In chemistry there are concepts of pure substances and mixtures. Pure ones contain molecules of only one substance. In nature, mixtures consisting of different substances predominate.
Concepts
All substances can be divided into two categories - pure and mixed. Pure substances include elements and compounds consisting of identical atoms, molecules or ions. These are substances with a constant composition that retain constant properties.
Examples of pure substances are:
- metals and noble gases consisting of atoms;
- water, consisting of water molecules;
- table salt, consisting of sodium cations and chlorine anions.
Rice. 1. Pure substances.
If you add sugar to water, it ceases to be a pure substance and a mixture is formed. Mixtures consist of several pure substances with different structures, which are called components. Mixtures can have any state of aggregation. For example, air is a mixture of various gases (oxygen, hydrogen, nitrogen), gasoline is a mixture of organic substances, brass is a mixture of zinc and copper.
Rice. 2. Mixtures.
Each substance retains its properties, so water with salt is salty, and an alloy with iron is attracted by a magnet. However, the properties of the mixture itself may vary in accordance with the quantitative and qualitative composition of the components. For example, distilled water that has undergone maximum purification, depending on the added substances, can acquire a sweet, sour, salty or sour-salty taste. Moreover, the higher the concentration of a certain substance, the more pronounced a certain taste is.
The structure of mixtures can be homogeneous or combine substances in different states of aggregation. In accordance with this, they distinguish:
- homogeneous or homogeneous - particles cannot be detected without chemical analysis, their indicator is the same anywhere in the sample (metal alloy);
- heterogeneous or heterogeneous - particles are easy to detect, their frequency is non-uniform in different places of the mixture (water with sand).
Heterogeneous mixtures include:
- suspensions - mixtures of solid and liquid substances (coal and water);
- emulsions - mixtures of liquids of different densities (oil and water).
If one component is ten times smaller in mass than another component, then it is called an impurity.
Cleaning methods
There are no absolutely pure substances. Pure substances are considered substances containing a small amount of impurities that do not affect the physical and chemical properties of the substance. To purify the substance as much as possible, we use methods for separating mixtures:
- sedimentation - sedimentation of heavy substances in liquids;
- filtration - separation of particles from liquid using filters;
- evaporation - heating the solution until the moisture evaporates;
- application of a magnet - selection using magnetization;
- distillation - separation of substances with different boiling points;
- adsorption is the accumulation of one substance on the surface of another.
Metals can be separated from non-metals using flotation. This is a process based on the ability of substances to become wet. In this way, iron is separated from sulfur: the iron gets wet and settles to the bottom, but the sulfur does not get wet and remains on the surface of the water.
Rice. 3. Flotation.
What have we learned?
From the 8th grade chemistry lesson we learned about the concepts of mixtures and pure substances. Elements and compounds consisting of homogeneous molecules, atoms or ions, and also having constant properties, are called pure. Mixtures include several pure substances of different concentrations and structures. Compounds can mix completely, forming homogeneous substances, or combine heterogeneously. Various methods are used to separate mixtures.
PURE SUBSTANCE PURE SUBSTANCE
PURE SUBSTANCE (ideally pure substance), a simple or complex substance that has only one inherent complex of permanent properties that are determined by a certain set of atoms and molecules. These properties include the requirement of chemical (absence of foreign atoms) purity and physical (absence of structural defects) perfection. In some cases, it can be supplemented by the requirement of isotopic purity, which provides for the absence in a pure substance of impurities of its isotopes, the decay products of which can change the desired properties. The requirements of physical perfection can be supplemented by the requirements of crystal chemical purity, expressed in the absence of polymorphic phases in a pure substance.
