Matter Around Us: Pure Substances PDF

C hapter 2 IS MATTER AROUND US PURE How do we judge whether milk, ghee, butter, physical process of evaporation. However, d salt, spices, mineral water or juice that we...

C hapter 2 IS MATTER AROUND US PURE How do we judge whether milk, ghee, butter, physical process of evaporation. However, d salt, spices, mineral water or juice that we sodium chloride is itself a substance and buy from the market are pure? cannot be separated by physical process into he its chemical constituents. Similarly, sugar is a substance because it contains only one kind of pure matter and its composition is the same throughout. Soft drink and soil are not single is substances. Whatever the source of a substance may be, it will always have the same characteristic properties. bl Therefore, we can say that a mixture contains more than one substance. Fig. 2.1: Some consumable items pu Have you ever noticed the word ‘pure’ 2.1.1 TYPES OF MIXTURES written on the packs of these consumables? Depending upon the nature of the be T For a common person pure means having no components that form a mixture, we can have adulteration. But, for a scientist all these things different types of mixtures. re are actually mixtures of different substances o R and hence not pure. For example, milk is Activity ______________ 2.1 actually a mixture of water, fat, proteins etc. When a scientist says that something is pure, Let us divide the class into groups A, tt E it means that all the constituent particles of B, C and D. that substance are the same in their chemical Group A takes a beaker containing nature. A pure substance consists of a single 50 mL of water and one spatula full of C type of particles. In other words, a substance copper sulphate powder. Group B takes 50 mL of water and two spatula full of is a pure single form of matter. copper sulphate powder in a beaker. no N As we look around, we can see that most Groups C and D can take different of the matter around us exist as mixtures of amounts of copper sulphate and two or more pure components, for example, potassium permanganate or common sea water, minerals, soil etc. are all mixtures. salt (sodium chloride) and mix the given © components to form a mixture. 2.1 What is a Mixture? Report the observations on the uniformity in colour and texture. Mixtures are constituted by more than one Groups A and B have obtained a kind of pure form of matter, known as a mixture which has a uniform substance. A substance cannot be separated composition throughout. Such into other kinds of matter by any physical mixtures are called homogeneous process. We know that dissolved sodium mixtures or solutions. Some other chloride can be separated from water by the examples of such mixtures are: (i) salt  in water and (ii) sugar in water. Compare the colour of the solutions of the two groups. Though both the groups have obtained copper sulphate solution but 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 d Fig. 2.2: Filtration heterogeneous mixtures. Mixtures of sodium chloride and iron filings, salt Now, we shall learn about solutions, he and sulphur, and oil and water are suspensions and colloidal solutions in the examples of heterogeneous mixtures. following sections. Q Activity ______________ 2.2 uestions is Let us again divide the class into four 1. What is meant by a substance? groups – A, B, C and D. 2. List the points of differences Distribute the following samples to between homogeneous and bl each group: heterogeneous mixtures. − Few crystals of copper sulphate to group A. pu − One spatula full of copper sulphate to group B. 2.2 What is a Solution? − Chalk powder or wheat flour to be T group C. A solution is a homogeneous mixture of two − Few drops of milk or ink to or more substances. You come across various re types of solutions in your daily life. Lemonade, o R group D. Each group should add the given soda water etc. are all examples of solutions. sample in water and stir properly using Usually we think of a solution as a liquid that contains either a solid, liquid or a gas tt E a glass rod. Are the particles in the mixture visible? dissolved in it. But, we can also have solid Direct a beam of light from a torch solutions (alloys) and gaseous solutions (air). In a solution there is homogeneity at the C through the beaker containing the mixture and observe from the front. particle level. For example, lemonade tastes the Was the path of the beam of light same throughout. This shows that particles of sugar or salt are evenly distributed in the no N visible? Leave the mixtures undisturbed for a solution. few minutes (and set up the filtration apparatus in the meantime). Is the Alloys: Alloys are mixtures of two or mixture stable or do the particles begin more metals or a metal and a non-metal © More to know to settle after some time? and cannot be separated into their Filter the mixture. Is there any residue components by physical methods. But on the filter paper? still, an alloy is considered as a mixture Discuss the results and form an because it shows the properties of its opinion. constituents and can have variable Groups A and B have got a solution. composition. For example, brass is a Group C has got a suspension. mixture of approximately 30% zinc and Group D has got a colloidal solution. 