I. New material

When preparing the lesson, the following materials were used by the author: N.K. Cheremisina,

chemistry teacher of secondary school No. 43

(Kaliningrad),

We live among chemicals. We inhale air, and this is a mixture of gases ( nitrogen, oxygen and others), exhale carbon dioxide. Let's wash ourselves water- This is another substance, the most common on Earth. We drink milk- mixture water with tiny drops of milk fat, and not only: there is also milk protein here casein, mineral salt, vitamins and even sugar, but not the kind with which they drink tea, but a special one, milk - lactose. We eat apples, which consist of a whole range of chemicals - here and sugar, And Apple acid, And vitamins... When chewed apple pieces enter the stomach, human digestive juices begin to act on them, which help to absorb all the tasty and healthy substances not only of the apple, but also of any other food. We not only live among chemicals, but we ourselves are made of them. Every person - his skin, muscles, blood, teeth, bones, hair is built of chemicals, like a house of bricks. Nitrogen, oxygen, sugar, vitamins are substances of natural, natural origin. Glass, rubber, steel is also a substance, more precisely, materials(mixtures of substances). Both glass and rubber are of artificial origin; they did not exist in nature. Absolutely pure substances are not found in nature or are found very rarely.

How do pure substances differ from mixtures of substances?

An individual pure substance has a certain set of characteristic properties (constant physical properties). Only pure distilled water has melting point = 0 °C, boiling point = 100 °C, and has no taste. Sea water freezes at a lower temperature and boils at a higher temperature; its taste is bitter and salty. The water of the Black Sea freezes at a lower temperature and boils at a higher temperature than the water of the Baltic Sea. Why? The fact is that sea water contains other substances, for example dissolved salts, i.e. it is a mixture of various substances, the composition of which varies widely, but the properties of the mixture are not constant. The definition of the concept “mixture” was given in the 17th century. English scientist Robert Boyle : “A mixture is an integral system consisting of heterogeneous components.”

Comparative characteristics of the mixture and pure substance

Signs of comparison	Pure substance	Mixture
Compound	Constant	Fickle
Substances	Same	Various
Physical properties	Permanent	Fickle
Energy change during formation	Happening	Not happening
Separation	Through chemical reactions	By physical methods

The mixtures differ from each other in appearance.

The classification of mixtures is shown in the table:

Let us give examples of suspensions (river sand + water), emulsions (vegetable oil + water) and solutions (air in a flask, table salt + water, small change: aluminum + copper or nickel + copper).

In suspensions, particles of a solid substance are visible, in emulsions - droplets of liquid, such mixtures are called heterogeneous (heterogeneous), and in solutions the components are not distinguishable, they are homogeneous (homogeneous) mixtures.

Methods for separating mixtures

In nature, substances exist in the form of mixtures. For laboratory research, industrial production, and for the needs of pharmacology and medicine, pure substances are needed.

Various methods for separating mixtures are used to purify substances.

These methods are based on differences in the physical properties of the components of the mixture.

Let's consider waysseparationheterogeneous And homogeneous mixtures .