The concept of an ideally pure substance is as abstract as, for example, absolute zero temperature or an ideal gas, and it is also impossible to obtain an ideally pure substance. The reason for this is limitations of a kinetic and thermodynamic nature. The first is manifested in the fact that the rate of purification of substances from impurities is directly proportional to the concentration and falls as it decreases. Therefore, the higher the degree of purity required to obtain a substance, the greater the expenditure of energy and time it will be associated with, i.e. it is possible to reduce the impurity content 10 times from 1 to 0.1% (by weight) much cheaper and faster than also by 10 times, but from 10 -4 to 10 -5% (by weight). A decrease in the content of one or another impurity by one order of magnitude, starting from 10 -3% (by weight), requires the use of special purification methods.
Maintaining the original purity is difficult due to the second limitation, since the process of contamination of a substance, i.e. disorder of the system occurs spontaneously: it is impossible to obtain an absolutely pure substance.
The purity of actually existing pure substances is relative. It is assessed by the content of foreign impurities in the substance. Their number can be quite large. Thus, in relatively pure gallium phosphide, with a total impurity concentration of 10 -5% (by mass), 72 impurities were detected by mass spectral analysis. As the sensitivity of the analysis increases, the amount of impurities detected in a pure substance increases accordingly.
Particularly pure substances are used mainly for special purposes - in the production of semiconductor materials and devices, in radio and quantum electronics. In this case, purity requirements apply to both the main materials and auxiliary materials used in the production process: water, gases, chemical reagents, container materials. Working with high-purity products containing impurities of the order of 10 -5% and below is quite difficult. The production of such products requires specially equipped rooms with carefully filtered air, the complete absence of metal objects, and the use of special types of plastic utensils. The use of distilled water (even twice or thrice distilled) is absolutely unacceptable - only water that has undergone additional purification using ion exchangers can be used. Strict measures are also taken to eliminate the possibility of any contamination from workers. For this purpose, in particular, lavsan overalls (lint-free), special shoes and rubber gloves are used.
Marking
According to the existing regulations in Russia, the qualifications “pure” (P), “pure for analysis” (analytical grade), “chemically pure” (CP) and “extra pure” (OSCH) are established for reagents, the latter is sometimes divided into several grades. Reagents with “pure” qualification are used in a wide variety of laboratory work, both educational and industrial. Analytical grade reagents, as the name suggests, are intended for analytical work performed with great precision. The content of impurities in analytical grade preparations is so low that it usually does not introduce noticeable errors into the analysis results. These reagents may well be used in research work. Finally, reagents classified as “chemically pure” are intended for responsible scientific research; they are also used in analytical laboratories as substances by which titers of working solutions are established. These three qualifications cover all general purpose reagents.
In foreign practice, in addition to expressing the concentration of an impurity in a pure substance in atomic percentages and percentages by mass, smaller units are often used: 0/00 - ppm, ppm - parts per million or gram per ton; ppb – part per billon (billion) or milligram per ton; very rarely ppT – parts per trillion.
Pure substances, the content of limited impurities in which is at a level from 10 -6 to 10 -7% (by mass), and the sum of the remaining impurities is 10 -3 10 -4% (by mass), belong to the class of ultrapure substances (OSP) . They are intended for special purposes only. Substances of special purity are divided into three classes. Class A is divided into subclasses A1 (main substance content 99.9%) and A2 (99.99% main substance). The number after the letter A characterizes the number of nines after the decimal point. According to the content of the main substance, subclasses B3, B4, B5 and B6 are distinguished. Finally, ultrapure substances will please class C, divided into subclasses C7-C10.
The purity of a substance is also characterized by the total content of a certain number of microimpurities. In addition to the name of the special purity grade, the marking of highly pure substances includes two numbers. The first indicates the amount of limited impurities, the second is an indicator of the negative degree of their sum, expressed as a percentage by mass. For example, for especially pure SiO2 ten impurities are standardized (Al, B, Fe, Ca, Mg, Na, P, Ti, Sn, Pb), and their total content does not exceed 1.10 -5%. For such a drug the index “OSCh-10-5” is established.
encyclopedic Dictionary. 2009 .
