Chemistry studies substances and their properties. When they are mixed, mixtures arise that acquire new valuable qualities.
What is a mixture
A mixture is a collection of individual substances. Their production is carried out not only by scientists in laboratories under certain conditions. Every day we start with aromatic tea or coffee to which we add sugar. Or we cook tasty soup, which must be salted. These are the real mixtures. Only we don’t think about it at all.
If it is impossible to distinguish particles of substances with the naked eye, you are looking at homogeneous mixtures (homogeneous). They can be obtained by dissolving the same sugar in tea or coffee.
But if you add sand to sugar, their particles can be easily distinguished. Such a mixture is considered heterogeneous or heterogeneous.
When making mixtures of this type, you can use substances that are in different states of aggregation: solid or liquid. Ground pepper mixture different types or other seasonings are most often precisely heterogeneous dry compounds.
If any liquid is used in the process of preparing a heterogeneous product, the resulting mass is called a suspension. Moreover, there are several types of them. When liquids and solids mix, suspensions are formed. An example of this is a mixture of water with sand or clay. When a builder makes cement, a cook mixes flour with water, a child brushes his teeth with paste - they all use suspensions.
Another type of heterogeneous mixture can be obtained by mixing two liquids. Naturally, if their particles are distinguishable. Drop vegetable oil into water and get an emulsion.
Video on the topic
Homogeneous mixtures
The best known of this group of substances is air. Every student knows that it contains a number of gases: nitrogen, oxygen, carbon dioxide, water vapor and impurities. Can they be seen and distinguished with the naked eye? Of course not.
Thus, both air and sweet water- homogeneous mixtures. They can be in different states of aggregation. But most often liquid homogeneous mixtures are used. They consist of a solvent and a solute. Moreover, the first component is either liquid or taken in a larger volume.
Substances cannot dissolve in infinite quantities. For example, you can only add two kilograms of sugar to a liter of water. This process simply will not happen further. This solution will become saturated.
An interesting phenomenon is represented by solid homogeneous mixtures. Thus, hydrogen is easily distributed into various metals. The intensity of the dissolution process depends on many factors. It increases with increasing temperature of the liquid and air, when substances are crushed and as a result of their mixing.
It is surprising that there are no absolutely insoluble substances in nature. Even silver ions are distributed between water molecules, forming a homogeneous mixture. Such solutions are found wide application in everyday life and human life. For example, everyone’s favorite and healthy milk is a homogeneous mixture.
Methods for separating mixtures
Sometimes there is a need not only to obtain homogeneous solutions, but also to separate homogeneous mixtures. Let's say there is only salt water in the house, but you need to get its crystals separately. To do this, such a mass is evaporated. Homogeneous mixtures, examples of which were given above, are most often separated in this way.
Distillation is based on differences in boiling point. Everyone knows that water begins to evaporate at 100 degrees Celsius, and ethyl alcohol at 78. The mixture of these liquids is heated. First, the alcohol vapor evaporates. They are condensed, that is, transferred to a liquid state, in contact with any cooled surface.
Using a magnet, mixtures containing metals are separated. For example, iron and wood filings. Vegetable oil and water can be obtained separately by settling.
Heterogeneous and homogeneous mixtures, examples of which are illustrated in the article, are of great economic importance. Minerals, air, The groundwater, seas, food products, Construction Materials, drinks, pastes - all this is a collection of individual substances, without which life would simply be impossible.
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: highlight essential features and properties, establish cause-and-effect relationships, classify, analyze, draw conclusions, perform experiments, observe, formalize observations in in the form of tables, schemes.
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 works, doing 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, solution copper sulfate, semolina, buckwheat.
DURING THE CLASSES
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 are made up of atoms of one chemical element, complex - from atoms of various 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. Black Sea water freezes at lower temperatures and boils at higher temperatures. high 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 various substances, the composition of which varies within wide limits, 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's consider distinctive features mixtures and pure substances. 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 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, 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 physical properties 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 the different solubility of substances in water and on various sizes particles.
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 filtering method is the basis of the work 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 acquainted with these methods of separating a mixture, let's help the Russian heroine 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 the grains by filtering through a sieve, if the grains have different sizes, or by agitation with water if the particles have different densities or different wettability with water. Let's take as example 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. It's about 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.
Chemistry studies substances and their properties. When they are mixed, mixtures arise that acquire new valuable qualities.
What is a mixture
A mixture is a collection of individual substances. Their production is carried out not only by scientists in laboratories under certain conditions. Every day we start with aromatic tea or coffee to which we add sugar. Or we cook a delicious soup, which must be salted. These are the real mixtures. Only we don’t think about it at all.
