were natural materials - wood, stone, clay. Over time, people learned to smelt metals and glass, to get lime and cement. In recent decades, traditional materials have been replaced by new ones, in particular various plastics.

Rice. 22. Building materials

What materials (plastic, glass, metal, fabric, wood) can a vase, necklace, plate be made of?

Aggregate states of matter.

A substance can exist in three states of aggregation- solid, liquid and gaseous.

When heated, solids melt, and liquids boil, turning into steam. downgrade temperature leads to reverse transformations. Some gases liquefy at high pressure. With all these phenomena, the smallest particles of matter are not destroyed. Thus, the substance, changing the state of aggregation, does not turn into another.

Everyone knows about the three states of aggregation of water in which it exists in nature: ice, water, water vapor. But not every substance can be solid, liquid and gaseous. For sugar, two are known: solid and liquid. When heated, sugar melts, then its melt darkens, and an unpleasant odor appears. This indicates the transformation of sugar into other substances. Hence, the gaseous state for sugar does not exist. And such a substance as graphite cannot be melted: at a temperature of 3500 0C, it immediately turns into steam.

Crystalline and amorphous substances.

If we look at salt and sugar through a magnifying glass, then we can see that the grains of salt have the shape of cubes, and sugar has a different shape, but also correct, symmetrical. Each such grain is a crystal. A crystal is natural, having flat edges (surfaces) and straight edges (face joints). Therefore, salt and sugar are crystalline substances. Such substances include citric acid, glucose, diamond, graphite, metals, etc. (Fig. 23). In many cases, the crystals of substances are so small that they can only be seen under a microscope.

Glass is not crystalline, but amorphous1 substance. If it is crushed, we get shapeless pieces that do not look alike. Amorphous substances are also starch, flour, polyethylene, etc. (Fig. 24).

Rice. 23. Crystalline substances
Rice. 24. Amorphous substances

Physical properties of substances.

All substances are extremely diverse; each has a set of certain properties.

The properties of a substance are the features by which a substance differs from another or is similar to it.

1 The term comes from the Greek prefix a- and the word morphe - form.

Iron is easily distinguished from wood by its color, special brilliance, and also by touch: metal always seems colder, as it conducts heat better. feature gland is that it is attracted to a magnet, but wood is not. Unlike iron, wood does not sink in water, since its density is less than the density of water, and the density of iron is greater. Iron withstands high temperatures, but wood first darkens, then blackens and lights up.

The properties of a substance that are determined by observation or measurement, without transforming it into another substance, are called physical.

The most important physical properties of matter:

Aggregate state at certain temperature and pressure;
color, luster (or lack thereof);
smell (or lack thereof);
solubility (or insolubility) in water;
melting temperature;
boiling temperature;
density;
thermal conductivity;
electrical conductivity (or non-electrical conductivity).

Scroll physical properties solids can be expanded to include hardness, ductility (or brittleness), and for crystalline, also the shape of the crystals. When characterizing a liquid, indicate whether it is mobile or oily.

Physical properties such as color, smell, taste, crystal shape can be determined visually, using the senses, and density, electrical conductivity, melting and boiling points are determined by measurement. Information about the physical properties of many substances is collected in special literature, in particular - in reference books.

Rice. 25. Heating iodine

The physical properties of a substance depend on its state of aggregation. For example, the density of ice, water and water vapor is different. Gaseous oxygen is colorless, while liquid oxygen is blue.

Knowledge of physical properties helps to "recognize" a lot of substances. For example, copper is the only red-colored metal. Only table salt has a salty taste. Iodine is an almost black solid that turns into a dark purple vapor when heated (Fig. 25). In most cases, to define a substance, several of its properties must be taken into account.

Laboratory experience No. 1

Introduction to the physical properties of substances

You have been given three test tubes containing saltpeter 1 , graphite and polyethylene 2 . You have a glass of water (or a washer) and glass rods at your disposal.

Describe substances. What is the nature of the particles of each substance (crystals, powder, small pieces of arbitrary shape)? Find out whether substances dissolve in water, whether they are lighter or heavier than it.

Write down the physical properties of substances in the table:

What property(s) distinguishes each substance from the other two?

name properties, the same for two (three) substances.

In addition to physical properties, every substance also has chemical properties. They will be discussed later.

1 Mineral fertilizer.
2 The teacher can replace graphite with sulfur, copper or iron filings, and polyethylene with another polymer.

conclusions

Matter is what the physical body is made of. The essential property of a substance is its mass.