See what “PURE SUBSTANCE” is in other dictionaries:
Pure substance elements or compounds, their solutions, alloys, mixtures, etc., characterized by an impurity content below a certain limit. This limit is determined by the properties, production or use of substances and how... ... Wikipedia
pure substance- grynoji medžiaga statusas T sritis fizika atitikmenys: engl. pure substance vok. reiner Substanz, f rus. pure substance, n pranc. substance pure, f … Fizikos terminų žodynas
highly pure substance- ypač gryna medžiaga statusas T sritis radioelektronika atitikmenys: engl. highly purified material vok. Reinststoff, m; Reinstsubstanz, f rus. extra pure substance, n pranc. substance de très grande pureté, f... Radioelektronikos terminų žodynas
Chemical substance- 2.6 chemical substance: A chemical element or chemical compound, existing in nature or obtained artificially. Source: GOST 30333 2007: Safety Data Sheet for Chemical Products. General requirements original document... Dictionary-reference book of terms of normative and technical documentation
This term has other meanings, see Chemistry (meanings). Chemistry (from Arabic کيمياء, presumably derived from the Egyptian word km.t (black), from which the name of Egypt, chernozem and lead “black” also originated... ... Wikipedia
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- (Berlinka) a very common paint, which in dry form is a dark blue non-crystalline mass with a reddish copper metallic luster, odorless and tasteless. Weak acids have no effect on it, caustic alkalis decompose with the release of oxides... Encyclopedic Dictionary F.A. Brockhaus and I.A. Efron
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CRYOCONDUCTORS, metals whose resistivity decreases smoothly when cooled, without jumps, and reaches low values at cryogenic temperatures (see Low temperatures (see LOW TEMPERATURES)). Reduced resistance of cryoconductors... ... encyclopedic Dictionary
Lesson type. Learning new material.
Lesson objectives. Educational– study the concepts of “pure substance” and “mixture”, homogeneous (homogeneous) and heterogeneous (heterogeneous) mixtures, consider ways to separate mixtures, teach students to separate mixtures into components.
Developmental– develop students’ intellectual and cognitive skills: identify essential features and properties, establish cause-and-effect relationships, classify, analyze, draw conclusions, perform experiments, observe, draw up observations in the form of tables and diagrams.
Educational– to promote in students the development of organization, accuracy when conducting experiments, the ability to organize mutual assistance when working in pairs, and the spirit of competition when performing exercises.
Teaching methods. Methods of organizing educational and cognitive activities– verbal (heuristic conversation), visual (tables, drawings, demonstrations of experiments), practical (laboratory work, exercises).
Methods for stimulating interest in learning– educational games, educational discussions.
Control methods– oral control, written control, experimental control.
Equipment and reagents.On students' desks- sheets of paper, spoons for substances, glass rods, glasses of water, magnets, sulfur and iron powders.
On the teacher's desk– spoons, test tubes, test tube holder, alcohol lamp, magnet, water, beakers, stand with ring, stand with claw, funnel, glass rods, filters, porcelain cup, separating funnel, test tube with gas outlet tube, receiver test tube, “glass” -refrigerator" with water, a ribbon of filter paper (2x10 cm), red ink, a flask, a sieve, iron and sulfur powders in a mass ratio of 7: 4, river sand, table salt, vegetable oil, copper sulfate solution, semolina, buckwheat.
DURING THE CLASSES
Organizing time
Mark those who are absent, explain the objectives of the lesson and introduce the lesson plan to students.
PLAN
1. Pure substances and mixtures. Distinctive features.
2. Homogeneous and heterogeneous mixtures.
3. Methods for separating mixtures.
Conversation on the topic “Substances and their properties”
Teacher. Remember what chemistry studies?.
Student. Substances, properties of substances, changes occurring with substances, i.e. transformation of substances.
Teacher. What is a substance called?
Student. Substance is what the physical body is made of.
Teacher. You know that substances can be simple and complex. Which substances are called simple and which are complex?
