pure substancecontains particles only one kind. Examples are silver (containing only silver atoms), sulfuric acid, and carbon monoxide ( IV) (contain only the molecules of the corresponding substances). All pure substances have constant physical properties, for example, melting point (T pl ) and boiling point ( T bale ).

A substance is not pure if it contains any amount of one or more other substances -impurities.

Contaminants lower the freezing point and raise the boiling point of a clean liquid. For example, if salt is added to water, the freezing point of the solution will decrease.

Mixes consist of two or more substances. Soil, sea water, air are all examples of different mixtures. Many mixtures can be separated into their component parts − Components - based on the difference in their physical properties.

Traditional methods that are used in laboratory practice to separate mixtures into individual components are:

filtration,

settling followed by decantation,

separation using a separating funnel,

centrifugation,

evaporation,

crystallization,

distillation (including fractional distillation),

chromatography,

sublimation and others.

Filtration. Filtration is used to separate liquids from fine solid particles suspended in it.(fig.37) , i.e. filtering liquid through finely porous materials -filters, which allow liquid to pass through and retain solid particles on their surface. The liquid that has passed through the filter and freed from the solid impurities in it is called filtrate.

Often used in laboratory practicesmooth and folded paper filters(fig.38) made from non-glued filter paper.

To filter hot solutions (for example, for the purpose of recrystallization of salts), a specialhot filter funnel(fig.39) with electric or water heating).

Often usedvacuum filtration. Vacuum filtration is used to speed up filtration and more completely free the precipitate from solution. For this purpose, a vacuum filtration device is assembled. (fig.40) . It consists ofBunsen flask, Buchner porcelain funnel, safety bottle and vacuum pump(usually water jet).

In the case of filtering a suspension of a sparingly soluble salt, the crystals of the latter can be washed with distilled water on a Buchner funnel to remove the initial solution from their surface. For this purpose, use washer(fig.41) .

Decantation. Liquids can be separated from insoluble solidsdecantation(fig.42) . This method can be used if the solid has a higher density than the liquid. For example, if river sand is added to a glass of water, then when settling, it will settle to the bottom of the glass, because the density of sand is greater than that of water. Then the water can be separated from the sand by simply draining. This method of settling and subsequent draining of the filtrate is called decantation.

Centrifugation.D To accelerate the process of separating very small particles that form stable suspensions or emulsions in a liquid, the method is used. centrifugation. This method can be used to separate mixtures of liquid and solid substances that differ in density. The division is carried out in manual or electric centrifuges(fig.43) .

Separation of two immiscible liquids, having different densities and not forming stable emulsions,can be done with a separating funnel (fig.44) . So you can separate, for example, a mixture of benzene and water. Benzene layer (density = 0.879 g/cm 3 ) located above a layer of water, which has a high density ( = 1.0 g/cm 3 ). By opening the stopcock of the separating funnel, you can carefully drain the bottom layer and separate one liquid from another.

Evaporation(fig.45) - this method involves the removal of a solvent, such as water, from a solution by heating it in an evaporating porcelain dish. In this case, the evaporated liquid is removed, and the dissolved substance remains in the evaporating dish.

Crystallization- this is the process of separating crystals of a solid when a solution is cooled, for example, after it has been evaporated. It should be borne in mind that large crystals are formed when the solution is slowly cooled. Upon rapid cooling (eg cooling under running water), fine crystals form.

Distillation- a method of cleaning a substance based on the evaporation of a liquid when heated, followed by condensation of the resulting vapors. Purification of water from salts (or other substances, for example, dyes) dissolved in it by distillation is called distillation, and the purified water itself is distilled.

Fractional distillation(fig.46) used to separate mixtures of liquids with different boiling points. A liquid with a lower boiling point boils faster and passes through fractional column(ordephlegmator). When this liquid reaches the top of the fractionation column, it entersfridge, cooled by water andallongegoing toreceiver(flask or test tube).

Fractional distillation can separate, for example, a mixture of ethanol and water. Boiling point of ethanol 78 0 C, and water 100 0 C. Ethanol evaporates more easily and enters the receiver through the condenser first.

Sublimation - This method is used to purify substances capable of changing from a solid state to a gaseous state when heated, bypassing the liquid state. Further, the vapors of the substance to be purified are condensed, and impurities that are not able to sublimate are separated.

Methods for separating mixtures

Most of the substances on our planet are not in their pure form, but in compounds and mixtures, along with other substances.

So, the composition of granite includes three substances that are visible to the naked eye.

But milk seems homogeneous to us until it turns sour. Sour

milk separates into a clear whey and a white solid precipitate - protein

casein. Man long ago uses these substances , included in milk, highlighting them

from the mixture. Curd is prepared from insoluble protein - casein, and soluble

whey proteins are used for clinical nutrition.

How can mixtures be separated?