Example of a mixture	Separation method
Suspension - a mixture of river sand and water	Advocacy Separation defending based on different densities of substances. Heavier sand settles to the bottom. You can also separate the emulsion: separate the oil or vegetable oil from the water. In the laboratory this can be done using a separatory funnel. Petroleum or vegetable oil forms the top, lighter layer.As a result of settling, dew falls out of the fog, soot settles out of the smoke, and cream settles in the milk. Separation of a mixture of water and vegetable oil by settling
A mixture of sand and table salt in water	Filtration What is the basis for the separation of heterogeneous mixtures using filtering?On different solubility of substances in water and on different particle sizes. Through Only particles of substances comparable to them pass through the pores of the filter, while larger particles are retained on the filter. This is how you can separate a heterogeneous mixture of table salt and river sand.Various porous substances can be used as filters: cotton wool, coal, baked clay, pressed glass and others. The filtration method is the basis for the operation of household appliances, such as vacuum cleaners. It is used by surgeons - gauze bandages; drillers and elevator workers - respiratory masks. Using a tea strainer to filter tea leaves, Ostap Bender, the hero of the work by Ilf and Petrov, managed to take one of the chairs from Ellochka the Ogress (“Twelve Chairs”).
Mixture of iron and sulfur powder	Action by magnet or water Iron powder was attracted by a magnet, but sulfur powder was not.. Non-wettable sulfur powder floated to the surface of the water, and heavy wettable iron powder settled to the bottom. Separating a mixture of sulfur and iron using a magnet and water
A solution of salt in water is a homogeneous mixture	Evaporation or crystallization The water evaporates, leaving salt crystals in the porcelain cup. When water is evaporated from lakes Elton and Baskunchak, table salt is obtained. This separation method is based on the difference in boiling points of the solvent and the solute. If a substance, for example sugar, decomposes when heated, then the water is not completely evaporated - the solution is evaporated, and then sugar crystals are precipitated from the saturated solution. Sometimes it is necessary to remove impurities from solvents with a lower temperature boiling, for example water from salt. In this case, the vapors of the substance must be collected and then condensed upon cooling. This method of separating a homogeneous mixture is called distillation or distillation. In special devices -distillers produce distilled water , whichused for the needs of pharmacology, laboratories, car cooling systems . At home, you can construct such a distiller: If you separate a mixture of alcohol and water, then the alcohol with boiling point = 78 °C will be distilled off first (collected in a receiving test tube), and water will remain in the test tube. Distillation is used to produce gasoline, kerosene, and gas oil from oil. Separation of homogeneous mixtures

A special method for separating components, based on their different absorption by a certain substance, is chromatography.

You can try the following experiment at home. Hang a strip of filter paper over a container of red ink, dipping only the end of the strip into it. The solution is absorbed by the paper and rises along it. But the paint rise boundary lags behind the water rise boundary. This is how two substances are separated: water and the coloring matter in the ink.

Using chromatography, the Russian botanist M. S. Tsvet was the first to isolate chlorophyll from the green parts of plants. In industry and laboratories, starch, coal, limestone, and aluminum oxide are used instead of filter paper for chromatography. Are substances with the same degree of purification always required?

For different purposes, substances with varying degrees of purification are required. Cooking water should be left to stand sufficiently to remove impurities and chlorine used to disinfect it. Water for drinking must first be boiled. And in chemical laboratories for preparing solutions and conducting experiments, in medicine, distilled water is needed, purified as much as possible from substances dissolved in it. Particularly pure substances, the content of impurities in which does not exceed one millionth of a percent, are used in electronics, semiconductor, nuclear technology and other precision industries.

Read L. Martynov’s poem “Distilled Water”:

Water
Favored
To pour!
She
Shined
So pure
No matter what to get drunk,
No washing.
And this was not without reason.
She missed
Willows, tala
And the bitterness of flowering vines,
She didn't have enough seaweed
And fish, fatty from dragonflies.
She missed being wavy
She missed flowing everywhere.
She didn't have enough life
Clean –
Distilled water!

Using distilled water

II. Tasks for consolidation

1) Work with simulators No. 1-4(necessarydownload the simulator, it will open in the Internet Explorer browser)

O.S.GABRIELYAN,
I.G. OSTROUMOV,
A.K.AKHLEBININ

START IN CHEMISTRY

7th grade

Continuation. For the beginning, see No. 1, 2, 3, 4, 5, 6/2006

Chapter 2. Mathematics in chemistry

(continuation)

§ 12. Pure substances and mixtures

One of the most favorite materials of sculptors and architects is marble (Fig. 59). The color of this rock is surprisingly varied: milky white, gray, pinkish. The whimsical pattern pleases the eye. Marble is obedient and pliable in the hands of a master; it is easily processed and perfectly polished to a mirror shine. Marble is a mineral, a material from which you can make a tile, a statue or a palace column. A tile, a statue, a column are physical bodies, products. But the basis of marble is a substance called calcium carbonate. The same substance is part of other minerals - chalk, limestone.

Let us trace the logical chain of relationship between the concepts “physical body” – “material” – “substance” using several more examples. A useful item - a ruler - is made of plastic material. Most likely, this plastic is polypropylene. The window frame is the physical body, wood is the material, cellulose is the main substance of wood. The blade of the knife is steel; steel is an alloy, the main component of which is iron.