If it is impossible to distinguish particles of substances with the naked eye, you are looking at homogeneous mixtures (homogeneous). They can be obtained by dissolving the same sugar in tea or coffee.
But if you add sand to sugar, their particles can be easily distinguished. Such a mixture is considered heterogeneous or heterogeneous.
When making mixtures of this type, you can use substances found in different solids or liquids. A mixture of ground pepper of different types or other seasonings are most often just heterogeneous dry compositions.
If any liquid is used in the process of preparing a heterogeneous product, the resulting mass is called a suspension. Moreover, there are several types of them. When liquids are mixed with, suspensions are formed. An example of this is a mixture of water with sand or clay. When a builder makes cement, a cook mixes flour with water, a child brushes his teeth with paste - they all use suspensions.
Another type of heterogeneous mixture can be obtained by mixing two liquids. Naturally, if their particles are distinguishable. Drop vegetable oil into water and get an emulsion.
Homogeneous mixtures
The best known of this group of substances is air. Every student knows that it contains a number of gases: nitrogen, oxygen, carbon dioxide, and impurities. Can they be seen and distinguished with the naked eye? Of course not.
Thus, both air and sweet water are homogeneous mixtures. They can be in different states of aggregation. But most often liquid homogeneous mixtures are used. They consist of a solvent and a solute. Moreover, the first component is either liquid or taken in a larger volume.
Substances cannot dissolve in infinite quantities. For example, you can only add two kilograms of sugar to a liter of water. This process simply will not happen further. This solution will become saturated.
An interesting phenomenon is represented by solid homogeneous mixtures. Thus, hydrogen is easily distributed in various metals. The intensity of the dissolution process depends on many factors. It increases with increasing temperature of the liquid and air, when substances are crushed and as a result of their mixing.
It is surprising that there are no absolutely insoluble substances in nature. Even silver ions are distributed between water molecules, forming a homogeneous mixture. Such solutions are widely used in everyday life and human life. For example, everyone’s favorite and healthy milk is a homogeneous mixture.
Methods for separating mixtures
Sometimes there is a need not only to obtain homogeneous solutions, but also to separate homogeneous mixtures. Let's say there is only salt water in the house, but you need to get its crystals separately. To do this, such a mass is evaporated. Homogeneous mixtures, examples of which were given above, are most often separated in this way.
Distillation is based on differences in boiling point. Everyone knows that water begins to evaporate at 100 degrees Celsius, and ethyl alcohol at 78. The mixture of these liquids is heated. First, the alcohol vapor evaporates. They are condensed, that is, transferred to a liquid state, in contact with any cooled surface.
Using a magnet, mixtures containing metals are separated. For example, iron and wood filings. Vegetable oil and water can be obtained separately by settling.
Heterogeneous and homogeneous mixtures, examples of which are illustrated in the article, are of great economic importance. Minerals, air, groundwater, seas, food products, building materials, drinks, pastes - all this is a collection of individual substances, without which life would simply be impossible.
>> Pure substances and mixtures. Advocacy. Separating a mixture of three solids
Pure substances and mixtures
The material in this paragraph will help you:
> realize that absolutely pure substances do not exist;
> distinguish between homogeneous and heterogeneous mixtures of substances;
> find out in which mixtures the physical properties of the components are preserved and in which they are not;
> select separation method mixtures of substances depending on its type.
Pure substances and mixtures.
Each substance always contains a certain amount of impurities. A substance in which there are almost no impurities is called pure. With such substances work in a science laboratory, school chemistry lab. Note that absolutely pure substances do not exist.
Each substance contained in a mixture is called a component.
Mixtures in which components cannot be detected by observation are called homogeneous.
Most metal alloys are also homogeneous mixtures. For example, in an alloy of gold and copper (it is used to make jewelry) there are no red copper particles and yellow gold particles.
Many items for various purposes are made from materials that are homogeneous mixtures of substances (Fig. 27).
All mixtures belong to homogeneous mixtures gases, including air. There are quite a few homogeneous mixtures liquids.
Rice. 27. Items made from homogeneous mixtures
This mixture is formed by mixing, for example, alcohol and water.
Give your example of a homogeneous mixture.
Homogeneous mixtures are also called solutions, even if they are solid or gaseous.
According to some physical properties homogeneous mixtures differ from their components. Thus, an alloy of tin and lead used for soldering melts at a lower temperature than pure metals. Water boils at a temperature of 100 °C, and water solution salt - at a higher temperature. If water is cooled to a temperature of 0 ° C, it will begin to turn into ice. The salt solution remains a liquid under these conditions (it freezes at temperatures below 0 °C). This can be seen in winter, when roads and sidewalks covered with ice are sprinkled with a mixture of salt and sand. Ice melts under the influence of salt; an aqueous solution of salt is formed that does not freeze in mild frost. And sand is needed so that the road is not slippery.
