Welcome dear students! Today we are going to learn about Is Matter Around Us Pure? from Class 9 Science. How do we judge whether milk, ghee, butter, salt, spices, mineral water or juice that we buy from the market are pure? You may have noticed the word pure written on the packs of these consumables. For a common person, pure means having no adulteration. But, for a scientist, all these things are actually mixtures of different substances and hence not pure. For example, milk is actually a mixture of water, fat, proteins, and so on. When a scientist says that something is pure, it means that all the constituent particles of that substance are the same in their chemical nature. A pure substance consists of a single type of particle. In other words, a substance is a pure single form of matter. As we look around, we can see that most of the matter around us exists as mixtures of two or more pure components, for example, sea water, minerals, soil, and so on, are all mixtures.
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Let us move to section two point one, What is a Mixture. Mixtures are constituted by more than one kind of pure form of matter. We know that dissolved sodium chloride can be separated from water by the physical process of evaporation. However, sodium chloride is itself a pure substance and cannot be separated by physical process into its chemical constituents. Similarly, sugar is a substance which contains only one kind of pure matter and its composition is the same throughout. Soft drink and soil are not single pure substances. Whatever the source of a pure substance may be, it will always have the same characteristic properties. Therefore, we can say that a mixture contains more than one pure substance. Depending upon the nature of the components that form a mixture, we can have different types of mixtures. Let us perform Activity two point one. Divide the class into groups A, B, C and D. Group A takes a beaker containing fifty millilitres of water and one spatula full of copper sulphate powder. Group B takes fifty millilitres of water and two spatula full of copper sulphate powder in a beaker. Groups C and D can take different amounts of copper sulphate and potassium permanganate or common salt, which is sodium chloride, and mix the given components to form a mixture. Report the observations on the uniformity in colour and texture.
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Groups A and B have obtained a mixture which has a uniform composition throughout. Such mixtures are called homogeneous mixtures or solutions. Some other examples of such mixtures are salt dissolved in water and sugar dissolved in water. Compare the colour of the solutions of the two groups. Though both the groups have obtained copper sulphate solution, the intensity of colour of the solutions is different. This shows that a homogeneous mixture can have a variable composition. Groups C and D have obtained mixtures, which contain physically distinct parts and have non-uniform compositions. Such mixtures are called heterogeneous mixtures. Mixtures of sodium chloride and iron filings, salt and sulphur, and oil and water are examples of heterogeneous mixtures. Now let us perform Activity two point two. Divide the class into four groups, A, B, C and D. Distribute the following samples to each group: a few crystals of copper sulphate to group A, one spatula full of copper sulphate to group B, chalk powder or wheat flour to group C, and a few drops of milk or ink to group D. Each group should add the given sample in water and stir properly using a glass rod. Are the particles in the mixture visible? Direct a beam of light from a torch through the beaker containing the mixture and observe from the front. Was the path of the beam of light visible?
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Leave the mixtures undisturbed for a few minutes, and set up the filtration apparatus in the meantime. Is the mixture stable or do the particles begin to settle after some time? Filter the mixture. Is there any residue on the filter paper? Discuss the results and form an opinion. In the filtration setup, we use a funnel fitted with a folded filter paper. The liquid passes through as filtrate, and the solid remains as residue. Groups A and B have got a solution. Group C has got a suspension. Group D has got a colloidal solution. Now we shall learn about solutions, suspensions and colloidal solutions in the following sections. Let us answer the questions following this activity. First, what is meant by a substance? A substance is a pure single form of matter that consists of a single type of particle and has the same characteristic properties throughout. Second, list the points of differences between homogeneous and heterogeneous mixtures. Homogeneous mixtures have a uniform composition throughout, while heterogeneous mixtures have physically distinct parts and non-uniform compositions. Homogeneous mixtures are also called solutions, whereas heterogeneous mixtures are not.