70% copper. I S MATTER A ROUND U S P URE 15  A solution has a solvent and a solute as proportion of the solute and solvent can be its components. The component of the solution varied. Depending upon the amount of solute that dissolves the other component in it present in a solution, it can be called a dilute, (usually the component present in larger concentrated or a saturated solution. Dilute amount) is called the solvent. The component and concentrated are comparative terms. In of the solution that is dissolved in the solvent activity 2.2, the solution obtained by group (usually present in lesser quantity) is called A is dilute as compared to that obtained by the solute. group B. Examples: Activity ______________ 2.3 d (i) A solution of sugar in water is a solid Take approximately 50 mL of water in liquid solution. In this solution, each in two separate beakers. sugar is the solute and water is the he Add salt in one beaker and sugar or solvent. barium chloride in the second beaker (ii) A solution of iodine in alcohol known with continuous stirring. as ‘tincture of iodine’, has iodine (solid) When no more solute can be dissolved, as the solute and alcohol (liquid) as heat the contents of the beaker to is the solvent. raise the temperature by about 5°C. (iii) Aerated drinks like soda water etc., are Start adding the solute again. gas in liquid solutions. These contain Is the amount of salt and sugar or barium bl carbon dioxide (gas) as solute and chloride, that can be dissolved in water at a water (liquid) as solvent. given temperature, the same? (iv) Air is a mixture of gas in gas. Air is a At any particular temperature, a solution pu homogeneous mixture of a number of that has dissolved as much solute as it is gases. Its two main constituents are: capable of dissolving, is said to be a saturated oxygen (21%) and nitrogen (78%). The solution. In other words, when no more solute be T other gases are present in very small can be dissolved in a solution at a given quantities. re temperature, it is called a saturated solution. o R The amount of the solute present in the Properties of a solution saturated solution at this temperature is A solution is a homogeneous mixture. called its solubility. tt E The particles of a solution are smaller If the amount of solute contained in a than 1 nm (10-9 metre) in diameter. So, solution is less than the saturation level, it is they cannot be seen by naked eyes. C called an unsaturated solution. Because of very small particle size, they What would happen if you were to take a do not scatter a beam of light passing saturated solution at a certain temperature through the solution. So, the path of no N and cool it slowly. light is not visible in a solution. We can infer from the above activity that The solute particles cannot be different substances in a given solvent have separated from the mixture by the different solubilities at the same temperature. process of filtration. The solute particles © The concentration of a solution is the amount do not settle down when left undisturbed, of solute present in a given amount (mass or that is, a solution is stable. volume) of solution, or the amount of solute dissolved in a given mass or volume of solvent. 2.2.1 CONCENTRATION OF A SOLUTION Concentration of solution = Amount of solute/ In activity 2.2, we observed that groups A and Amount of solution B obtained different shades of solutions. So, Or we understand that in a solution the relative Amount of solute/Amount of solvent 16 SCIENCE  There are various ways of expressing the The particles of a suspension can be concentration of a solution, but here we will seen by the naked eye. learn only two methods. The particles of a suspension scatter a (i) Mass by mass percentage of a solution beam of light passing through it and Mass of solute make its path visible. = ×100 Mass of solution The solute particles settle down when (ii) Mass by volume percentage of a solution a suspension is left undisturbed, that is, a suspension is unstable. They can Mass of solute be separated from the mixture by the = ×100 Volume of solution process of filtration. When the particles d settle