Now let's think about why marble comes in different colors? Why does it have a unique pattern on its surface? Because in addition to calcium carbonate, it contains impurities that give color. Likewise, rulers come in different colors, which depend on what dye is added to the plastic. The rubber material used to make car tires includes 24 components, the most important of which is the chemical substance rubber.

So it turns out that there are very few pure substances in nature, in technology, in everyday life. Much more common mixtures- a combination of two or more substances. Air is a mixture of various gases; oil – a natural mixture of organic substances (hydrocarbons); Any minerals or rocks are also solid mixtures of various substances.

Mixtures differ in the size of the particles of substances included in their composition. Sometimes these particles are so large that they can be seen with the naked eye. If you mix river sand with sugar, you can easily distinguish individual crystals from each other, especially if you use a magnifying glass for this purpose. Such mixtures include, for example, washing powder, culinary mixtures for baking pancakes or cakes, and construction mixtures.

Sometimes the particles of components in mixtures are smaller and not visible to the eye. For example, flour contains grains of starch and protein that cannot be seen with the naked eye. Milk is also an aqueous mixture that contains small droplets of fat, protein, lactose and other substances. You can see droplets of fat in milk if you examine a drop of milk under a microscope.

The physical state of substances in a mixture may be different. Toothpaste, for example, is a mixture of solid and liquid components (Fig. 60).

Mixtures in which particles of their constituent substances are visible to the naked eye or under a microscope are called heterogeneous or heterogeneous.

There are mixtures in the formation of which substances “penetrate each other” so much that they are broken into tiny particles that are not distinguishable even under a microscope. No matter how you peer into the air, you will not be able to distinguish the gases that make it up. It is also useless to look for “heterogeneity” in solutions of acetic acid or table salt in water.

A mixture in which particles of its constituent substances cannot be seen even with the help of magnifying instruments is called uniform or homogeneous.

Homogeneous mixtures according to their state of aggregation are divided into gaseous, liquid and solid.

Mixture any gases always homogeneous. For example, clean air is a homogeneous mixture of nitrogen, oxygen, carbon dioxide and noble gases, and water vapor. But dusty air is a heterogeneous mixture of the same gases, only containing dust particles. You've probably seen more than once how early in the morning the sun's rays make their way into the room through loosely drawn curtains. Their paths are often marked with luminous paths: these dust particles suspended in the air scatter sunlight. Smog over a city or over an industrial enterprise is also a heterogeneous mixture: air that contains not only dust particles, but also soot from smoke, droplets of various liquids, etc. (Fig. 61).

Natural gas and associated petroleum gas are also natural mixtures of gaseous substances, the main component of which is methane CH4. The same methane enters our apartments through pipelines and burns in the kitchen with a cheerful blue flame. But household gas is also a mixture. Strong-smelling substances are specially introduced into its composition so that the slightest gas leak can be detected by smell. Why is this necessary? The fact is that both air (necessary for the breathing of all living things) and natural gas (an irreplaceable fuel and raw material for the chemical industry) are a great blessing for humanity, but their mixture turns into a formidable destructive force due to its extreme explosiveness. From media reports, you are certainly aware of the tragedies associated with methane explosions in coal mines, domestic gas explosions as a result of criminal negligence or failure to comply with basic safety standards. If you smell gas in an apartment or in the entrance of your house, you must immediately turn off the taps and valves, ventilate the room, and call a specialized emergency service by calling 04. In this case, it is strictly forbidden to use open fire or turn on or off electrical appliances.

TO liquid natural mixtures refers to oil. It contains hundreds of different components, mainly carbon compounds. Oil is called the “blood of the Earth”, “black gold”, and you are well aware of how significant a role the extraction, refining and export of oil and petroleum products plays in the economy of our state and many other countries.

Of course, the most common liquid mixture, or rather solution, is the water of the seas and oceans. You already know that one liter of sea water contains on average 35 g of salts, the main part of which is sodium chloride. Unlike pure sea water, it has a bitter-salty taste and freezes not at 0 °C, but at –1.9 °C.