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Moving to section two point two, What is a Solution. A solution is a homogeneous mixture of two or more substances. You come across various types of solutions in your daily life. Lemonade, soda water, and so on, are all examples of solutions. Usually we think of a solution as a liquid that contains either a solid, liquid or a gas dissolved in it. But, we can also have solid solutions, which are alloys, and gaseous solutions like air. In a solution there is homogeneity at the particle level. For example, lemonade tastes the same throughout. This shows that particles of sugar or salt are evenly distributed in the solution. Let us look at an important note on alloys. Alloys are mixtures of two or more metals or a metal and a non-metal and cannot be separated into their components by physical methods. But still, an alloy is considered as a mixture because it shows the properties of its constituents and can have variable composition. For example, brass is a mixture of approximately thirty percent zinc and seventy percent copper. A solution has a solvent and a solute as its components. The component of the solution that dissolves the other component in it, usually the component present in larger amount, is called the solvent. The component of the solution that is dissolved in the solvent, usually present in lesser quantity, is called the solute.
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Let us look at some examples. First, a solution of sugar in water is a solid in liquid solution. In this solution, sugar is the solute and water is the solvent. Second, a solution of iodine in alcohol known as tincture of iodine, has iodine as the solute and alcohol as the solvent. Third, aerated drinks like soda water are gas in liquid solutions. These contain carbon dioxide as solute and water as solvent. Fourth, air is a mixture of gas in gas. Air is a homogeneous mixture of a number of gases. Its two main constituents are oxygen at twenty one percent and nitrogen at seventy eight percent. The other gases are present in very small quantities. Now, let us list the properties of a solution. A solution is a homogeneous mixture. The particles of a solution are smaller than one nanometre, which is ten to the power of minus nine metres, in diameter. So, they cannot be seen by naked eyes. Because of very small particle size, they do not scatter a beam of light passing through the solution. So, the path of light is not visible in a solution. The solute particles cannot be separated from the mixture by the process of filtration. The solute particles do not settle down when left undisturbed, that is, a solution is stable.
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Next is section two point two point one, Concentration of a Solution. In activity two point two, we observed that groups A and B obtained different shades of solutions. So, we understand that in a solution the relative proportion of the solute and solvent can be varied. Depending upon the amount of solute present in a solution, it can be called dilute, concentrated or saturated solution. Dilute and concentrated are comparative terms. In activity two point two, the solution obtained by group A is dilute as compared to that obtained by group B. Let us perform Activity two point three. Take approximately fifty millilitres of water each in two separate beakers. Add salt in one beaker and sugar or barium chloride in the second beaker with continuous stirring. When no more solute can be dissolved, heat the contents of the beaker to raise the temperature by about five degrees Celsius. Start adding the solute again. Is the amount of salt and sugar or barium chloride, that can be dissolved in water at a given temperature, the same? At any particular temperature, a solution that has dissolved as much solute as it is capable of dissolving, is said to be a saturated solution. In other words, when no more solute can be dissolved in a solution at a given temperature, it is called a saturated solution. The amount of the solute present in the saturated solution at this temperature is called its solubility. If the amount of solute contained in a solution is less than the saturation level, it is called an unsaturated solution.
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What would happen if you were to take a saturated solution at a certain temperature and cool it slowly. We can infer from the above activity that different substances in a given solvent have different solubilities at the same temperature. The concentration of a solution is the amount, either mass or volume, of solute present in a given amount, either mass or volume, of solution. There are various ways of expressing the concentration of a solution, but here we will learn only three methods. First, mass by mass percentage of a solution equals mass of solute divided by mass of solution, multiplied by one hundred. Second, mass by volume percentage of a solution equals mass of solute divided by volume of solution, multiplied by one hundred. Third, volume by volume percentage of a solution equals volume of solute divided by volume of solution, multiplied by one hundred. Let us solve Example two point one. A solution contains forty grams of common salt in three hundred twenty grams of water. Calculate the concentration in terms of mass by mass percentage of the solution. Solution: Mass of solute, which is salt, equals forty grams. Mass of solvent, which is water, equals three hundred twenty grams. We know, mass of solution equals mass of solute plus mass of solvent, which is forty grams plus three hundred twenty grams, equaling three hundred sixty grams. Mass percentage of solution equals mass of solute divided by mass of solution, multiplied by one hundred. This is forty divided by three hundred sixty, multiplied by one hundred, which equals eleven point one percent.