Student. Simple substances consist of atoms of one chemical element, complex substances - of atoms of different chemical elements.
Teacher. What physical properties do substances have?
Student. Physical state, melting and boiling points, electrical and thermal conductivity, solubility in water, etc..
Explanation of new material
Pure substances and mixtures.
Distinctive features
Teacher. Only pure substances have constant physical properties. Only pure distilled water has t pl = 0 °C, t boil = 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.”
Let us consider the distinctive features of the mixture and the pure substance. To do this, we will perform the following experiments.
Experience 1. Using the instructions for the experiment, study the essential physical properties of iron and sulfur powders, prepare a mixture of these powders and determine whether these substances retain their properties in the mixture.
Discussion with students of the results of the experiment.
Teacher. Describe the state of aggregation and color of sulfur.
Student. Sulfur is a yellow solid.
Teacher. What is the physical state and color of iron in powder form?
Student. Iron is a hard gray matter.
Teacher. How do these substances relate: a) to a magnet; b) to the water?
Student. Iron is attracted by a magnet, but sulfur is not; Iron powder sinks in water, because... iron is heavier than water, and sulfur powder floats to the surface of the water because it is not wetted by water.
Teacher. What can you say about the ratio of iron and sulfur in the mixture?
Student. The ratio of iron and sulfur in the mixture can be different, i.e. fickle.
Teacher. Are the properties of iron and sulfur preserved in the mixture?
Student. Yes, the properties of each substance in the mixture are preserved.
Teacher. How can you separate a mixture of sulfur and iron?
Student. This can be done by physical methods: a magnet or water.
Teacher . Experience 2. Now I will show the reaction between sulfur and iron. Your task is to carefully observe this experiment and determine whether iron and sulfur retain their properties in the iron(II) sulfide obtained as a result of the reaction and whether iron and sulfur can be isolated from it by physical methods.
I thoroughly mix iron and sulfur powders in a mass ratio of 7: 4:
m(Fe ): m( S ) = А r ( Fe ): А r ( S ) = 56: 32 = 7: 4,
I place the mixture in a test tube, heat it in the flame of an alcohol lamp, heat it very hot in one place and stop heating when a violent exothermic reaction begins. After the test tube has cooled, I carefully break it, after wrapping it in a towel, and remove the contents. Take a close look at the resulting substance – iron(II) sulfide. Are gray iron powder and yellow sulfur powder visible in it separately?
Student. No, the resulting substance is dark gray in color.
Teacher. Then I test the resulting substance with a magnet. Are iron and sulfur separable?
Student. No, the resulting substance is not magnetized.
Teacher. I place iron(II) sulfide in water. What do you observe?
Student. Iron(II) sulfide sinks in water.
Teacher. Do sulfur and iron retain their properties when they are part of iron(II) sulfide?
Student. No, the new substance has properties different from the properties of the substances taken for the reaction.
Teacher. Is it possible to separate iron(II) sulfide into simple substances by physical methods?
Student. No, neither a magnet nor water can separate iron(II) sulfide into iron and sulfur.
Teacher. Is there a change in energy when a chemical is formed?
Student. Yes, for example, when iron and sulfur interact, energy is released.
Teacher. Let us enter the results of the discussion of the experiments in the table.
Table
Comparative characteristics of the mixture and pure substance
To reinforce this part of the lesson, do the exercise: determine where in the picture(see p. 34) depicts a simple substance, a complex substance or a mixture.
Homogeneous and heterogeneous mixtures
Teacher. Let's find out whether the mixtures differ in appearance from each other.
The teacher demonstrates examples of suspensions (river sand + water), emulsions (vegetable oil + water) and solutions (air in a flask, table salt + water, coin: aluminum + copper or nickel + copper).
Teacher. 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. Consider the scheme for classifying mixtures(Scheme 1).
Scheme 1
Give examples of each type of mixture: suspensions, emulsions and solutions.