A substance can exist in three states of aggregation: solid, liquid and gaseous. Solids are either crystalline or amorphous.

The properties of a substance are the features by which it differs from another substance or is similar to it.

The physical properties of a substance are determined by observation or measurement, without transforming it into another substance.

?

19. What is a physical body, substance, material?
20. Find a match:

Substance physical body
1) gold; a) a thermometer;
2) mercury; b) ring;
3) paper; c) showcase;
4) glass; d) notebook.

21.. Choose among the given words and phrases those that relate to substances: table, copper, ice, plastic bottle, alcohol, newspaper, water vapor, silver chain.

22. Which of the substances are building materials: carbon dioxide, reinforced concrete, glass, paper, nylon, steel?

23. Give examples of: a) several objects made from the same material; b) an object made of several materials; c) two materials from which similar items are made.

24. Describe the physical properties of chalk.

25. What substances that you have at home can be identified by smell?

26. Vessels without labels contain perfume, vegetable oil, table salt, pieces of iron, marble. What are the properties of each substance?

27. Name a few solids that you can easily distinguish from the rest.

28. Taking into account the physical properties of substances, explain why screwdrivers and pliers usually have plastic handles.

Experimenting at home

Properties of some foods

Write on separate pieces of paper the names of the substances: flour, Extra salt, powdered sugar, starch. Pour on each leaflet a few grams of the corresponding substance.

Describe the appearance of substances.

Rub a pinch of each substance with your fingers (determine how small its particles are).

Taste the substances (it is strictly forbidden to do this with the substances available in the chemical laboratory).

Find out if substances dissolve in water.

Record the results of research and observations in a table similar to that shown on p. 32.

State	Properties
gaseous	1. The ability to take the volume and shape of the vessel. 2. Compressibility. 3. Fast diffusion (chaotic movement of molecules). 4. E kinetic. > E pot.
	1. The ability to take the form of that part of the vessel that the substance occupies. 2. Inability to expand until the vessel is full. 3. Slight compressibility. 4. Slow diffusion. 5. Fluidity. 6. E kinetic. = E pot.
	1. The ability to maintain the proper shape and volume. 2. Slight compressibility (under pressure). 3. Very slow diffusion due to oscillatory motions of particles. 4. Lack of fluidity. 5. E kinetic.< Е потенц.

The aggregate state of a substance is determined by the forces acting between the molecules, the distance between the particles and the nature of their movement.

AT solid particles occupy a certain position relative to each other. It has low compressibility, mechanical strength, since the molecules do not have freedom of movement, but only vibrations. Molecules, atoms, or ions that form a solid are called structural units. Solids are divided into amorphous and crystalline(Table 27 ).

Table 33

Comparative characteristics of amorphous and crystalline substances

Substance	Characteristic
amorphous	1. Short-range order of particles arrangement. 2. Isotropy of physical properties. 3. No specific melting point. 4. Thermodynamic instability (large supply of internal energy). 5. Fluidity. Examples: amber, glass, organic polymers, etc.
crystalline	1. Long-range order of particle arrangement. 2. Anisotropy of physical properties. 3. Specific melting point. 4. Thermodynamic stability (small internal energy). 5. There are elements of symmetry. Examples: metals, alloys, hard salts, carbon (diamond, graphite), etc.

Crystalline substances melt at a strictly defined temperature (T pl), amorphous ones do not have a pronounced melting point; when heated, they soften (characterized by a softening interval) and pass into a liquid or viscous state. The internal structure of amorphous substances is characterized by a random arrangement of molecules . The crystalline state of matter implies the correct arrangement in space of the particles that make up the crystal, and the formation crystalline (spatial)gratings. The main feature of crystalline bodies is their anisotropy - unevenness of properties (thermal, electrical conductivity, mechanical strength, dissolution rate, etc.) in different directions, while amorphous bodies isotropic .

Solidcrystals- three-dimensional formations characterized by strict repeatability of the same structural element (elementary cell) in all directions. elementary cell- represents the smallest volume of a crystal in the form of a parallelepiped, repeated in the crystal an infinite number of times.

Basic parameters of the crystal lattice:

The energy of the crystal lattice (E cr. , kJ/mol) – this is the energy that is released during the formation of 1 mol of a crystal from microparticles (atoms, molecules, ions) that are in a gaseous state and are separated from each other by a distance that excludes their interaction.

