How to separate a mixture of sand and water. Topic: "Methods for separating mixtures" (Grade 8)

If dispersed particles are released slowly from the medium or it is necessary to pre-clarify an inhomogeneous system, methods such as flocculation, flotation, classification, coagulation, etc. are used.

Coagulation is the process of sticking together of particles in colloidal systems (emulsions or suspensions) with the formation of aggregates. Sticking occurs due to the collision of particles during Brownian motion. Coagulation refers to a spontaneous process that tends to move into a state that has a lower free energy. The coagulation threshold is the minimum concentration of an injected substance that causes coagulation. Artificial coagulation can be accelerated by adding special substances - coagulators to the colloidal system, as well as by applying an electric field to the system (electrocoagulation), mechanical action (vibration, mixing), etc.

During coagulation, coagulant chemicals are often added to the heterogeneous mixture to be separated, which destroy the solvated shells, while reducing the diffusion part of the electrical double layer located near the surface of the particles. This facilitates the agglomeration of particles and the formation of aggregates. Thus, due to the formation of larger fractions of the dispersed phase, particle settling is accelerated. Salts of iron, aluminum or salts of other polyvalent metals are used as coagulants.

Peptization is the reverse process of coagulation, which is the breakdown of aggregates into primary particles. Peptization is carried out by adding peptizing substances to the dispersion medium. This process contributes to the disaggregation of substances into primary particles. Peptizing agents can be surface-active substances (surfactants) or electrolytes such as humic acids or ferric chloride. The peptization process is used to obtain liquid dispersion systems from pastes or powders.

In turn, flocculation is a kind of coagulation. In this process, small particles that are suspended in gas or liquid media form flocculent aggregates, which are called floccules. Soluble polymers, such as polyelectrolytes, are used as flocculants. Flocculating substances can be easily removed by filtration or settling. Flocculation is used for water treatment and the separation of valuable substances from wastewater, as well as for mineral processing. In the case of water treatment, flocculants are used in low concentrations (from 0.1 to 5 mg/l).

In order to destroy aggregates in liquid systems, additives are used that induce charges on particles that prevent their convergence. This effect can also be achieved by changing the pH of the medium. This method is called deflocculation.

Flotation is the process of separating solid hydrophobic particles from a liquid continuous phase by selectively fixing them at the interface between the liquid and gaseous phases (the contact surface of liquid and gas or the surface of bubbles in the liquid phase). The resulting system of solid particles and gas inclusions is removed from the surface of the liquid phase. This process is used not only to remove particles of the dispersed phase, but also to separate different particles due to differences in their wettability. In this process, hydrophobic particles are fixed at the interface and separated from hydrophilic particles that settle to the bottom. The best flotation results occur when the particle size is between 0.1 and 0.04 mm.

There are several types of flotation: foam, oil, film, etc. The most common is froth flotation. This process allows the particles treated with reagents to be carried to the surface of the water with the help of air bubbles. This allows the formation of a foam layer, the stability of which is controlled by a foaming agent.

The classification is used in devices of variable cross section. With its help, it is possible to separate a certain amount of small particles from the main product, consisting of large particles. Classification is carried out using centrifuges and hydrocyclones due to the effect of centrifugal force.

The separation of suspensions using magnetic processing systems is a very promising method. Water that has been treated in a magnetic field retains changed properties for a long time, for example, reduced wetting ability. This process makes it possible to intensify the separation of suspensions.

This lesson is a hands-on exercise in which you will learn how to separate different mixtures of substances using the techniques you have learned. The lesson describes in detail the experiments on the separation of the following mixtures: 1) sugar with sawdust; 2) table salt with river sand; 3) vegetable oil with water.

Topic: Initial chemical ideas

Lesson: Practice 2. Separation of mixtures of substances

The separation of any mixture of substances is based on knowledge of the differences in the properties of its constituent components. Thus, in order to divide the proposed mixture into its constituent components, you need to know how they differ from each other.

EXPERIENCE 1. Separation of a mixture of sugar and sawdust. As you know, sugar dissolves well in water, but sawdust does not. In addition, sawdust does not sink in water, so settling can be used to separate this mixture.