Methods for separating mixtures
Teacher. 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.
To purify substances, various methods of separating mixtures are used (Scheme 2).
Scheme 2
These methods are based on differences in the physical properties of the components of the mixture.
Consider separation methods heterogeneous mixtures.
How can you separate a suspension - a mixture of river sand and water, i.e., clean the water from sand?
Student. By settling and then filtering.
Teacher. Right. 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. (The teacher demonstrates the corresponding experiments.)
As a result of settling, dew falls out of the fog, soot settles out of the smoke, and cream settles in the milk.
What is the basis for the separation of heterogeneous mixtures using filtering?
Student. On different solubility of substances in water and on different particle sizes.
Teacher. That’s right, 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.
Student shows experience: pours water into a mixture of sand and salt, mixes, and then passes the suspension (suspension) through a filter - a solution of salt in water passes through the filter, and large particles of water-insoluble sand remain on the filter.
Teacher. What substances can be used as filters?
Student. Various porous substances can be used as filters: cotton wool, coal, baked clay, pressed glass and others.
Teacher. What examples of the use of filtering in human life can you give?
Student. 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”).
Teacher. And now, having become familiar with these methods of separating a mixture, let’s help the heroine of the Russian folk tale “Vasilisa the Beautiful”.
Student. In this tale, Baba Yaga ordered Vasilisa to separate the rye from the nigella and the poppy from the ground. The heroine of the fairy tale was helped by pigeons. We can now separate grains by filtering through a sieve, if the grains have different sizes, or by shaking with water, if the particles have different densities or different wettability with water. Let's take as an example a mixture consisting of grains of various sizes: a mixture of semolina and buckwheat.(The student shows how semolina with smaller particle sizes passes through a sieve, and buckwheat remains on it.)
Teacher. But today you have already become acquainted with a mixture of substances that have different wettability with water. What mixture am I talking about?
Student. We are talking about a mixture of iron and sulfur powders. We conducted a laboratory experiment with this mixture.
Teacher. Remember how you separated such a mixture.
Student. By settling in water and using a magnet.
Teacher. What did you observe when you separated a mixture of iron and sulfur powders using water?
Student. Non-wettable sulfur powder floated to the surface of the water, and heavy wettable iron powder settled to the bottom.
Teacher. How was this mixture separated using a magnet?
Student. Iron powder was attracted by a magnet, but sulfur powder was not..
Teacher. So, we got acquainted with three methods of separating heterogeneous mixtures: sedimentation, filtration and magnetic action. Now let's look at the separation methods homogeneous (uniform) mixtures. Remember, after separating the sand by filtering, we obtained a solution of salt in water - a homogeneous mixture. How to isolate pure salt from a solution?
Student. Evaporation or crystallization.
The teacher demonstrates the experiment: the water evaporates, and salt crystals remain in the porcelain cup.
Teacher. 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 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 boiling point, such as salt from water. 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.
The teacher shows the distillation of a solution of copper sulfate, the water evaporates when t kip = 100 °C, then the vapors condense in a receiving test tube cooled with water in a glass.
Teacher. In special devices - distillers, distilled water is obtained, which is used for the needs of pharmacology, laboratories, and car cooling systems.
The student demonstrates a drawing of a “device” he designed for distilling water.
Teacher. 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.
A special method for separating components, based on their different absorption by a certain substance, is chromatography.
The teacher demonstrates experience. He hangs 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.
Teacher. 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?
Student. 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.
Teacher. Listen to 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!
To consolidate and check the mastery of the material, students answer the following questions.
1. When ore is crushed at mining and processing plants, fragments of iron tools fall into it. How can they be extracted from the ore?
2. Before recycling household waste, as well as waste paper, it is necessary to get rid of iron objects. What's the easiest way to do this?
3. The vacuum cleaner sucks in air containing dust and releases clean air. Why?
4. Water after washing cars in large garages turns out to be contaminated with machine oil. What should you do before draining it into the sewer?