Crystal lattice constant ( d , [ A 0 ]) – the smallest distance between the center of two particles in a crystal connected by a chemical bond.

Coordination number (c.h.) - the number of particles surrounding the central particle in space, connected with it by a chemical bond.

The points where the crystal particles are located are called lattice nodes

Despite the variety of forms of crystals, they can be classified. Systematization of crystal forms was introduced A.V. Gadolin(1867), it is based on the features of their symmetry. In accordance with the geometric shape of crystals, the following systems (syngonies) are possible: cubic, tetragonal, orthorhombic, monoclinic, triclinic, hexagonal, and rhombohedral (Fig. 18).

The same substance can have different crystalline forms, which differ in internal structure, and hence the physical and chemical properties. Such a phenomenon is called polymorphism . isomorphism – two substances of different nature form crystals of the same structure. Such substances can replace each other in the crystal lattice, forming mixed crystals.

Rice. 18. Basic systems of crystals.

Depending on the type of particles located at the nodes of the crystal lattice and the type of bonds between them, crystals are of four types: ionic, atomic, molecular and metallic(rice . 19).

Rice. 19. Types of crystals

Characteristics of crystal lattices are presented in table. 34.

Matter properties	Oxygen	Acetic acid	Aluminum
1. Aggregate state under normal conditions	Gas	Liquid	Solid
2. Color	no color	no color	Silver white
3. Taste	Tasteless	Sour	Tasteless
4. Smell	Doesn't have	sharp specific	Doesn't have
5. Solubility in water	Poorly soluble	Soluble	Practically insoluble
6. Thermal conductivity	Low	small	high
7. Electrical conductivity	Missing	Malaya	high

Knowledge of the properties of substances is necessary for their practical application. For example, Figure 6 shows the areas of application of aluminum, due to the properties of this metal.

1. What subjects are natural?

2. Give examples of positive human impact on the environment.

3. Give examples of the negative impact of man on nature.

4. What does chemistry study?

5. From the following list of names, write out the bodies and substances separately: snowflake, dew drop, water, ice, granulated sugar, sugar cube, chalk, school chalk. How many bodies and how many substances are named in this list?

6. Compare the properties of substances (that is, establish the common and different between them):

a) carbon dioxide and oxygen

b) nitrogen and carbon dioxide;

c) sugar and salt;

d) acetic and citric acids.

7. What properties of aluminum underlie its application?

8. Why begin to study chemistry later than biology, geography, physics?

New programs and textbooks guide us towards achieving an organic unity in the education and development of students. The task of the teacher is not only to equip students with knowledge and practical skills, but also with mental operations.

One of the most important mental operations by which knowledge is acquired is comparison. In logical terms, comparison is presented as the basis of generalization, on the one hand, and as the unity of such logical operations as analysis and synthesis, on the other. But in order to form a comparison among students as a method of their mental activity, it is necessary to use comparison as a method of teaching (didactic method). The use of comparison as a didactic device is an indispensable condition for the formation of analytical and synthetic activity in schoolchildren.

The use of comparison in the learning process was considered by K.D. Ushinsky, I.G. Pestalozzi, Ya.A. Komensky. Russian scientists have deepened and specified the understanding of the role of comparison and the possibility of its application. “An important didactic technique for the formation of concepts,” notes G.I. Shchukina, “are comparisons that help to better understand the similarities and differences between objects and phenomena.” According to L.V. Zankov, the comparison more accurately and correctly defines the peculiar features of the object.

Comparison, like every technique, is formed in stages. If we consider the stages as sequential, interrelated actions, then the method of comparison can be defined as follows: comparison is academic work and the method of thinking, in the process of implementation of which the mental activity of students is aimed at:

Identification of signs by which phenomena, substances or other given objects can be compared;
establishing similarities or differences between them;
summarizing the comparison results in the form of a conclusion.

I begin the formation of the method of comparison and the simplest types of generalization in one of the first lessons in the 8th grade when studying the physical properties of substances. I specifically allocate time in the lesson to familiarize students with the method of comparison, briefly defining it as follows: comparison is the establishment of similarities or differences between substances, phenomena or other given objects. Here I talk about the meaning of the reception and its types:

a) in case of incomplete comparison on the grounds of similarity or on the grounds of differences;
b) with a full comparison with the identification of both signs of similarity and signs of difference.