1. If the substance is insoluble in water, such as cereals (rice, buckwheat, semolina, etc.), river sand, chalk, clay, then you can use the filtration method.

Filtration-filtering liquids (gases) through a filter in order to purify them from solid impurities.

1. Putting up a filter. We place it in a funnel, slightly wetting it with water.

2. Insert the funnel with the filter into the flask.

3. Pass a mixture of undissolved matter and water through the filter.

Conclusion. Water purified by filtration freely passed through the filter; a substance insoluble in water remains on the filter.

2. If the solid is soluble in water (table salt, sugar, citric acid), then for separationmixtures, the evaporation method can be used.

Evaporation- separation of solids dissolved in a liquid by converting it into vapor.

In a glass of water, the salt did not disappear, although it became invisible - the solution is transparent. Evaporation made it possible to isolate a substance dissolved in water from a mixture of substances (water and salt). Salt crystals are visible on the glass. This confirms the conclusion that that each substance (both water and salt) of the mixture retains its properties.

Conclusion. Soluble substances can be isolated from a solution.

3 .To separate liquids soluble in each other, to obtain pure (without impurities) water, the distillation method is used

(or distillation)

Distillation-distillation, separation of substances contained in liquid mixtures according to boiling points, followed by cooling of the vapor.

In nature, water in its pure form (without salts) does not occur. Oceanic, sea, river, well and spring water are varieties of salt solutions in water. However, often people need clean water that does not contain salts (used in car engines; in chemical production to obtain various solutions and substances; in the manufacture of photographs). Such water is called distilled, and the method of obtaining it is called distillation.

We heat tap water over the flame of an alcohol lamp in a test tube, closed with a cork with a gas outlet tube. We lower the end of the tube into a clean, dry test tube placed in a glass with ice. Drops of distilled (purified from salts and impurities) water will appear on the bottom and walls of a test tube in a glass with ice.

Exercise

1. Look into an empty kettle in which water is boiled. Are there white deposits (scale) on the walls and bottom of substances that have been dissolved in water?

2. Droplets of water flow from the lid of the kettle in which water is boiled. Which water - on the lid or in the kettle itself - contains more salts? Explain your answer.

3. What is the name of the process shown in the picture?

4. If the mixture contains iron, then a magnet can be used to isolate it, because. iron and its alloys are attracted by a magnet.

5. To separate two immiscible liquids (oil and water, sunflower oil and water), you need to use a separating funnel.

The liquid with a higher density will merge into a glass, and a lighter liquid will remain in the separating funnel.

Every substance contains impurities. A substance is considered pure if it contains almost no impurities.

Mixtures of substances are either homogeneous or heterogeneous. In a homogeneous mixture, the components cannot be detected by observation, but in an inhomogeneous mixture it is possible.

Some physical properties of a homogeneous mixture differ from those of the components.

In a heterogeneous mixture, the properties of the components are preserved.

Heterogeneous mixtures of substances are separated by settling, filtering, sometimes by the action of a magnet, and homogeneous mixtures are separated by evaporation and distillation (distillation).

Pure substances and mixtures

We live among chemical substances. We inhale air, and this is a mixture of gases (nitrogen, oxygen and others), we exhale carbon dioxide. We wash ourselves with water - this is another substance, the most common on Earth. We drink milk - a mixture of water with the smallest droplets of milk fat, and not only: there is also casein milk protein, mineral salts, vitamins and even sugar, but not the one with which they drink tea, but a special milk - lactose. We eat apples, which consist of a whole range of chemicals - sugar, malic acid, vitamins... apple, but also 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 are built of chemicals, like a house of bricks. Nitrogen, oxygen, sugar, vitamins are substances of natural, natural origin. Glass, rubber, steel are also substances, more precisely, materials (mixtures of substances). Both glass and rubber are of artificial origin; they did not exist in nature. Completely pure substances are not found in nature or are very rare.

Each substance always contains a certain amount of impurities. A substance that contains almost no impurities is called pure. They work with such substances in a scientific laboratory, a school chemistry room. Note that absolutely pure substances do not exist.

An individual pure substance has a certain set of characteristic properties (constant physical properties). Only pure distilled water has tmelt = 0 °С, tboil = 100 °С, and has no taste. Sea water freezes at a lower temperature, and boils at a higher temperature, its taste is bitter-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 point is that in sea ​​water contains other substances, such as dissolved salts, i.e. it is a mixture of various substances, the composition of which varies over a wide range, but the properties of the mixture are not constant. The concept of "mixture" was defined in the 17th century. English scientist Robert Boyle: "A mixture is an integral system consisting of heterogeneous components."

Almost all natural substances, foodstuffs (except for salt, sugar, and some others), many medicinal and cosmetic products, goods are mixtures. household chemicals, Construction Materials.

Comparative characteristics of a mixture and a pure substance

Each substance contained in a mixture is called a component.