down, the suspension breaks Example 2.1 A solution contains 40 g of and it does not scatter light any more. he common salt in 320 g of water. Calculate the concentration in terms of mass by mass percentage of the 2.2.3 WHAT IS A COLLOIDAL SOLUTION? solution. The mixture obtained by group D in activity 2.2 Solution: is called a colloid or a colloidal solution. The is particles of a colloid are uniformly spread Mass of solute (salt) = 40 g throughout the solution. Due to the relatively Mass of solvent (water) = 320 g bl We know, smaller size of particles, as compared to that of Mass of solution = Mass of solute + a suspension, the mixture appears to be Mass of solvent homogeneous. But actually, a colloidal solution pu = 40 g + 320 g is a heterogeneous mixture, for example, milk. = 360 g Because of the small size of colloidal particles, we cannot see them with naked be T Mass percentage of solution eyes. But, these particles can easily scatter a re Mass of solute beam of visible light as observed in activity o R = ×100 2.2. This scattering of a beam of light is called Massof solution the Tyndall effect after the name of the scientist who discovered this effect. tt E 40 = × 100 =1 1.1% Tyndall effect can also be observed when a 36 0 fine beam of light enters a room through a small C hole. This happens due to the scattering of light 2.2.2 What is a suspension? by the particles of dust and smoke in the air. Non-homogeneous systems, like those no N obtained by group C in activity 2.2, 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. (a) (b) Properties of a Suspension Fig. 2.3: (a) Solution of copper sulphate does not show Suspension is a heterogeneous Tyndall effect, (b) mixture of water and milk mixture. shows Tyndall effect. I S MATTER A ROUND U S P URE 17  Tyndall effect can be observed when They cannot be separated from the sunlight passes through the canopy of a dense mixture by the process of filtration. But, forest. In the forest, mist contains tiny droplets a special technique of separation known of water, which act as particles of colloid as centrifugation (perform activity 2.5), dispersed in air. 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 d dispersed phase, and the component in which the dispersed phase is suspended is known he as the dispersing medium. Colloids are classified according to the state (solid, liquid or gas) of the dispersing medium and the dispersed phase. A few common examples are is given in Table 2.1. From this table you can see that they are very common everyday life. Q Fig. 2.4: The Tyndall effect bl uestions Properties of a colloid 1. Differentiate between homogen- pu A colloid is a heterogeneous mixture. eous and heterogeneous mixtures The size of particles of a colloid is too with examples. small to be individually seen by naked 2. How are sol, solution and be T eyes. suspension different from each other? Colloids are big enough to scatter a re 3. To make a saturated solution, o R beam of light passing through it and 36 g of sodium chloride is dissolved make its path visible. in 100 g of water at 293 K. They do not settle down when left Find its concentration at this tt E undisturbed, that is, a colloid is quite temperature. stable. C Table 2.1: Common examples of colloids Dispersed Dispersing Type Example no N phase Medium Liquid Gas Aerosol Fog, clouds, mist Solid Gas Aerosol Smoke, automobile exhaust © Gas Liquid Foam Shaving cream Liquid Liquid Emulsion Milk, face cream Solid Liquid Sol Milk of magnesia, mud Gas Solid Foam Foam, rubber, sponge, pumice Liquid Solid Gel Jelly, cheese, butter Solid Solid Solid Sol Coloured gemstone, milky glass 18 SCIENCE  2.3 Separating the Components Now answer of a Mixture What do you think has got evaporated We have learnt that most of the natural from the watch glass? substances are not chemically pure. Different Is there a residue on the watch glass? methods of separation are used to get What is your interpretation? Is ink a individual components from a mixture. single substance (pure) or is it a Separation makes it possible to study and use the individual components of a mixture. mixture? Heterogeneous mixtures can be separated into their respective constituents by simple We find that ink is a mixture of a dye in d physical methods like handpicking, sieving, water. Thus, we can separate the volatile filtration that we use in our day-to-day life. component (solvent) from its non-volatile he Sometimes special techniques have to be used solute by the method of evaporation. for the separation of the components of a mixture. 