First, the mixture must be placed in water (the sugar will dissolve), and then carefully remove the sawdust from the surface of the water. In order to isolate the sugar dissolved in water in its pure form, you need to evaporate the solution, leaving some water, and then cool it - as a result, sugar crystals will precipitate (evaporation and crystallization methods are used here).

Rice. 1. Separation of a mixture of sugar and sawdust

EXPERIENCE 2. Purification of table salt from the admixture of river sand. Easily soluble substances can be easily removed from mechanical impurities by filtering the solution. First, contaminated table salt must be placed in water, the salt will dissolve in water, but impurities will not. Filtration can be used to separate mechanical impurities. Note that when the contaminant is soluble, the filter will not be able to separate it. How to get dry table salt from a purified solution? To do this, you need to use the evaporation method: the water will evaporate, and crystals of purified salt will remain on the walls of the porcelain cup.

Rice. 2. Purification of common salt from the admixture of river sand (stage of evaporation of the filtrate)

Purification of table salt from river sand is carried out based on such properties of salt as good solubility in water and non-volatility (when water is evaporated from a solution).

EXPERIENCE 3. Separation of a mixture of vegetable oil and water. If the liquids are insoluble in each other and have different densities, then they can be separated using the settling method.

Pour water into a separating funnel and add a few drops of vegetable oil. Shake the funnel. It turns out a white emulsion: a mixture of oil and water. Let the mixture stand. Gradually, the emulsion separates into two layers: the heavy bottom layer is water. Upper, lighter - vegetable oil. Carefully drain the water through the faucet of the separating funnel. Vegetable oil remains in the funnel.

Rice. 3. Using the settling method to separate a mixture of vegetable oil and water

Settling can be used to separate mixtures of other slightly or insoluble liquids with different densities. For example, a mixture of gasoline and water; mixture of oil and water.

1. Collection of tasks and exercises in chemistry: 8th grade: to the textbook by P.A. Orzhekovsky and others. "Chemistry, Grade 8" / P.A. Orzhekovsky, N.A. Titov, F.F. Hegel. - M.: AST: Astrel, 2006.

2. Ushakova O.V. Chemistry workbook: 8th grade: to the textbook by P.A. Orzhekovsky and others. “Chemistry. Grade 8” / O.V. Ushakova, P.I. Bespalov, P.A. Orzhekovsky; under. ed. prof. P.A. Orzhekovsky - M .: AST: Astrel: Profizdat, 2006. (p. 10-11)

3. Chemistry: 8th grade: textbook. for general institutions / P.A. Orzhekovsky, L.M. Meshcheryakova, L.S. Pontak. M.: AST: Astrel, 2005.(§4)

4. Chemistry: inorg. chemistry: textbook. for 8 cells. general institutions / G.E. Rudzitis, FuGyu Feldman. - M.: Enlightenment, JSC "Moscow textbooks", 2009. (§ 2)

5. Encyclopedia for children. Volume 17. Chemistry / Chapter. edited by V.A. Volodin, leading. scientific ed. I. Leenson. - M.: Avanta +, 2003.

Additional web resources

1. A single collection of digital educational resources ().

2. Electronic version of the journal "Chemistry and Life" ().

Homework

Do a home experiment. Tint 50 ml of water with ink. Add 2-3 activated charcoal tablets. Mix well. Let the mixture stand or filter it. Describe your observations. What water purification method did you use?

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 chemicals. 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 milk protein casein, mineral salts, vitamins and even sugar, but not the one with which they drink tea, but a special, milky one - 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 water 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 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, food products (except salt, sugar, and some others), many medicinal and cosmetic products, household chemicals, and building materials are mixtures.

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 in it evenly mixed tiny particles these substances.

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, turbid water, milk and other foods, some drugs 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 types of heterogeneous mixtures have special names: foam (for example, foam, soap suds), 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 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.

Consider methods for separating heterogeneous and homogeneous mixtures.

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.

In our article, we will consider what pure substances and mixtures are, methods for separating mixtures. AT Everyday life each of us uses them. Do pure substances occur in nature at all? And how to distinguish them from mixtures?