You come across liquid mixtures in everyday life all the time. Shampoos and drinks, potions and household chemicals are all mixtures of substances. Even tap water cannot be considered a pure substance: it contains dissolved salts and tiny insoluble impurities; it is disinfected by chlorination. This water should not be drunk unboiled; it is not recommended to use it for cooking. Special household filters will help purify tap water not only from solid particles, but also from some dissolved impurities. Even reagent solutions cannot be prepared using tap water. For this purpose, water is purified by distillation, which you will learn about a little later.

Widespread and solid mixtures. As we have already said, rocks are mixtures of several substances. Soil, clay, sand are also mixtures. Solid mixtures include glass, ceramics, and alloys. Everyone is familiar with culinary mixtures or mixtures that form washing powders.

Tell me, is the composition of the air that we inhale and exhale the same? Of course not. The latter contains less oxygen, but more carbon dioxide. But “more or less” are relative concepts. The composition of mixtures can be expressed quantitatively, i.e. in numbers. How? This will be discussed in the next paragraph.

1. What is the difference between a material and a chemical?

2. Can water in various states of aggregation be a material? Give examples.

3. What is a mixture? Give examples of natural mixtures of different states of aggregation. Name the components of these mixtures.

4. Give examples of household mixtures of various states of aggregation. Name the components of these mixtures.

5. What mixtures are called heterogeneous? Give examples of such natural and household mixtures and name their components.

6. What mixtures are called homogeneous? Give examples of such natural and household mixtures and name their components.

7. Which air can be considered as a homogeneous mixture, and which as a heterogeneous one?

Due to its low density and compressibility.

The normal speed of flame propagation is the linear speed of movement of the combustion zone relative to the fresh homogeneous combustible mixture in the direction normal to the flame front. Combustion with a clearly defined flame front is typical for conditions when the combustible mixture is stationary or moves laminarly. The speed of flame propagation under such conditions for a given composition of the combustible mixture can be considered as a physicochemical characteristic that depends only on pressure and temperature.

Due to the wide variety of types of fuel and oxidizer, the specific characteristics and areas of use of gas are very different. The most important factor determining the basic properties of gas is the state of aggregation of the fuel and oxidizer. Based on the state of aggregation of the fuel and oxidizer, they are distinguished: 1) homogeneous - gases and vaporous combustibles in a gaseous oxidizer (including air oxygen) 2) heterogeneous - liquid and solid combustibles in a gaseous oxidizer, as well as gas in the system liquid flammable mixture - a liquid oxidizer (for example, acids) 3) G. explosives and gunpowders, which are essentially a condensed homogeneous system.

So far we have considered the propagation of flame through a homogeneous combustible mixture. Another type of flame occurs when combustion occurs at the contact surface of two gases capable of forming a combustible mixture. Such flames are familiar from everyday experience; it is enough to name the flame of a match or candle, coal, wood, or a gas jet used for lighting. Since these flames are formed in the process of mutual diffusion of two gases, they are called diffusion flames. Diffusion phenomena play, of course, a role in all combustion processes; however, the difference between ordinary and diffusion flames is not difficult to understand. As such, it is impossible to indicate a sharp boundary between these two types of flame, since there must be a continuous transition from one to the other, as can be observed if the primary air supply is gradually reduced in a Bunsen burner. Another example of transitional phenomena between ordinary and diffusion flames can be the above-mentioned flames in very dilute mixtures of hydrogen with air and balls of flame formed in mixtures below the propagation limit (Chapter VII). The term diffusion flame, however, appears to be useful.

Ignition is characteristic of any combustible systems - homogeneous, heterogeneous and more complex systems. There are, however, two methods (types) of ignition: self-ignition and the so-called. forced ignition - ignition. During self-ignition, the conditions for self-acceleration of the reaction described above are created throughout the entire volume of this flammable mixture. For example, during thermal self-ignition, a gaseous mixture is heated either from the hot wall of a vessel (bomb), or by rapid compression of the mixture, or by rapid mixing of previously heated components of the mixture. In this case, the corresponding value of the initial temperature is fixed, at which ignition occurs, and this temperature is called the self-ignition temperature.