2.3.2 H O W CAN W E SEPARATE CREAM FROM MILK? 2.3.1 H OW is CAN WE OBTAIN COLOURED COMPONENT ( DYE ) FROM BLUE/ Now-a-days, we get full-cream, toned and double-toned varieties of milk packed in poly- BLACK INK? bl packs or tetra packs in the market. These varieties of milk contain different amounts Activity ______________ 2.4 of fat. pu Fill half a beaker with water. Put a watch glass on the mouth of the Activity ______________ 2.5 beaker (Fig. 2.5). be T Put few drops of ink on the watch glass. Take some full-cream milk in a test Now start heating the beaker. We do tube. re Centrifuge it by using a centrifuging o R not want to heat the ink directly. You will see that evaporation is taking place machine for two minutes. If a from the watch glass. centrifuging machine is not available Continue heating as the evaporation in the school, you can do this activity tt E goes on and stop heating when you do at home by using a milk churner, used not see any further change on the in the kitchen. watch glass. If you have a milk dairy nearby, visit it C Observe carefully and record your and ask (i) how they separate cream observations. from milk and (ii) how they make cheese (paneer) from milk. no N Now answer What do you observe on churning the © milk? Explain how the separation of cream from milk takes place. Sometimes the solid particles in a liquid are very small and pass through a filter paper. For such particles the filtration technique Fig. 2.5: Evaporation cannot be used for separation. Such mixtures I S MATTER A ROUND U S P URE 19  are separated by centrifugation. The principle Applications is that the denser particles are forced to the To separate mixture of oil and water. bottom and the lighter particles stay at the In the extraction of iron from its ore, top when spun rapidly. the lighter slag is removed from the top by this method to leave the molten Applications iron at the bottom in the furnace. Used in diagnostic laboratories for blood and urine tests. The principle is that immiscible liquids Used in dairies and home to separate separate out in layers depending on their butter from cream. densities. d Used in washing machines to squeeze out water from wet clothes. 2.3.4 HOW CAN WE SEPARATE A MIXTURE he OF SALT AND AMMONIUM CHLORIDE ? 2.3.3 HOW CAN WE SEPARATE A MIXTURE We have learnt in chapter 1 that ammonium OF TWO IMMISCIBLE LIQUIDS? chloride changes directly from solid to is gaseous state on heating. So, to separate such Activity ______________ 2.6 mixtures that contain a sublimable volatile component from a non-sublimable impurity bl Let us try to separate kerosene oil (salt in this case), the sublimation process is from water using a separating funnel. Pour the mixture of kerosene oil and used (Fig. 2.7). Some examples of solids which water in a separating funnel (Fig. 2.6). sublime are ammonium chloride, camphor, pu Let it stand undisturbed for sometime naphthalene and anthracene. so that separate layers of oil and water are formed. be T Open the stopcock of the separating re funnel and pour out the lower layer of o R water carefully. Close the stopcock of the separating funnel as the oil reaches the stop-cock. tt E C no N © Fig. 2.7: Separation of ammonium chloride and salt Fig. 2.6: Separation of immiscible liquids by sublimation 20 SCIENCE  2.3.5 IS THE DYE IN BLACK INK A SINGLE This process of separation of components of a mixture is known as chromatography. COLOUR? Kroma in Greek means colour. This technique was first used for separation of colours, so Activity ______________ 2.7 this name was given. Chromatography is the technique used for separation of those solutes Take a thin strip of filter paper. Draw a line on it using a pencil, that dissolve in the same solvent. approximately 3 cm above the lower With the advancement in technology, edge [Fig. 2.8 (a)]. newer techniques of chromatography have Put a small drop of ink (water soluble, been developed. You will study about that is, from a sketch pen or fountain chromatography in higher classes. d pen) at the centre of the line. Let it dry. Lower the filter paper into a jar/glass/ Applications beaker/test tube containing water so he that the drop of ink on the paper is just To separate above the water level, as shown in Fig. colours in a dye 2.8(b) and leave it undisturbed. pigments from natural colours Watch carefully, as the water rises up on the filter paper. Record your drugs from blood. is observations. 