Classification of mixtures

There are homogeneous and heterogeneous mixtures.

Homogeneous mixtures (homogeneous)

Add a small portion of sugar to a glass of water and stir until all the sugar is dissolved. The liquid will taste sweet. Thus, the sugar did not disappear, but remained in the mixture. Ho, we will not see its crystals, even when examining a drop of liquid in a powerful microscope. The prepared mixture of sugar and water is homogeneous; the smallest particles of these substances are evenly mixed in it.

Mixtures in which components cannot be detected by observation are called homogeneous.

Most metal alloys are also homogeneous mixtures. For example, an alloy of gold and copper (used to make jewelry) lacks red copper particles and yellow gold particles.

From materials that are homogeneous mixtures of substances, many items for various purposes are made.

All mixtures of gases, including air, belong to homogeneous mixtures. There are many homogeneous mixtures of liquids.

Homogeneous mixtures are also called solutions, even if they are solid or gaseous.

Let us give examples of solutions (air in a flask, table salt + water, small change: aluminum + copper or nickel + copper).

Heterogeneous mixtures (heterogeneous)

You know that chalk does not dissolve in water. If its powder is poured into a glass of water, then chalk particles can always be found in the resulting mixture, which are visible to the naked eye or through a microscope.

Mixtures in which components can be detected by observation are called heterogeneous.

Heterogeneous mixtures include most minerals, soil, building materials, living tissues, cloudy water, milk and other foodstuffs, some medicines and cosmetics.

In a heterogeneous mixture, the physical properties of the components are preserved. So, iron filings mixed with copper or aluminum do not lose their ability to be attracted to a magnet.

Some species heterogeneous mixtures have special names: foam (for example, foam, soap foam), suspension (a mixture of water with a small amount of flour), emulsion (milk, well-shaken vegetable oil with water), aerosol (smoke, fog).

Methods for separating mixtures

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

There are many methods for separating mixtures. They are chosen taking into account the type of mixture, state of aggregation and differences in physical properties components.

Methods for separating mixtures

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

Consider methods for separating heterogeneous and homogeneous mixtures.

Blend example	Separation method
Suspension - a mixture of river sand with water	settling Separation by settling is based on different densities of the substances. Heavier sand settles to the bottom. You can also separate the emulsion: to separate oil or vegetable oil from water. In the laboratory, this can be done using a separating funnel. Oil or vegetable oil forms the top, lighter layer. As a result of settling, dew falls out of the fog, soot is deposited from smoke, cream is settled in milk.
A mixture of sand and table salt in water	Filtration The separation of heterogeneous mixtures by filtration is based on the different solubility of substances in water and on different particle sizes. Only particles of substances commensurate with them pass through the pores of the filter, while larger particles are retained on the filter. So you can separate a heterogeneous mixture of table salt and river sand. Various porous substances can be used as filters: cotton wool, coal, fired clay, pressed glass, and others. The filtering method is the basis for the operation of household appliances, such as vacuum cleaners. It is used by surgeons - gauze bandages; drillers and workers of elevators - respiratory masks. With the help of a tea strainer for filtering tea leaves, Ostap Bender - the hero of the work of Ilf and Petrov - managed to take one of the chairs from Ellochka the Cannibal ("The Twelve Chairs").
A mixture of iron powder and sulfur	Action by magnet or water Iron powder was attracted by a magnet, but sulfur powder was not. The non-wettable sulfur powder floated to the surface of the water, while the heavy wettable iron powder settled to the bottom.
A solution of salt in water is a homogeneous mixture	Evaporation or crystallization The water evaporates and salt crystals remain 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 the boiling points of the solvent and the solute. If a substance, such as sugar, decomposes when heated, then the water is not completely evaporated - the solution is evaporated, and then sugar crystals are precipitated from a saturated solution. Sometimes it is required to remove impurities from solvents with a lower boiling point, 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, distilled water is obtained, which is used for the needs of pharmacology, laboratories, and car cooling systems. At home, you can design such a distiller. If, however, a mixture of alcohol and water is separated, then the first to be distilled off (collected in a receiving test tube) is alcohol with tboil = 78 °C, and water will remain in the test tube. Distillation is used to obtain gasoline, kerosene, gas oil from oil.

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

If you hang a strip of filter paper over a vessel with red ink, immersing only the end of the strip in them. The solution is absorbed by the paper and rises along it. But the border of the rise of the paint lags behind the border of the rise of the water. This is how the separation of two substances occurs: water and the coloring matter in the ink.

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

For different purposes, substances with different degrees of purification are needed. Cooking water is sufficiently settled to remove impurities and chlorine used to disinfect it. Drinking water must first be boiled. And in chemical laboratories for the preparation of solutions and experiments, in medicine, distilled water is needed, as purified as possible from the substances dissolved in it. Highly 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.