For a similar mixture. However, liquid droplets measuring 5 µm are too small, making microphotography very difficult. Therefore, a new version of the expansion unit was designed specifically to study the flame structure. The installation diagram is shown in Fig. 9.10. Its main elements are a combustion chamber with a volume of 1000 cm3 and an auxiliary compartment, separated from each other by a free-floating piston with a glass window. The piston starts to move when the auxiliary compartment is connected to a container in which reduced pressure is created. Using a device that allows you to regulate the stroke of the piston, the degree of expansion, as in previous experiments, is set to 1.25. A homogeneous mixture of fuel vapor and air, intended for expansion, is prepared in an evaporation tank having a water jacket connected to a thermostat. The mixture circulates through a circuit that includes a combustion chamber. A hygrometer is used to judge whether a mixture is saturated.

Reactors in which the fuel and moderator form a homogeneous mixture are called homogeneous reactors. One of such reactors is described below, in which the critical mass of uranium-235 combustible nuclei is only 800 g. In the reactor core there is a solution of highly enriched uranium sulfate in heavy water (at 6 hours 1 hour. The solution is placed in a spherical container, which is surrounded by protection consisting of lead (10 cm), cadmium (several millimeters) and concrete (150 cm). The reactor is cooled by water circulating through pipes in the form of a coil located inside the container. Control rods are made of cadmium. The reactor consists of the fact that the chain reaction is maintained in it at a given level without the help of control rods. This is due to changes in the neutron multiplication factor even with slight fluctuations in the concentration of nuclear fuel. With increasing temperature, the concentration of nuclear fuel decreases due to its thermal expansion, causing a decrease in the multiplication factor and the cessation of the chain reaction reaction until the temperature of the uranium solution drops to the calculated value.

A number of researchers believe that flame extinguishing is due to homogeneous inhibition, which consists of interaction with the active centers of gaseous particles formed during the evaporation and decomposition of powders. Another, most numerous group associates flame extinguishing with heterogeneous recombination of radicals and atomic particles on the surface of powders and. finally, the third group believes that when a flame is extinguished, both homogeneous and heterogeneous inhibition occurs. Homogeneous inhibition by powders is considered most thoroughly in the work. The effect of various powders on the speed of flame propagation of a methane-air mixture was studied. Carbonates, bicarbonates and halides of alkali and alkaline earth metals with particles smaller than 10 microns were used in the experiments. It was found that when using the most effective salts (carbonates and bicarbonates of potassium and sodium), the combustion rate decreased to a minimum value when their concentration was less than 1 mg/cm. When only 0.86% CH3C was added to the combustible mixture, the effectiveness of the powders sharply decreased.

Lehner confirmed his conclusion about the heterogeneous origin of a homogeneous reaction with this experiment. A mixture of ethylene (80%) and oxygen (20%) was passed through a capillary heated to 450° (c = 2 mm) into a Pyrex vessel (30 x 2.5 cm), filled with narrow Pyrex tubes in such a way that the total volume of free space in the tubes equal to 5.5 cm. The residence time of the combustible mixture in the heated zone was 70 seconds. Under such conditions there was no reaction. Then the reaction vessel, still filled with tubes, was connected to a 1-liter flask using a tube (2.6x1.b). The temperature of the capillary and vessel with tubes, as before, was 450°, and the temperature of the empty flask was 315°. The contact time was 27 seconds. Under these conditions the reaction proceeded quickly. Its products turned out to be 1.5% ethylene oxide, 34% formic aldehyde and 6.9% formic acid; part of the reacted ethylene turned into water and carbon dioxide.

The presented idea of ​​the combustion mechanism of liquid drops, apparently, is applicable only to drops of sufficiently large sizes. In the case of droplets smaller than a few hundredths of a millimeter, the picture changes, since droplets of such sizes can have time to evaporate before entering the combustion region. In this case, flammable fog, which is a mixture of fine droplets with air, enters the combustion area in the form of a homogeneous mixture of fuel vapor and air. Therefore, the combustion of such fog in its characteristics should be close to the combustion of pre-prepared gas mixtures, which is confirmed by observations of the combustion of fine aerosols.