2.3.6 HOW CAN WE SEPARATE A MIXTURE OF TWO MISCIBLE LIQUIDS? bl Activity ______________ 2.8 pu Let us try to separate acetone and water from their mixture. Take the mixture in a distillation flask. be T Fit it with a thermometer. Arrange the apparatus as shown in re Fig. 2.9. o R Fig. 2.8: Separation of dyes in black ink using Heat the mixture slowly keeping a close chromatography watch at the thermometer. The acetone vaporises, condenses in tt E Now answer the condenser and can be collected from the condenser outlet. What do you observe on the filter paper Water is left behind in the distillation C flask. as the water rises on it? Do you obtain different colours on the no N filter paper strip? What according to you, can be the reason for the rise of the coloured spot on the paper strip? © The ink that we use has water as the solvent and the dye is soluble in it. As the water rises on the filter paper it takes along with it the dye particles. Usually, a dye is a mixture of two or more colours. The coloured component that is more soluble in water, rises faster and in this way the colours get Fig.2.9: Separation of two miscible liquids by separated. distillation I S MATTER A ROUND U S P URE 21  Now answer 2.3.7 HOW CAN WE OBTAIN DIFFERENT GASES FROM AIR ? What do you observe as you start heating the mixture? Air is a homogeneous mixture and can be At what temperature does the separated into its components by fractional thermometer reading become distillation. The flow diagram (Fig. 2.11) constant for some time? shows the steps of the process. What is the boiling point of acetone? Why do the two components separate? d This method is called distillation. It is used for the separation of components of a mixture he containing two miscible liquids that boil without decomposition and have sufficient difference in their boiling points. To separate a mixture of two or more miscible liquids for which the difference in is boiling points is less than 25 K, fractional distillation process is used, for example, for bl the separation of different gases from air, different factions from petroleum products etc. The apparatus is similar to that for simple pu distillation, except that a fractionating column is fitted in between the distillation flask and the condenser. be T A simple fractionating column is a tube packed with glass beads. The beads provide re surface for the vapours to cool and condense o R repeatedly, as shown in Fig. 2.10. tt E Fig. 2.11: Flow diagram shows the process of obtaining gases from air C If we want oxygen gas from air (Fig. 2.12), we have to separate out all the other gases present in the air. The air is compressed by no N increasing the pressure and is then cooled by decreasing the temperature to get liquid air. This liquid air is allowed to warm-up slowly in a fractional distillation column, © where gases get separated at different heights depending upon their boiling points. Answer the following: Arrange the gases present in air in increasing order of their boiling points. Which gas forms the liquid first as the Fig. 2.10: Fractional distillation air is cooled? 22 SCIENCE  d he Fig. 2.12: Separation of components of air is 2.3.8 HOW CAN WE OBTAIN PURE COPPER it. To remove these impurities, the process of bl crystallisation is used. Crystallisation is a SULPHATE FROM AN IMPURE SAMPLE? process that separates a pure solid in the form of its crystals from a solution. Crystallisation Activity ______________ 2.9 pu technique is better than simple evaporation technique as – Take some (approximately 5 g) impure some solids decompose or some, like sample of copper sulphate in a china be T sugar, may get charred on heating to dish. dryness. re Dissolve it in minimum amount of some impurities may remain dissolved o R water. in the solution even after filtration. On Filter the impurities out. evaporation these contaminate the Evaporate water from the copper solid. tt E sulphate solution so as to get a saturated solution. Applications Cover the solution with a filter paper Purification of salt that we get from sea C and leave it undisturbed at room temperature to cool slowly for a day. water. You will obtain the crystals of copper Separation of crystals of alum (phitkari) from impure samples. no N sulphate in the china dish. This process is called crystallisation. Thus, by choosing one of the above methods according to the nature of the Now answer components of a mixture, we get a pure substance. With advancements in technology © What do you observe in the china dish? many more methods of separation techniques Do the crystals look alike? have been devised. In cities, drinking water is supplied from How will you separate the crystals from water works. A flow diagram of a typical water the liquid in the china dish? works is shown in Fig. 2.13. From this figure The crystallisation method is used to write down the processes involved to get the purify solids. For example, the salt we get supply of drinking water to your home from from sea water can have many impurities in the water works and discuss it in your class. I S MATTER A ROUND U S P URE 23  d Fig. 2.13: Water purification system in water works Q he uestions Both water and cooking oil are liquid but their chemical characteristics are different. 