Knowledge of reception is better absorbed by students if they use the action plan, which I propose to draw them up in the form of a table:

To reduce the time for the image of the table in notebooks when students use it repeatedly, I propose to arrange it as follows:

Comparison of substances (phenomena)

Depending on the task, students make a conclusion on the basis of an incomplete or complete comparison or the most significant features by which the studied objects are compared and contrasted.

The use, especially in the first lessons, of this plan, which reflects all the stages of the formation of the comparison method, helps students quickly memorize the necessary order of actions.

In the same lesson, first with my help, and then on their own, students complete tasks, comparing the properties of substances well known to them (salt and sugar, chalk and coal, water and sunflower oil, iron and sulfur, copper and aluminum, etc.), and respond according to the proposed plan orally or in writing. In necessary cases, I use visual aids and TCO.

However, knowledge of the technique is not yet a skill. The ability, in my opinion, can be formed only when, in subsequent lessons, students can easily and with sufficient independence, taking into account all stages of the action, apply knowledge and perform similar and more difficult tasks. In grade 8, as such tasks, I propose a comparison of simple and complex substances, pure substances and mixtures, types of chemical reactions, etc., for example, hydrogen and oxygen, ozone and oxygen, sulfur and iron, a mixture of sulfur and iron, compound reactions and expansions, etc.

The development of a skill requires a longer time than its formation, and is carried out mainly with the help of tasks of increasing difficulty, which provide for a deeper and wider transfer of knowledge, as well as greater independence of actions in establishing various kinds relationships in educational material. Complicating tasks using the comparison technique, I carry out in several directions, which in general look like this:

Complexity options at various stages
formation of the comparison technique

Options increasing degree difficulties assignments	Formation and development comparison reception
	Stage I - establishing list compared signs	Stage II - comparison	Stage III - generalization in the form output
	the list of features is offered in finished form	comparison of two substances (phenomena)	conclusion as a result of incomplete comparison (comparison or opposition)
	the list of signs is remembered or partially established by students	comparison of groups of substances (phenomena)	output from full comparison
	the list of signs is determined by the students themselves	essential comparison	conclusion on the most significant features characterizing a given substance (phenomenon)

I will give examples of some tasks, taking the following numbering for them: I-1; I-2; I-3; II-1; II-2; II-3; III-1 etc. The Roman numeral in this designation corresponds to certain stage formation of the reception, and the Arabic numeral indicates the degree of difficulty of the task, the level of the task and the level of the action performed when solving the task, or, in other words, the smaller Arabic numeral indicates an easier version of the task, the larger digit indicates the task of increased difficulty. Thus, by number, I can easily determine the degree of difficulty of the task and select the tasks necessary for work, for example:

I-1. Compare (orally) the properties of: a) sulfur and angles; b) copper and zinc; c) oxygen and carbon dioxide. Record the answer about comparing one pair of substances in the table. When completing the task, consider where it is required to indicate the following signs of comparison: state of aggregation, crystalline or amorphous structure of a substance, density, color, luster, transparency, smell, solubility, taste, melting or boiling point, density, thermal or electrical conductivity.

I-2. In vessels without labels are given: a) powder of sulfur and iron; b) powdered sugar and starch; c) table salt and naphthalene; d) aluminum (molten) and mercury. By what characteristics can these substances be distinguished?

I-3. Coal dust got into the powdered sugar. List all the operations that you would have to follow in order to purify the sugar.

II-1. List some of the similar physical properties of aluminum and copper that make these metals equally useful.

II-2. Read the material “Chemical elements” in the textbook of chemistry. Compare the properties of metals and nonmetals. To do this, first compare the properties of the three metals you have chosen, then the properties of the three non-metals (orally); then carry out the opposition of the properties of these groups of metals and non-metals according to the most characteristic features (II-III). Record your answer and your conclusions in the table.

When completing the above tasks, I suggest that students complete the following table:

Comparison of physical properties of both metals and non-metals

III-1. List the signs: a) similarities; b) differences between compound and substitution reactions, taking into account the number of substances taken and received, as well as whether these substances are simple or complex. Compare decomposition and substitution reactions.

III-2. A copper plate was dipped into a colorless solution of sublimate. Add the equation for the corresponding reaction HgCl 2 + Cu > ? if it is known that in this case new simple and complex substances are obtained. What signs of reaction can be assumed in this case?

III-3. Compare decomposition and substitution reactions. Notice any similarities between them. What signs indicate a significant difference between these reactions?