§ 13. MIXTURES AND THEIR COMPOSITION

In everyday life we ​​rarely encounterpure substances. As a fewexamples of pure substances include sugar,potassium manganate (potassium permanganate), table salt (andthen, if various additives are not added to it, for examplemeasures containing iodine for disease preventionthyroid gland)(Fig. 7).Much more often than ussurround mixtures of substances that contain two or more individual compounds, called mixture components.

Fig.7. Sugar (a), potassium permanganate (b), salt (c) - examples
pure substances used in everyday life

Mixtures differ in the size of the particles of substances included in their composition. Sometimes these particles are quite large: if you mix river sand with sugar, you can easily distinguish individual crystals from each other.

Mixtures , in which particles of their constituent substances are visible to the naked eye or under a microscope are called heterogeneous , orheterogeneous . Such mixtures include, for example, washing powder, culinary mixtures for baking pancakes or cakes, and construction mixtures.
There are mixtures, during the formation of which substances are crushed into tiny particles (molecules, ions), which are not distinguishable even under a microscope. No matter how you peer into the air, you will not be able to visually distinguish the molecules of the gases that make it up. It is useless to look for “heterogeneity” in solutions of acetic acid or table salt in water. Such mixtures are called homogeneous , or homogeneous .
Homogeneous mixtures, like chemical substances, can be divided according to their state of aggregation into gaseous, liquid and solid. The most familiar natural mixtures of gases are air, the already familiar natural and associated petroleum gases.
Of course, the most common liquid mixture on Earth, or rather a solution, is the water of the seas and oceans. One liter of sea water contains on average 35 g of salts, the main part of which is sodium chloride. Unlike pure water, sea water has a bitter-salty taste and freezes not at 0 °C, but at –1.9 °C.
You come across liquid mixtures in everyday life all the time. Shampoos and drinks, potions and household chemicals are all mixtures of substances. Even
Tap water cannot be considered a pure substance: it contains dissolved salts, tiny insoluble impurities and microorganisms, which are partially removed by chlorination or ozonation. However, in this case it is recommended to boil the water. Special household filters will help make water suitable for drinking and purify it not only of solid particles, but also of some dissolved impurities. Solid mixtures are also widespread. As we have already said, rocks are a mixture of several substances. Soil, clay, sand are also mixtures. Solid artificial mixtures include glass, ceramics, and alloys. Everyone is familiar with culinary mixtures or mixtures that form washing powders.
As you know from biology, the composition of the air that we inhale and then exhale is not the same. There is less oxygen in the exhaled air, but more carbon dioxide and water vapor. But “more” and “less” are relative concepts.
The composition of mixtures can be expressed quantitatively, i.e. in numbers. The composition of a gas mixture is expressed by the volume fraction of each of its components.
Volume fraction of gas in the mixture is the ratio of the volume of a given gas to the total volume of the mixture, expressed in fractions of a unit or percentage.
ϕ(gas) = V ( gas ) X 100 (%). V ( mixtures )
The volume fraction of gas in the mixture is denoted by the letter ϕ (phi). This value shows how much of the total volume of the mixture is occupied by a particular gas. For example, you know that the volume fraction of oxygen in the air is 21%, nitrogen - 78%. The remaining 1% comes from noble gases, carbon dioxide and other air components.
Obviously, the sum of the volume fractions of all gases in the mixture is 100%.
The composition of liquid and solid mixtures is usually expressed by a value called the mass fraction of the component.
Mass fraction of the substance in the mixture is the ratio of the mass of a given substance to the total mass of the mixture, expressed in fractions of a unit or percentage.
ω(substances) = m (in-va) X 100 (%). m ( mixtures )

Almost any tablet in a home medicine cabinet is a compressed mixture of one or more medicinal substances and a filler, which can be gypsum, starch, or glucose. Construction and culinary mixtures, perfumery compositions and paints, fertilizers and plastics have a composition that can be expressed in mass fractions of the components that form them.
Substances with impurities are also mixtures. Only in such mixtures is it customary to isolate the main (main) substance, and the extraneous components are called in one word - impurities. The fewer there are, the purer the substance.