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Now, section two point two point two, What is a Suspension. Non-homogeneous systems, like those obtained by group C in activity two point two, in which solids are dispersed in liquids, are called suspensions. A suspension is a heterogeneous mixture in which the solute particles do not dissolve but remain suspended throughout the bulk of the medium. Particles of a suspension are visible to the naked eye. Let us list the properties of a suspension. Suspension is a heterogeneous mixture. The particles of a suspension can be seen by the naked eye. The particles of a suspension scatter a beam of light passing through it and make its path visible. The solute particles settle down when a suspension is left undisturbed, that is, a suspension is unstable. They can be separated from the mixture by the process of filtration. When the particles settle down, the suspension breaks and it does not scatter light any more. Next, section two point two point three, What is a Colloidal Solution. The mixture obtained by group D in activity two point two is called a colloid or a colloidal solution. The particles of a colloid are uniformly spread throughout the solution. Due to the relatively smaller size of particles, as compared to that of a suspension, the mixture appears to be homogeneous. But actually, a colloidal solution is a heterogeneous mixture, for example, milk. Because of the small size of colloidal particles, we cannot see them with naked eyes. But, these particles can easily scatter a beam of visible light as observed in activity two point two. This scattering of a beam of light is called the Tyndall effect after the name of the scientist who discovered this effect.
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Tyndall effect can also be observed when a fine beam of light enters a room through a small hole. This happens due to the scattering of light by the particles of dust and smoke in the air. In Figure two point three, we see two test tubes. Part a shows a solution of copper sulphate which does not show the Tyndall effect, meaning the light beam is not visible. Part b shows a mixture of water and milk which shows the Tyndall effect, meaning the light beam is clearly visible as it scatters. Tyndall effect can be observed when sunlight passes through the canopy of a dense forest. In the forest, mist contains tiny droplets of water, which act as particles of colloid dispersed in air. Figure two point four illustrates this phenomenon, showing light rays scattering through mist in a forest. Let us list the properties of a colloid. A colloid is a heterogeneous mixture. The size of particles of a colloid is too small to be individually seen with naked eyes. Colloids are big enough to scatter a beam of light passing through it and make its path visible. They do not settle down when left undisturbed, that is, a colloid is quite stable. They cannot be separated from the mixture by the process of filtration. But, a special technique of separation known as centrifugation, which you will perform in activity two point five, can be used to separate the colloidal particles. The components of a colloidal solution are the dispersed phase and the dispersion medium. The solute-like component or the dispersed particles in a colloid form the dispersed phase, and the component in which the dispersed phase is suspended is known as the dispersing medium.
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Colloids are classified according to the state, which can be solid, liquid or gas, of the dispersing medium and the dispersed phase. Let us go through the common examples from the table. When the dispersed phase is liquid and the dispersing medium is gas, it is an aerosol, with examples being fog, clouds, and mist. When the dispersed phase is solid and the dispersing medium is gas, it is an aerosol, with examples being smoke and automobile exhaust. When the dispersed phase is gas and the dispersing medium is liquid, it is a foam, with the example being shaving cream. When the dispersed phase is liquid and the dispersing medium is liquid, it is an emulsion, with examples being milk and face cream. When the dispersed phase is solid and the dispersing medium is liquid, it is a sol, with examples being milk of magnesia and mud. When the dispersed phase is gas and the dispersing medium is solid, it is a foam, with examples being foam rubber, sponge, and pumice. When the dispersed phase is liquid and the dispersing medium is solid, it is a gel, with examples being jelly, cheese, and butter. When the dispersed phase is solid and the dispersing medium is solid, it is a solid sol, with examples being coloured gemstone and milky glass. Let us answer the questions. First, differentiate between homogeneous and heterogeneous mixtures with examples. Homogeneous mixtures have uniform composition throughout and no visible boundaries, like salt in water or sugar in water. Heterogeneous mixtures have non-uniform composition with visible boundaries, like sand and salt or oil and water.