5. Flour is cleared of bran by sifting. Why do they do this?
6. How to separate tooth powder and table salt? Gasoline and water? Alcohol and water?
Literature
Alikberova L.Yu. Entertaining chemistry. M.: AST-Press, 1999; Gabrielyan O.S., Voskoboynikova N.P., Yashukova A.V. Teacher's handbook. Chemistry. 8th grade. M.: Bustard, 2002; Gabrielyan O.S. Chemistry.
8th grade. M.: Bustard, 2000; Guzey L.S., Sorokin V.V., Surovtseva R.P. Chemistry. 8th grade. M.: Bustard, 1995; Ilf I.A., Petrov E.P. The twelve Chairs. M.: Education, 1987; Kuznetsova N.E., Titova I.M., Gara N.N., Zhegin A.Yu. Chemistry. Textbook for 8th grade students of general education institutions. M.: Ventana-Graf, 1997; Rudzitis G.E., Feldman F.G. Chemistry. Textbook for 8th grade of general education institutions. M.: Education, 2000; Tyldsepp A.A., Kork V.A.. We are studying chemistry. M.: Education, 1998.
In our article we will look at what pure substances and mixtures are, and methods for separating mixtures. Each of us uses them in everyday life. Are pure substances even found in nature? And how to distinguish them from mixtures?
Pure substances and mixtures: methods for separating mixtures
Substances that contain only certain types of particles are called pure. Scientists believe that they practically do not exist in nature, since they all, albeit in insignificant proportions, contain impurities. Absolutely all substances are also soluble in water. Even if, for example, a silver ring is immersed in this liquid, the ions of this metal will go into solution.
A sign of pure substances is the constancy of composition and physical properties. During their formation, the amount of energy changes. Moreover, it can both increase and decrease. A pure substance can only be separated into its individual components using a chemical reaction. For example, only distilled water has the boiling and freezing point typical for this substance, and lacks taste and smell. And its oxygen and hydrogen can only be decomposed by electrolysis.
How do their aggregates differ from pure substances? Chemistry will help us answer this question. Methods for separating mixtures are physical, since they do not lead to a change in the chemical composition of the substances. Unlike pure substances, mixtures have variable composition and properties, and they can be separated by physical methods.
What is a mixture
A mixture is a collection of individual substances. An example of this is sea water. Unlike distilled, it has a bitter or salty taste, boils at a higher temperature, and freezes at a lower temperature. Methods for separating mixtures of substances are physical. Thus, pure salt can be obtained from sea water by evaporation and subsequent crystallization.
Types of mixtures
If you add sugar to water, after a while its particles will dissolve and become invisible. As a result, they will be impossible to distinguish with the naked eye. Such mixtures are called homogeneous or homogeneous. Examples of them are also air, gasoline, broth, perfume, sweet and salt water, an alloy of copper and aluminum. As you can see, they can be in different states of aggregation, but liquids are most common. They are also called solutions.
In inhomogeneous or heterogeneous mixtures, particles of individual substances can be distinguished. Iron and wood filings, sand and table salt are typical examples. Heterogeneous mixtures are also called suspensions. Among them, suspensions and emulsions are distinguished. The former consists of a liquid and a solid. So, an emulsion is a mixture of water and sand. An emulsion is a combination of two liquids with different densities.
There are heterogeneous mixtures with special names. So, an example of foam is polystyrene foam, and aerosols include fog, smoke, deodorants, air fresheners, and antistatic agents.
Methods for separating mixtures
Of course, many mixtures have more valuable properties than the individual substances included in their composition. But even in everyday life, situations arise when they need to be separated. And in industry, entire productions are based on this process. For example, as a result of oil refining, gasoline, gas oil, kerosene, fuel oil, diesel and engine oil, rocket fuel, acetylene and benzene are obtained. Agree, it is more profitable to use these products than to mindlessly burn oil.