1. How will you separate a mixture They differ in odour and inflammability. We containing kerosene and petrol know that oil burns in air whereas water is (difference in their boiling points extinguishes fire. It is this chemical property is more than 25ºC), which are of oil that makes it different from water. Burning is a chemical change. During this bl miscible with each other? 2. Name the technique to separate process one substance reacts with another (i) butter from curd, to undergo a change in chemical composition. (ii) salt from sea-water, Chemical change brings change in the pu (iii) camphor from salt. chemical properties of matter and we get new 3. What type of mixtures are substances. A chemical change is also called a chemical reaction. be T separated by the technique of crystallisation? During burning of a candle, both physical re and chemical changes take place. Can you o R Q 2.4 Physical and Chemical distinguish these? Changes uestions tt E To understand the difference between a pure 1. Classify the following as substance and a mixture, let us understand chemical or physical changes: C the difference between a physical and a cutting of trees, chemical change. melting of butter in a pan, In the previous chapter, we have learnt rusting of almirah, no N about a few physical properties of matter. The boiling of water to form steam, properties that can be observed and specified passing of electric current, like colour, hardness, rigidity, fluidity, through water and the water density, melting point, boiling point etc. are breaking down into hydrogen © the physical properites. and oxygen gases, The interconversion of states is a physical dissolving common salt in water, change because these changes occur without making a fruit salad with raw a change in composition and no change in fruits, and the chemical nature of the substance. burning of paper and wood. Although ice, water and water vapour all look 2. Try segregating the things different and display different physical around you as pure substances properties, they are chemically the same. or mixtures. 24 SCIENCE  2.5 What are the Types of Pure The number of elements known at Substances? present are more than 100. Ninety-two elements are naturally On the basis of their chemical composition, occurring and the rest are man- made. More to know substances can be classified either as elements or compounds. Majority of the elements are solid. Eleven elements are in gaseous 2.5.1 ELEMENTS state at room temperature. Two elements are liquid at room Robert Boyle was the first scientist to use the temperature–mercury and term element in 1661. Antoine Laurent bromine. d Lavoisier (1743-94), a French chemist, was Elements, gallium and cesium the first to establish an experimentally useful become liquid at a temperature he definition of an element. He defined an slightly above room temperature element as a basic form of matter that cannot (303 K). be broken down into simpler substances by chemical reactions. 2.5.2 COMPOUNDS is Elements can be normally divided into metals, non-metals and metalloids. A compound is a substance composed of two Metals usually show some or all of the or more elements, chemically combined with bl following properties: one another in a fixed proportion. They have a lustre (shine). What do we get when two or more elements They have silvery-grey or golden-yellow are combined? pu colour. They conduct heat and electricity. Activity _____________ 2.10 They are ductile (can be drawn into Divide the class into two groups. Give be T wires). 5 g of iron filings and 3 g of sulphur They are malleable (can be hammered re powder in a china dish to both the o R into thin sheets). groups. They are sonorous (make a ringing Group I sound when hit). Mix and crush iron filings and sulphur tt E Examples of metals are gold, silver, copper, powder. iron, sodium, potassium etc. Mercury is the only metal that is liquid at room temperature. Group II C Non-metals usually show some or all of the Mix and crush iron filings and sulphur powder. Heat this mixture strongly till following properties: red hot. Remove from flame and let the They display a variety of colours. no N mixture cool. They are poor conductors of heat and electricity. Groups I and II They are not lustrous, sonorous or Check for magnetism in the material obtained. Bring a magnet near the malleable. © material and check if the material is Examples of non-metals are hydrogen, attracted towards the magnet. oxygen, iodine, carbon (coal, coke), Compare the texture and colour of the bromine, chlorine etc. Some elements have material obtained by the groups. intermediate properties between those of Add carbon disulphide to one part of metals and non-metals, they are the material obtained. Stir well and called metalloids; examples are boron, filter. silicon, germanium etc. Add dilute sulphuric acid or dilute hydrochloric acid to the other part of I S MATTER A ROUND U S P URE 25  Table 2.2: Mixtures and Compounds Mixtures Compounds 1. Elements or compounds just mix 1. Elements react to form new compounds. together to form a mixture and no new compound is formed. 