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Second, how are sol, solution and suspension different from each other. A solution has particles smaller than one nanometre, is homogeneous, does not scatter light, and is stable. A suspension has particles visible to the naked eye, is heterogeneous, scatters light, is unstable, and settles down. A sol, which is a type of colloid, has particles between one and one thousand nanometres, appears homogeneous but is actually heterogeneous, scatters light showing the Tyndall effect, and is stable. Third, to make a saturated solution, thirty six grams of sodium chloride is dissolved in one hundred grams of water at two hundred ninety three kelvin. Find its concentration at this temperature. Mass of solute is thirty six grams. Mass of solvent is one hundred grams. Mass of solution is thirty six plus one hundred, which equals one hundred thirty six grams. Concentration in mass by mass percentage equals thirty six divided by one hundred thirty six, multiplied by one hundred, which equals approximately twenty six point four seven percent. Now, section two point three, Physical and Chemical Changes. The properties that can be observed and specified like colour, hardness, rigidity, fluidity, density, melting point, boiling point, and so on, are the physical properties. The interconversion of states is a physical change because these changes occur without a change in composition and no change in the chemical nature of the substance. Although ice, water and water vapour all look different and display different physical properties, they are chemically the same. Both water and cooking oil are liquid but their chemical characteristics are different. They differ in odour and inflammability. We know that oil burns in air whereas water extinguishes fire. It is this chemical property of oil that makes it different from water. Burning is a chemical change. During this process one substance reacts with another to undergo a change in chemical composition. Chemical change brings change in the chemical properties of matter and we get new substances. A chemical change is also called a chemical reaction. During burning of a candle, both physical and chemical changes take place. Can you distinguish these?
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Let us answer the questions. First, classify the following as chemical or physical changes. Cutting of trees is a physical change. Melting of butter in a pan is a physical change. Rusting of almirah is a chemical change. Boiling of water to form steam is a physical change. Passing of electric current through water and the water breaking down into hydrogen and oxygen gases is a chemical change. Dissolving common salt in water is a physical change. Making a fruit salad with raw fruits is a physical change. Burning of paper and wood is a chemical change. Second, try segregating the things around you as pure substances or mixtures. Pure substances include distilled water, gold, silver, iron, sodium chloride, sugar, and oxygen. Mixtures include milk, air, soil, sea water, brass, and ink. Moving to section two point four, What are the Types of Pure Substances. On the basis of their chemical composition, substances can be classified either as elements or compounds. Section two point four point one covers Elements. Robert Boyle was the first scientist to use the term element in sixteen sixty one. Antoine Laurent Lavoisier, a French chemist who lived from seventeen forty three to seventeen ninety four, was the first to establish an experimentally useful definition of an element. He defined an element as a basic form of matter that cannot be broken down into simpler substances by chemical reactions. Elements can be normally divided into metals, non-metals and metalloids.
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Metals usually show some or all of the following properties. They have a lustre, which means shine. They have silvery grey or golden yellow colour. They conduct heat and electricity. They are ductile, meaning they can be drawn into wires. They are malleable, meaning they can be hammered into thin sheets. They are sonorous, meaning they make a ringing sound when hit. Examples of metals are gold, silver, copper, iron, sodium, potassium, and so on. Mercury is the only metal that is liquid at room temperature. Non-metals usually show some or all of the following properties. They display a variety of colours. They are poor conductors of heat and electricity. They are not lustrous, sonorous or malleable. Examples of non-metals are hydrogen, oxygen, iodine, carbon in forms like coal and coke, bromine, chlorine, and so on. Some elements have intermediate properties between those of metals and non-metals, they are called metalloids. Examples are boron, silicon, germanium, and so on. Let us look at the More to know section. The number of elements known at present are more than one hundred. Ninety two elements are naturally occurring and the rest are man made. Majority of the elements are solid. Eleven elements are in gaseous state at room temperature. Two elements are liquid at room temperature, which are mercury and bromine. Elements gallium and cesium become liquid at a temperature slightly above room temperature, which is three hundred three kelvin.
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Now, section two point four point two, Compounds. A compound is a substance composed of two or more elements, chemically combined with one another in a fixed proportion. What do we get when two or more elements are combined? Let us perform Activity two point four. Divide the class into two groups. Give five grams of iron filings and three grams of sulphur powder in a china dish to both the groups. Group one should mix and crush iron filings and sulphur powder. Group two should mix and crush iron filings and sulphur powder, then heat this mixture strongly till red hot, remove from flame and let the mixture cool. Both groups should then check for magnetism in the material obtained. Bring a magnet near the material and check if the material is attracted towards the magnet. Compare the texture and colour of the material obtained by the groups. Add carbon disulphide to one part of the material obtained. Stir well and filter. Add dilute sulphuric acid or dilute hydrochloric acid to the other part of the material obtained. Note that teacher supervision is necessary for this activity. Perform all the above steps with both the elements, iron and sulphur, separately. Now answer the questions. Did the material obtained by the two groups look the same? No, they looked different. Which group has obtained a material with magnetic properties? Group one, because the iron filings were not chemically changed. Can we separate the components of the material obtained? Yes, from group one using a magnet, but not from group two. On adding dilute sulphuric acid or dilute hydrochloric acid, did both the groups obtain a gas? Did the gas in both the cases smell the same or different? Yes, both obtained a gas, but they smelled different.