Pure substances and mixtures: ways to separate mixtures

Pure substances are substances that contain particles of only a certain type. Scientists believe that they practically do not exist in nature, since all of them, albeit in negligible 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. In the process of their formation, a change in the amount of energy occurs. Moreover, it can both increase and decrease. The only way to separate a pure substance into its individual components is to chemical reaction. For example, only distilled water has a typical boiling and freezing point for this substance, the absence of taste and smell. And its oxygen and hydrogen can be decomposed only by electrolysis.

And how do they differ from pure substances in their totality? Chemistry will help us answer this question. Methods for separating mixtures are physical because they do not change chemical composition 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 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. Yes, from sea ​​water you can get pure salt 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 cannot be distinguished with the naked eye. Such mixtures are called homogeneous or homogeneous. Air, gasoline, broth, perfume, sweet and salt water, and an alloy of copper and aluminum are also examples of these. As you can see, they can be in different states of aggregation, but liquids are most common. They are also called solutions.

In heterogeneous, 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 foam, and aerosols include fog, smoke, deodorants, air fresheners, antistatic agents.

Methods for separating mixtures

Of course, many mixtures have more valuable properties than individual individual substances that make up their composition. But even in everyday life there are situations when they need to be separated. And in industry, entire industries are based on this process. For example, from oil as a result of its processing, gasoline, gas oil, kerosene, fuel oil, solar oil and machine oil, rocket fuel, acetylene and benzene are obtained. Agree, it is more profitable to use these products than mindlessly burning oil.

Now let's see if there is such a thing as chemical methods for separating mixtures. Suppose 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 at the same time, there will be no transformation of one substance into another. This means that the basis of this process are physical phenomena.

Methods for separating mixtures depend on the state of aggregation, the ability to dissolve, the difference in boiling point, the density and composition of its components. Let's consider each of them in more detail with specific examples.

Filtration

This separation method is suitable for mixtures containing 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 freely pass through it, and the sand will remain.

settling

Some methods of separating mixtures are based on the action of gravity. In this way, suspensions and emulsions can be decomposed. 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 be 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 work, a denser liquid is drained into a vessel, and a light one remains.

Settling is characterized by a low speed of the process. It takes a certain amount of time for the precipitate to form. In industrial conditions, this method is carried out in special structures called sedimentation tanks.

Magnet action

If the mixture contains metal, then it can be separated using a magnet. For example, to separate iron and But do all metals have such properties? Not at all. For this method, only mixtures containing ferromagnets are suitable. In addition to iron, these include nickel, cobalt, gadolinium, terbium, dysprosium, holmium, and erbium.

Distillation

This name is translated from Latin means "drip". Distillation is a method of separating mixtures based on the difference in boiling points of substances. Thus, even at home, alcohol and water can be separated. The first substance begins to evaporate already at a temperature of 78 degrees Celsius. Touching the cold surface, the alcohol vapor condenses, turning into a liquid state.

In industry, oil refining products, aromatic substances, and pure metals are obtained in this way.

Evaporation and crystallization

These separation methods are suitable for liquid solutions. The substances that make up their composition differ in their boiling point. Thus, it is possible to obtain crystals of salt or sugar from the water in which they are dissolved. To do this, the solutions are heated and evaporated to a saturated state. In this case, the crystals are deposited. If you need to get clean water, then the solution is brought to a boil, followed by vapor condensation on a colder surface.

Methods for separating gas mixtures

Gaseous mixtures are separated by laboratory and industrial methods, since this process requires special equipment. The raw material of natural origin is air, coke, generator, associated and natural gas, which is a combination of hydrocarbons.

The physical methods for separating mixtures in the gaseous state are as follows:

• Condensation is the process of gradual cooling of a mixture, during which the condensation of its constituents occurs. In this case, first of all, high-boiling substances, which are collected in separators, pass into the liquid state. In this way, hydrogen is obtained from and also ammonia is separated from the unreacted part of the mixture.
• Sorption is the absorption of some substances by others. This process has opposite components, between which equilibrium is established during the reaction. The forward and reverse processes require different conditions. 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 in which the property of semi-permeable partitions is used to selectively pass molecules of various substances.
• Reflux - the process of condensation of high-boiling parts of mixtures as a result of their cooling. In this case, the temperature of the transition to the liquid state of the individual components should differ significantly.