I use some of the above tasks at various stages of reception training. In this case, I also make the numbering of tasks triple and more complex. Each stage of the formation of comparison among students, especially in the 8th grade, has its own difficulties.

Stage I - the formation of the ability to distinguish signs, phenomena. Eighth graders still do not know how to identify essential features. They often make a comparison on one (and insignificant) feature. For example, they indicate the similarity of metals and non-metals in taste, physical state, but do not note electrical conductivity and thermal conductivity. A sign of the similarity of sugar and table salt called the absence of smell, but their solubility and crystal structure are forgotten.

Quite often, students compare substances or phenomena according to incomparable features. So, when comparing the two proposed substances, the answers were given: “Copper is red, and aluminum is silver” (color and luster are confused); “Water is colorless, and sunflower oil is dark” (color and shade); “Fatty oil and fresh water” (fat content and taste); “Salt consists of small, like cereals, crystals, and sugar is lumpy,” etc.

To avoid such mistakes, students are helped by work on a comparison plan and exercises in which, first, comparison is carried out according to a ready-made list of features with or without visual aids, then using a list of features, which is remembered and partially established by students, and, finally, comparison using a list signs independently identified by students, or a comparison according to the identified essential features.

Stage II - the formation and development of the ability to compare. Students of the 8th grade easily memorize the types of comparisons: comparison, contrast, complete and incomplete comparison. They quite easily master the comparison of two substances according to the proposed plan. But in chemistry lessons, you often have to compare groups of substances or two substances according to several criteria. This needs to be specifically taught to students.

If students have difficulty comparing groups of substances, I recommend this workflow. First, compare 2-3 substances belonging to each of the groups with each other, identify the most characteristics similarities between them, and then establish signs of similarities and differences between groups. I offer such exercises when studying metals and non-metals, comparing the composition and properties of oxides, acids, bases, as well as when summarizing the material and repeating the classification of inorganic compounds.

Students who have insufficient knowledge and ability to apply the methods of mental actions make comparisons not only on incomparable features, but also on those that are not provided for in the task. So, comparing the physical properties of sulfur and iron, some students answered: “Sulfur and iron are simple substances, solid, but differ in color” (contrasting the composition and state of aggregation with physical properties); or: “Large pieces of sulfur and iron sink, while small ones float on water” (an incorrect conclusion about the density of substances as a result of erroneous observations); or: “Sulfur is poisonous, but iron is not; sulfur burns, but iron is not. Sulfur is used in gunpowder, but iron is not” (instead of contrasting physical properties with reference to chemical properties and uses). In such cases, I explain to students that their answer is not to the question of the task and cannot be taken into account.

Stage III - the formation of generalization techniques. Students of the 8th grade find it difficult to generalize the material. Often, instead of making a conclusion after comparing substances or phenomena, they again list previously established signs of similarity or difference. In this case, to form the ability to compare and make generalizations based on comparison, I use tasks of increasing difficulty; a) tasks in which conclusions are answers to control questions; b) tasks in which the word “conclusion” resembles a generalization, and, finally, c) tasks in which an independent generalization is provided educational material students.

The ability to draw a conclusion as a result of comparison according to essential features is easier to form when the list of compared features is clearly defined in the task. In some cases, I give the list of characteristics to be compared to the students ready-made or previously compiled by the students with some help from me. I explain what was said with examples of tasks and answers of students.

Exercise 1. Compare the physical properties of hydrogen and oxygen, add the answer to the table.

Comparison of the physical properties of hydrogen and oxygen

Task 2. Contrast the chemical properties of hydrogen and oxygen, write the answer in the table.

Comparison of chemical properties of hydrogen and oxygen

Thus, comparison as a method of mental activity can be formed in two ways. The first way is spontaneous, determined by such a formulation of the educational process, when comparison does not act as a special subject of assimilation, the formation of this technique occurs in the course of assimilation of knowledge, in the process of solving problems. Experience shows that learning proceeds in the second way: through a system of tasks that require students to systematically use a comparison that is becoming more complex in its content.

Any method of mental activity should be, first of all, repeatedly used in the teacher's explanation, in educational texts. However, this is not enough. Reception is formed only on the basis of exercises and creative tasks, own activities of trainees. That is why we not only widely use this technique in explaining, but also developed a system of special tasks in which comparison, being a necessary method of activity for the assimilation of specific material, would also act as a special subject of assimilation.