Now let's figure out whether there is such a thing as chemical methods for separating mixtures. Let's say we need to obtain pure substances from an aqueous solution of salt. To do this, the mixture must be heated. As a result, the water will turn into steam and the salt will crystallize. But in this case there will be no transformation of some substances into others. This means that the basis of this process is physical phenomena.
Methods for separating mixtures depend on the state of aggregation, solubility, difference in boiling point, density and composition of its components. Let's look at each of them in more detail using specific examples.
Filtration
This separation method is suitable for mixtures that contain a liquid and an insoluble solid. For example, water and river sand. This mixture must be passed through a filter. As a result, clean water will pass through it freely, but the sand will remain.
Advocacy
Some methods for separating mixtures rely on gravity. In this way, suspensions and emulsions can be separated. If vegetable oil gets into the water, the mixture must first be shaken. Then leave it for a while. As a result, the water will end up at the bottom of the vessel, and the oil will cover it in the form of a film.
In laboratory conditions, they are used for settling. As a result of its operation, the denser liquid is drained into the vessel, and the lighter liquid remains.
Settlement is characterized by a low speed of the process. It takes a certain amount of time for a precipitate to form. In industrial conditions, this method is carried out in special structures called settling tanks.
Action by magnet
If the mixture contains metal, it can be separated using a magnet. For example, separate iron and wood filings. But do all metals have these properties? Not at all. Only mixtures containing ferromagnets are suitable for this method. In addition to iron, these include nickel, cobalt, gadolinium, terbium, dysprosium, holmium, and erbium.
Distillation
This name translated from Latin means “dripping down”. Distillation is a method of separating mixtures based on differences in boiling points of substances. Thus, even at home you can separate alcohol and water. The first substance begins to evaporate already at a temperature of 78 degrees Celsius. Touching a cold surface, alcohol vapor condenses, turning into a liquid state.
In industry, petroleum products, aromatic substances, and pure metals are obtained in this way.
Evaporation and crystallization
These methods of separating mixtures are suitable for liquid solutions. The substances that make up them differ in their boiling point. In this way, salt or sugar crystals can be obtained from the water in which they are dissolved. To do this, the solutions are heated and evaporated to a saturated state. In this case, crystals are deposited. If it is necessary to obtain clean water, then the solution is brought to a boil, followed by condensation of vapors on a colder surface.
Methods for separating gas mixtures
Gaseous mixtures are separated by laboratory and industrial methods, since this process requires special equipment. Raw materials of natural origin are air, coke oven, generator, associated and natural gas, which is a combination of hydrocarbons.
Physical methods for separating mixtures in a gaseous state are as follows:
- Condensation is the process of gradual cooling of a mixture, during which condensation of its components occurs. In this case, first of all, high-boiling substances, which are collected in separators, pass into a liquid state. In this way, hydrogen is obtained from and ammonia is also separated from the unreacted part of the mixture.
- Sorbing is the absorption of some substances by others. This process has opposite components, between which equilibrium is established during the reaction. Different conditions are required for the forward and reverse processes. In the first case, it is a combination of high pressure and low temperature. This process is called sorption. Otherwise, the opposite conditions are used: low pressure at high temperature.
- Membrane separation is a method that uses the property of semi-permeable partitions to selectively allow molecules of various substances to pass through.
- Refluxation is the process of condensation of high-boiling parts of mixtures as a result of their cooling. In this case, the temperature of transition to the liquid state of individual components should differ significantly.
Chromatography
The name of this method can be translated as “I write with color.” Imagine adding ink to water. If you dip the end of a filter paper into this mixture, it will begin to be absorbed. In this case, water will be absorbed faster than ink, which is due to the different degrees of sorption of these substances. Chromatography is not only a method for separating mixtures, but also a method for studying such properties of substances as diffusion and solubility.