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The gas obtained by Group one is hydrogen, it is colourless, odourless and combustible. It is not advised to do the combustion test for hydrogen in the class. The gas obtained by Group two is hydrogen sulphide. It is a colourless gas with the smell of rotten eggs. You must have observed that the products obtained by both the groups show different properties, though the starting materials were the same. Group one has carried out the activity involving a physical change whereas in case of Group two, a chemical change, which is a chemical reaction, has taken place. The material obtained by group one is a mixture of the two substances. The substances given are the elements, iron and sulphur. The properties of the mixture are the same as that of its constituents. The material obtained by group two is a compound. On heating the two elements strongly we get a compound, which has totally different properties compared to the combining elements. The composition of a compound is the same throughout. We can also observe that the texture and the colour of the compound are the same throughout. Let us look at the differences between mixtures and compounds from the table. First, in mixtures, elements or compounds just mix together to form a mixture and no new compound is formed. In compounds, elements react to form new compounds. Second, a mixture has a variable composition. The composition of each new substance is always fixed. Third, a mixture shows the properties of the constituent substances. The new substance has totally different properties. Fourth, the constituents can be separated fairly easily by physical methods. The constituents can be separated only by chemical or electrochemical reactions.
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We can summarise the physical and chemical nature of matter. Matter can be solid, liquid or gas. It is divided into pure substances and mixtures. Mixtures have no fixed composition and are divided into homogeneous and heterogeneous. Homogeneous mixtures have uniform composition, like sugar in water. Heterogeneous mixtures have non-uniform composition, like sand and salt. Pure substances are divided into elements and compounds. Elements cannot be broken down to simpler substances, for example copper, oxygen, iron, hydrogen, and mercury. Compounds have fixed composition and can be broken down into elements by chemical or electrochemical reactions, for example water, methane, sugar, and salt. Now let us review what we have learnt. A mixture contains more than one substance, which can be elements or compounds, mixed in any proportion. Mixtures can be separated into pure substances using appropriate separation techniques. A solution is a homogeneous mixture of two or more substances. The major component of a solution is called the solvent, and the minor, the solute. The concentration of a solution is the amount of solute present per unit volume or per unit mass of the solution. Materials that are insoluble in a solvent and have particles that are visible to naked eyes, form a suspension. A suspension is a heterogeneous mixture. Colloids are heterogeneous mixtures in which the particle size is too small to be seen with the naked eye, but is big enough to scatter light. Colloids are useful in industry and daily life. The particles are called the dispersed phase and the medium in which they are distributed is called the dispersion medium. Pure substances can be elements or compounds. An element is a form of matter that cannot be broken down by chemical reactions into simpler substances. A compound is a substance composed of two or more different types of elements, chemically combined in a fixed proportion. Properties of a compound are different from its constituent elements, whereas a mixture shows the properties of its constituting elements or compounds.
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Now, let us solve the exercises completely. Exercise one: Write the steps you would use for making tea. Use the words solution, solvent, solute, dissolve, soluble, insoluble, filtrate and residue. First, take a cup of water, which acts as the solvent, and heat it in a pan. Add sugar, which is a soluble solute, and tea leaves, which are insoluble. The sugar will dissolve in the hot water to form a solution. Add milk, which mixes with the water. Boil the mixture for a few minutes. Then, filter the mixture using a strainer. The tea leaves that remain on the strainer are the residue. The liquid that passes through is the filtrate, which is your prepared tea. Exercise two: Pragya tested the solubility of three different substances at different temperatures. Part a: What mass of potassium nitrate would be needed to produce a saturated solution of potassium nitrate in fifty grams of water at three hundred thirteen kelvin? From the table, at three hundred thirteen kelvin, the solubility of potassium nitrate is sixty two grams per one hundred grams of water. For fifty grams of water, we need half of that, which is thirty one grams. Part b: Pragya makes a saturated solution of potassium chloride in water at three hundred fifty three kelvin and leaves the solution to cool at room temperature. What would she observe as the solution cools? Explain. As the solution cools, the solubility of potassium chloride decreases. Therefore, some of the dissolved potassium chloride will crystallize out of the solution and settle at the bottom. Part c: Find the solubility of each salt at two hundred ninety three kelvin. Which salt has the highest solubility at this temperature? At two hundred ninety three kelvin, potassium nitrate is thirty two grams, sodium chloride is thirty six grams, potassium chloride is thirty five grams, and ammonium chloride is thirty seven grams. Ammonium chloride has the highest solubility at this temperature. Part d: What is the effect of change of temperature on the solubility of a salt? Generally, the solubility of salts increases with an increase in temperature.