Chromatography

The name of this method can be translated as "I write with color." Imagine that ink is added to the water. If you lower the end of the filter paper into such a mixture, it will begin to be absorbed. In this case, water will be absorbed faster than ink, which is associated with a different degree 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 first are elements or compounds consisting only of particles of a certain type. Their examples are salt, sugar, distilled water. Mixtures are a collection of individual substances. A number of methods are used to separate them. The way they are separated depends on the physical properties of its constituents. The main ones are settling, evaporation, crystallization, filtration, distillation, magnetization and chromatography.

In order to establish the properties of a substance, it is necessary to have it in its pure form, but substances in nature do not occur in a pure form. 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.

Almost all natural substances, foodstuffs (except for salt, sugar, and some others), building materials, household chemicals, many medicines and cosmetics are mixtures.

Natural substances are mixtures, sometimes consisting of a very large number of different substances. For example, natural water always contains salts and gases dissolved in it. Sometimes a very small impurity content can lead to a very strong change in some properties of a substance. For example, the content in zinc of only hundredths of iron or copper accelerates its interaction with hydrochloric acid hundreds of times. When one of the substances is in the mixture in a predominant amount, the entire mixture usually bears its name.

• 
  A component is each substance contained in a mixture.

uniform mixtures.

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. But we will not see its crystals, even when examining a drop of liquid through a powerful microscope.

Rice. 3. Homogeneous mixture ( water solution Sahara)

The prepared mixture of sugar and water is homogeneous (Fig. 3); the smallest particles of these substances are evenly mixed in it.

• 
  Mixtures in which components cannot be detected with the naked eye are called homogeneous.

Water mixed with sand, chalk or clay freezes at 0 0 C and boils at 100 0 C.

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

Rice. 5. Heterogeneous mixtures:
a - a mixture of water and sulfur;
b - a mixture of vegetable oil and water;
c - a mixture of air and water

Exist different ways separation of mixtures. The choice of method for separating a mixture is influenced by the properties of the substances that form this mixture.

Let's take a closer look at each method:

• 
  settling- a common method of purifying or liquids from water-insoluble mechanical impurities, or liquid substances that are insoluble in each other, having different densities.

Settling is used in the preparation of water for technological and household needs, sewage treatment, dehydration and desalination of crude oil, in many chemical engineering processes. It is an important stage in the natural self-purification of natural and artificial reservoirs.

• 
  Filtration- separation of liquid from solid insoluble impurities in it; liquid molecules pass through the pores of the filter, and large particles of impurities are retained.

Imagine that you have a mixture of river sand and water. Determine the type of mixture. ( heterogeneous). Compare the physical properties of river sand and water. (These are substances that are insoluble in each other, having different densities). Suggest a method for separating this mixture ( filtering).

• 
  Magnet action- this is a method of separating inhomogeneous mixtures, when one of the substances in the mixture is able to be attracted by a magnet

• 
  Evaporation - this is a method of separating homogeneous mixtures, in this case, a solid soluble substance is released from a solution, when heated, water evaporates, and solid crystals remain.

• 
  Distillation (Latin meaning "dropping") – This is a method of separating homogeneous mixtures, in which case liquid mixtures are separated into fractions differing in composition. It is carried out by partial evaporation of the liquid, followed by vapor condensation. The distilled fraction (distillate) is enriched with relatively more volatile (low-boiling) substances, and the non-distilled liquid (distillation residue) is enriched with relatively less volatile (high-boiling) substances.

In the laboratory, distillation is carried out on a special installation (Fig. 6). 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. 6. Laboratory installation for distillation: a - conventional; b - simplified
1 - a mixture of liquids with different boiling points;
2 - thermometer;
3 - water cooler;
4 - receiver

Consider how some methods separation of mixtures.

The filtering process underlies the operation of a respirator, a device that protects the lungs of a person working in a heavily dusty environment. The respirator has filters that prevent dust from entering the lungs (Fig. 7). The simplest respirator is a bandage made of several layers of gauze. A filter that extracts dust from the air is also in the vacuum cleaner.

Rice. 7. Worker in a respirator

Draw a conclusion by what methods it is possible to separate a mixture of soluble and insoluble substances in water.