So, we got acquainted with such concepts as “pure substances” and “mixtures”. The former are elements or compounds consisting only of particles of a certain type. Examples of these are salt, sugar, distilled water. Mixtures are a collection of individual substances. A number of methods are used to separate them. The method of their separation depends on the physical properties of its components. The main ones include settling, evaporation, crystallization, filtration, distillation, magnetic action and chromatography.
SECTION I. GENERAL CHEMISTRY
6. Mixtures of substances. Solutions
6.2. Mixtures, their types, names, composition, separation methods
Mixtures are a collection of different substances from which one physical body can be formed. Each substance contained in a mixture is called a component. When mixed, a new substance does not appear. All substances that are part of the mixture retain their inherent properties. But the physical properties of the mixture, as a rule, differ from the physical properties of the individual components. Mixtures can be homogeneous or heterogeneous.
Homogeneous (homogeneous) mixtures are mixtures in which the components are mixed at the molecular level (single-phase material); they cannot be detected when viewed with the naked eye or even when using powerful optical instruments. For example, aqueous solutions of sugar, table salt, alcohol, acetic acid, metal alloys, air.
Inhomogeneous (heterogeneous) mixtures form so-called dispersed systems. They are formed by mixing two or more substances that do not dissolve in each other (do not form homogeneous systems) and do not react chemically. The components of disperse systems are called dispersion medium and dispersed phase; there is an interface between them.
Based on the particle size of the dispersed phase, systems are divided into:
Coarse (> 10 -5 m);
Microheterogeneous (10 -7 -10 -5 m);
Ultramicroheterogeneous (10 -9 -10 -7 m), or sols (colloidal systems) 1.
If the dispersed phase particles have the same size, the systems are called monodisperse; if different, they are polydisperse (almost all natural systems are such). Depending on the state of aggregation of the dispersion medium and the dispersed phase, the following simple disperse systems are distinguished:
|
Dispersed phase |
Dispersive medium |
Designations |
Name |
Example |
|
gaseous |
gaseous |
y/y |
not formed* |
|
|
liquid |
y/y |
gas emulsion, foam |
sea, soap foam |
|
|
hard |
g/t |
porous body (solid foam)** |
pumice, activated carbon |
|
|
liquid |
gaseous |
y/y |
aerosol |
clouds, fog |
|
liquid |
y/y |
emulsion |
milk, oil |
|
|
hard |
r/t |
capillary systems |
foam sponge soaked in water |
|
|
hard |
gaseous |
t/y |
aerosol |
smoke, sandstorm |
|
liquid |
t/y |
suspension, sol, suspension |
paste, a suspension of clay in water |
|
|
hard |
t/t |
solid heterogeneous system |
rocks, concrete, alloys |
* Gases form homogeneous mixtures (gaseous solutions).
** Porous bodies are divided into:
Microporous (2 nm);
Lesoporous (2-50 nm);
Macroporous (> 50 nm).
Mixtures are separated using physical methods. To separate heterogeneous mixtures, sedimentation, filtration, flotation, and sometimes the action of a magnet are used.
|
Advocacy |
To separate a mixture containing solid particles insoluble in water or liquids insoluble in each other. Solid insoluble particles or drops of liquid settle to the bottom of the vessel or float to the surface of the mixture. Use a separatory funnel to separate liquids that do not mix. |
clay and water; copper filings, sawdust and water; oil and water |
|
Filtration |
To separate a mixture of soluble and insoluble substances in a solvent. Solid insoluble particles remain on the filter |
water + sand; water + sawdust |
|
Flotation |
For separating mixtures of substances with different wettability indices |
Mineral beneficiation |
|
Action of a magnet |
For separating mixtures containing iron or other metals ( Ni, Co ), which are attracted by a magnet (ferromagnets) |
iron + sulfur; iron + sand |
To separate homogeneous mixtures, evaporation and distillation (distillation) are used.
_____________________________________________________________
1 If the particle sizes of the dispersed phase do not exceed the sizes of molecules or ions (up to 1 nm), such systems are called true solutions.