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Exercise three: Explain the following giving examples. Part a: Saturated solution. A saturated solution is a solution in which no more solute can be dissolved at a given temperature. For example, if you keep adding sugar to water at room temperature and it stops dissolving, it becomes saturated. Part b: Pure substance. A pure substance consists of only one type of particle and has a fixed chemical composition. Examples include distilled water, gold, and sodium chloride. Part c: Colloid. A colloid is a heterogeneous mixture where particles are too small to be seen with the naked eye but large enough to scatter light. Examples include milk, fog, and smoke. Part d: Suspension. A suspension is a heterogeneous mixture where solute particles do not dissolve but remain suspended and are visible to the naked eye. Examples include chalk powder in water and muddy water. Exercise four: Classify each of the following as a homogeneous or heterogeneous mixture. Soda water is homogeneous. Wood is heterogeneous. Air is homogeneous. Soil is heterogeneous. Vinegar is homogeneous. Filtered tea is homogeneous. Exercise five: How would you confirm that a colourless liquid given to you is pure water? We can confirm it by measuring its boiling point. Pure water boils exactly at one hundred degrees Celsius at standard atmospheric pressure. If it boils at a different temperature or leaves a residue upon evaporation, it is not pure water.
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Exercise six: Which of the following materials fall in the category of a pure substance? Ice is pure water, so it is a pure substance. Milk is a mixture. Iron is an element, so it is a pure substance. Hydrochloric acid is a compound, so it is a pure substance. Calcium oxide is a compound, so it is a pure substance. Mercury is an element, so it is a pure substance. Brick is a mixture. Wood is a mixture. Air is a mixture. So the pure substances are ice, iron, hydrochloric acid, calcium oxide, and mercury. Exercise seven: Identify the solutions among the following mixtures. Soil is not a solution. Sea water is a solution. Air is a solution. Coal is not a solution. Soda water is a solution. So sea water, air, and soda water are solutions. Exercise eight: Which of the following will show Tyndall effect? Salt solution will not. Milk will. Copper sulphate solution will not. Starch solution will. So milk and starch solution will show the Tyndall effect. Exercise nine: Classify the following into elements, compounds and mixtures. Sodium is an element. Soil is a mixture. Sugar solution is a mixture. Silver is an element. Calcium carbonate is a compound. Tin is an element. Silicon is an element. Coal is a mixture. Air is a mixture. Soap is a mixture. Methane is a compound. Carbon dioxide is a compound. Blood is a mixture. Exercise ten: Which of the following are chemical changes? Growth of a plant is a chemical change. Rusting of iron is a chemical change. Mixing of iron filings and sand is a physical change. Cooking of food is a chemical change. Digestion of food is a chemical change. Freezing of water is a physical change. Burning of a candle is a chemical change.
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Finally, the Group Activity. Take an earthen pot, some pebbles and sand. Design a small scale filtration plant that you could use to clean muddy water. To do this, you can invert the earthen pot. Place a layer of cloth or cotton at the narrow bottom opening. Then, add a thick layer of fine sand. Above the sand, add a layer of coarse pebbles or gravel. Pour the muddy water from the top. The pebbles will trap large debris, the sand will filter out finer suspended particles, and the cloth will prevent the sand from escaping. The clean water will collect at the bottom. This mimics natural groundwater filtration. Thank you for listening! Keep revising and practicing. Goodbye! [CHAPTER_COMPLETE]