Rice. 28. Heterogeneous mixture of chalk and water
You know that chalk does not dissolve in water. If its powder is poured into a glass of water, then in the resulting mixture you can always find chalk particles that are visible to the naked eye or through a microscope (Fig. 28).
Mixtures in which components can be detected by observation are called heterogeneous.
Heterogeneous mixtures (Fig. 29) include most minerals, soil, building materials, living tissues, muddy water, milk and other food products, some medicinal and cosmetical tools.
Give your example of a heterogeneous mixture.
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.
Rice. 29. Heterogeneous mixtures:
a - a mixture of water and sulfur;
b - a mixture of vegetable oil and water;
c - a mixture of air and water
Water mixed with sand, chalk or clay freezes at a temperature of 0C and boils at 100C.
Some types of heterogeneous mixtures have special names: foam (for example, polystyrene foam, lather), suspension (a mixture of water with a small amount of flour), emulsion (milk, well-shaken vegetable oil and water), aerosol (smoke, fog).
What are the components in each named mixture?
The material presented above is summarized in Scheme 3.
Scheme 3. Substances and mixtures
It often becomes necessary to separate a mixture in order to obtain its components or to purify a substance from impurities.
There are many methods for separating mixtures. They are selected taking into account the type of mixture, state of aggregation and differences in the physical properties of the components (Scheme 4). You know some methods from your natural history course.
Scheme 4. Methods for separating mixtures
Explain due to what properties of the components it is possible to separate each heterogeneous mixture indicated in the diagram.
Rice. 30. Worker in a respirator
Let's look at how some use methods separation of mixtures.
The filtration process underlies the operation of a respirator - a device that protects the lungs of a person working in a very dusty room. The respirator has filters that prevent dust from entering the lungs (Fig. 30). The simplest respirator is a bandage made of several layers of gauze. A vacuum cleaner also has a filter that removes dust from the air.
With the help of a magnet, iron ore - magnetite - is enriched in industry.
Thanks to the ability to be attracted to a magnet, ore is separated from sand, clay, earth, etc. In this way, iron is extracted from industrial and household waste.
An important method for separating homogeneous mixtures of liquids is distillation, or distillation1. This method allows you to purify natural water from impurities. The resulting pure (distilled) water is used in research laboratories and in the production of substances for modern technology, in medicine for the preparation of medicines.
1 The term comes from the Latin word distillatio - dripping down.
In industry, the distillation of oil (a mixture of many substances, mainly liquids) produces gasoline, kerosene, and diesel fuel.
In the laboratory, distillation is carried out using a special installation (Fig. 31). When a mixture of liquids is heated, the substance with the lowest boiling point boils first. Its vapor leaves the vessel, cools, condenses1, and the resulting liquid flows into the receiver. When this substance is no longer in the mixture, the temperature will begin to rise, and over time, another liquid component will boil. Non-volatile liquids remain in the vessel.
Rice. 31.Laboratory distillation unit:
a - ordinary;
1 - a mixture of liquids with different boiling points;
2 - thermometer;
3 - water refrigerator;
4 - receiver
6 - simplified
Separation various mixtures happens in nature too. Dust particles settle from the air, and during rain and snow - water droplets and snowflakes. As a result of settling, cloudy water becomes clear. Water is also cleared of insoluble substances when passing through sand. After the water evaporates, the salts that were dissolved in it remain on the banks of the estuaries. Dissolved gases are released from the water flowing from the well.
1 The term comes from the Latin word condensatio - thickening, compaction.
conclusions
Every substance contains impurities. A substance is considered pure if it contains almost no impurities.
Mixtures of substances can be homogeneous or heterogeneous. In a homogeneous mixture, the components cannot be detected by observation, but in a heterogeneous mixture this is possible.
Some physical properties of a homogeneous mixture differ from the properties of the components. In a heterogeneous mixture, the properties of the components are preserved.
Heterogeneous mixtures of substances are separated by settling, filtering, and sometimes by the action of a magnet, and homogeneous mixtures are separated by evaporation and distillation (distillation).
?
29. What types of mixtures exist and how do they differ?
30. Write down the given words and phrases in the appropriate columns of the table below: aluminum, ash, newsprint, mercury, air, iodine tincture, granite, ice from clean water, carbon dioxide, reinforced concrete.
| Pure substances | Mixtures | |
| homogeneous | heterogeneous | |
31. Name several food products that are solutions.
32. Which popular drink, depending on the method of preparation, is a homogeneous or heterogeneous mixture?
33. Is it possible to turn an aqueous solution of table salt into a heterogeneous mixture? If possible, how to do it?
34. What mixtures can be separated by filtration: a) a mixture of sand and clay; b) a mixture of alcohol and copper filings; c) a mixture of water and gasoline; d) a mixture of water and pieces of plastic? Name the substances that will remain on the filter.