In some areas of technology, the use of insufficiently pure substances is unacceptable. In nuclear energy, increased demands are placed not only on the purity of nuclear fuel, but also on the substances from which the installations themselves are made. A computer chip cannot be made without a particularly pure silicon crystal. The light signal in the fiberglass cable will “go out” when it encounters foreign impurities.
To separate the components of a mixture or to purify the main substance from impurities, various techniques and methods are used. As a rule, substances in a mixture retain their physical properties: boiling point, melting point, solubility in various solvents. Since the properties of one substance differ
from the properties of another, it is possible to separate the mixture into individual components. The transition of substances from one state of aggregation to another is often used.
The separation of mixtures of liquid substances is based on the difference in their boiling points. This process, as you know from the example of oil refining, is called rectification, or distillation. You already know that any gases are mixed in any ratio. Is it possible to isolate individual components from a mixture of gases? The task is not easy. But scientists have proposed a very effective solution. A mixture of gases can be turned into a liquid and subjected to distillation. For example, air is liquefied by intense cooling and compression, and then the individual components are allowed to boil off one by one, since they have different boiling points. The first of
Nitrogen evaporates in liquid air; it has the lowest boiling point (–196 °C). Argon (–186 °C) can then be removed from the liquid mixture of oxygen and argon.
What remains is almost pure oxygen (its boiling point is –183 °C, Fig. 8), which is quite suitable for gas welding, chemical production, and also for medical purposes.
Distillation is used not only to separate mixtures into individual components, but also to purify substances.
Tap water is clean, transparent, odorless... But is this substance pure from the point of view of a chemist? Look into the kettle: scale and brownish deposits remain in
as a result of repeated boiling of water in it. What about limescale on taps? Both natural and tap water are a mixture, a solution of solid and gaseous substances.

Rice. 8. In liquid form
oxygen is colored light
blue

Of course, their content in water is very small, but these impurities can lead not only to scale formation, but also to more serious consequences. It is no coincidence that injection medications, reagent solutions, and electrolyte for a car battery are prepared only using purified water, called distilled water.
Where did this name come from? The thing is that distillation is also called distillation. The essence of distillation is that the mixture is heated to a boil, the resulting vapors of the pure substance are removed, cooled and converted back into liquid. But it no longer contains contaminants.
In laboratory conditions, distillation is carried out using a special installation (Fig. 9). The mixture to be separated, for example water with substances dissolved in it, is poured into a distillation flask equipped with a thermometer and heated to a boil. The flask is connected to a downward condenser - a device for condensing vapors of a boiling substance. For this purpose, cold water is supplied into the refrigerator jacket through rubber hoses. Drops of a pure substance condensed in the refrigerator fall into the receiving flask.

Rice. 9. Laboratory installation for distillation of liquids:
1 – distillation flask; 2 – thermometer; 3 – refrigerator;
4 – receiver

What should you do if you want to isolate from a solution not a liquid, but a solid dissolved in it? For this purpose, the crystallization method is used. A solid can be isolated from a solution by crystallization by evaporating the solvent. Special porcelain cups are designed for this (Fig. 10).

Rice. 10. Evaporation
solution in porcelain
cup

This method is widely used to extract salt from concentrated solutions of salt lakes.
There is wormwood and the taste of quinine all around,
And, with strong salt soda,
The plain colored by the rays
A smooth wave licks a little.
N. Ushakov
In nature, salt lakes are like giant bowls. Due to the evaporation of water on the shores of such lakes, a huge amount of salt crystallizes, which, after purification, ends up on our table (Fig. 11).

Rice. 11. Extracting salt from salt lakes
When performing crystallization, it is not necessary to evaporate the solvent. It is known that when heated, the solubility of most solids in water increases; when a solution saturated by heating is cooled, a certain amount of crystals will precipitate.
Laboratory experiments: To 5 g of orange crystals of potassium dichromate, add several crystals of potassium permanganate (potassium permanganate) as an impurity. The mixture is dissolved in 8–10 ml of boiling water. When the solution is cooled, the solubility of potassium dichromate sharply decreases, and the substance precipitates. Crystals of dichromate purified from potassium permanganate are separated and washed with several milliliters of ice water. If you dissolve the purified substance in water, then by the color of the solution you can determine that it does not contain potassium permanganate, it remains in the original solution.
To isolate insoluble substances from liquids, the method is used defending . It is based on different densities of substances. If the solid particles are large enough, they quickly settle to the bottom, and the liquid becomes transparent (Fig. 12). It can be carefully drained from the sediment. The smaller the size of the solid particles in the liquid, the longer the mixture will settle.