35. How would you divide a mixture of: a) table salt and chalk; b) alcohol and water? What differences in the properties of substances make it possible to use the method you choose?
36. Consider an experiment to separate a mixture of table salt, sand, iron and sawdust. Make a plan, briefly describe each stage of the experiment and talk about the expected results.
Experimenting at home
Advocacy
Pour water into two glasses. Pour 1/2 teaspoon of sand into one glass, and the same amount of starch into the other. Mix both mixtures at the same time. Do particles of substances settle at the same speed in water? If not, which particles settle faster and why?
Write down your observations in your notebook.
Separating a mixture of three solids
Mix small amounts of crushed foam, sand and table salt.
What methods can be used to separate this mixture?
Divide the mixture 1. If heating is necessary, use it very carefully.
Describe each stage of the experiment in your notebook.
Popel P. P., Kryklya L. S., Chemistry: Pidruch. for 7th grade zagalnosvit. navch. closing - K.: VC "Academy", 2008. - 136 p.: ill.
Every substance contains impurities. A substance is considered pure if it contains almost no impurities.
Mixtures of substances can be homogeneous or heterogeneous. In a homogeneous mixture, the components cannot be detected by observation, but in a heterogeneous mixture this is possible.
Some physical properties of a homogeneous mixture differ from the properties of the components.
In a heterogeneous mixture, the properties of the components are preserved.
Heterogeneous mixtures of substances are separated by settling, filtering, and 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, which is a mixture of gases (nitrogen, oxygen and others), and 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 tiny droplets of milk fat, and not only: there is also the milk protein casein, mineral salts, vitamins and even sugar, but not the one with which you drink tea, but a special milk protein - lactose. We eat apples, which consist of a whole set of chemicals - here there is sugar, malic 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 useful material not only apples, but 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 origin. Glass, rubber, steel are also substances, or rather, 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.
Each substance always contains a certain amount of impurities. A substance in which there are almost no impurities is called pure. They work with such substances in a scientific laboratory or school chemistry lab. 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 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.”
Mixtures include almost all natural substances, food products (except salt, sugar, some others), many medicines and cosmetics, goods household chemicals, Construction Materials.
Comparative characteristics of the mixture and 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 has dissolved. The liquid will taste sweet. Thus, the sugar did not disappear, but remained in the mixture. But we will not see its crystals, even when examining a drop of liquid through a powerful microscope. The prepared mixture of sugar and water is homogeneous; the smallest particles of these substances are evenly mixed.
Mixtures in which components cannot be detected by observation are called homogeneous.
Most metal alloys are also homogeneous mixtures. For example, in an alloy of gold and copper (used to make jewelry), there are no red copper particles and yellow gold particles.
Many items for various purposes are made from materials that are homogeneous mixtures of substances.
Homogeneous mixtures include all mixtures of gases, including air. 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 in the resulting mixture you can always find chalk particles that 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, muddy water, milk and other food products, some medicines and cosmetics.
In a heterogeneous mixture, the physical properties of the components are preserved. Thus, iron filings mixed with copper or aluminum do not lose their ability to be attracted to a magnet.
Some types of heterogeneous mixtures have special names: foam (for example, polystyrene foam, soap suds), suspension (a mixture of water with a small amount of flour), emulsion (milk, well-shaken vegetable oil and water), aerosol (smoke, fog).
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.
There are many methods for separating mixtures. They are selected taking into account the type of mixture, state of aggregation and differences in the physical properties of the components.
Methods for separating mixtures
These methods are based on differences in the physical properties of the components of the mixture.
Let's consider ways to separate heterogeneous and homogeneous mixtures.
Example of a mixture |
Separation method |
Suspension - a mixture of river sand and water |
Advocacy Separation by settling is 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. |
A mixture of sand and table salt in water |
Filtration The separation of heterogeneous mixtures by filtration is based on different solubilities of substances in water and different particle sizes. Only particles of substances comparable to them pass through the pores of the filter, while larger particles are retained on the filter. This way 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. |
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 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. In special devices - distillers, distilled water is obtained, which is used for the needs of pharmacology, laboratories, and car cooling systems. You can construct such a distiller at home. 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 particular substance, is chromatography.
If you hang a strip of filter paper over a container of red ink, immersing only the end of the strip in 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.