Rice. 12. Soil settling in water

LABORATORY EXPERIMENT: Pour a little dishwashing powder into a glass beaker and add half a glass of water. A cloudy mixture forms.
The liquid will become clear only the next day. Why does this mixture sit for so long? Mixtures of two liquids that are insoluble in each other are also separated by settling. If water gets into the car's lubrication system, the oil will have to be drained. However, after some time the mixture will separate. Water, which has a higher density, forms the bottom layer, with a layer of oil on top.A mixture of water and oil, water and vegetable oil is settled in the same way.

To separate such mixtures it is convenient to use
special laboratory glassware called a separating funnel (Fig. 13).

Rice. 13. Separating two immiscible liquids using a separating funnel
Laboratory experiments. Equal volumes of water and vegetable oil are poured into a conical flask. Vigorous shaking breaks water and oil into small droplets, forming a cloudy mixture. It is poured into a separatory funnel. After some time, the mixture separates into a heavier water layer and oil that floats to the top. By opening the separating funnel tap, the water layer is separated from the oil layer.
Particles of solid insoluble matter can be separated from a liquid by filtration. In the laboratory, special porous paper called filter paper is used for this. Solid particles do not pass through the pores of the paper and remain on the filter. The liquid with substances dissolved in it (called filtrate) freely seeps through it and becomes completely transparent.
Filtration - a very common process in everyday life, in technology, and in nature. At water treatment plants, water is filtered through a layer of clean sand, which retains silt, oil impurities, soil and clay particles. Fuel and oil in a car engine must pass through filter elements. Cell membranes, the walls of the intestines or stomach are also unique biological filters, the pores of which allow certain substances to pass through and retain others.
It is not only liquid mixtures that can be filtered. More than once you have seen people wearing gauze bandages, and you probably had to use them yourself. Several layers of gauze with cotton wool sandwiched between them purify the inhaled air from particles of dust, smog, and pathogens (Fig. 14). In industry, special devices called respirators are used to protect the respiratory system from dust. The air entering the car engine is also cleaned of dust using fabric or paper filters.

Rice. 14. Doctors and microbiologists protect the respiratory system with special bandages.

? 1. What is a mixture? What types of mixtures are distinguished based on the state of aggregation of the substances that form them, on the basis of homogeneity?
2. Is the phrase “air molecules” correct? Why? Name the constant, variable and random components of air. Make an assumption about the relative content of individual components in the air after a thunderstorm, in deep gorges and on mountain peaks, in a forested area and near a large industrial enterprise.

3. What volume of oxygen is contained in 500 m3 (n.s.) of air?
4. In the natural gas of a certain field, the volume fractions of saturated hydrocarbons are equal: methane - 85%, ethane - 10%, propane - 4% and butane - 1%. What volume of each gas can be produced from 125 liters of natural gas (n.o.)?
5. The composition of the dry cement mixture for plastering work includes 25% cement and 75% sand. How many kilograms of each component do you need to take to prepare 150 kg of such a mixture?
6. Name the methods known to you for separating mixtures. What is the basis of each of them? Suggest a method for separating the following mixtures:
a) iron and copper filings;
b) sand and sawdust;
c) gasoline and water;
d) chalk whitewash (divided into chalk and water);
e) a solution of ethyl alcohol in water.
7. During a flu epidemic, doctors recommend wearing gauze bandages. For what? How to make such a bandage? How long can it be worn? How to restore the protective properties of the bandage?
8. Prospectors separated gold sand from ordinary sand by stirring up the soil in water and draining the muddy liquid from the sediment. This is where the expression “panning for gold” comes from. What property do you think of gold sand is based on its separation from grains of waste rock?
9. Prepare messages on the topics: “Paints in the hands of the artist” and “Famous perfumers” using Internet resources.