2. A mixture has a variable composition. 2. The composition of each new substance d is always fixed. he 3, A mixture shows the properties of the 3. The new substance has totally different constituent substances. properties. 4. The constituents can be seperated 4. The constituents can be separated only fairly easily by physical methods. by chemical or electrochemical is reactions. bl the material obtained.(Note: teacher You must have observed that the products supervision is necessary for this obtained by both the groups show different activity). pu properties, though the starting materials were Perform all the above steps with both the same. Group I has carried out the activity the elements (iron and sulphur) separately. involving a physical change whereas in case be T of Group II, a chemical change (a chemical reaction) has taken place. Now answer re o R The material obtained by group I is a Did the material obtained by the two mixture of the two substances. The groups look the same? substances given are the elements– iron tt E and sulphur. Which group has obtained a material The properties of the mixture are the with magnetic properties? same as that of its constituents. C Can we separate the components of the The material obtained by group II is a material obtained? compound. no N On adding dilute sulphuric acid or On heating the two elements strongly we dilute hydrochloric acid, did both the get a compound, which has totally groups obtain a gas? Did the gas in different properties compared to the both the cases smell the same or combining elements. © different? The composition of a compound is the The gas obtained by Group I is hydrogen, same throughout. We can also observe it is colourless, odourless and combustible– that the texture and the colour of the it is not advised to do the combustion test for compound are the same throughout. hydrogen in the class. The gas obtained by Thus, we can summarise the physical Group II is hydrogen sulphide. It is a colourless and chemical nature of matter in the gas with the smell of rotten eggs. following graphical organiser : 26 SCIENCE  d he What is you have bl learnt A mixture contains more than one substance (element and/or pu compound) mixed in any proportion. Mixtures can be separated into pure substances using be T appropriate separation techniques. A solution is a homogeneous mixture of two or more substances. re The major component of a solution is called the solvent, and o R the minor, the solute. The concentration of a solution is the amount of solute present tt E per unit volume or per unit mass of the solution/solvent. Materials that are insoluble in a solvent and have particles that are visible to naked eyes, form a suspension. A suspension C is a heterogeneous mixture. Colloids are heterogeneous mixtures in which the particle size no N 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. I S MATTER A ROUND U S P URE 27  Exercises 1. Which separation techniques will you apply for the separation of the following? (a) Sodium chloride from its solution in water. (b) Ammonium chloride from a mixture containing sodium chloride and ammonium chloride. (c) Small pieces of metal in the engine oil of a car. (d) Different pigments from an extract of flower petals. d (e) Butter from curd. (f) Oil from water. he (g) Tea leaves from tea. (h) Iron pins from sand. (i) Wheat grains from husk. is (j) Fine mud particles suspended in water. 2. Write the steps you would use for making tea. Use the words solution, solvent, solute, dissolve, soluble, insoluble, filtrate bl and residue. 3. Pragya tested the solubility of three different substances at different temperatures and collected the data as given below pu (results are given in the following table, as grams of substance dissolved in 100 grams of water to form a saturated solution). be T Substance Dissolved Temperature in K re 283 293 313 333 353 o R Solubility Potassium nitrate 21 32 62 106 167 tt E Sodium chloride 36 36 36 37 37 Potassium chloride 35 35 40 46 54 C Ammonium chloride 24 37 41 55 66 no N (a) What mass of potassium nitrate would be needed to produce a saturated solution of potassium nitrate in 50 grams of water at 313 K? (b) Pragya makes a saturated solution of potassium chloride © in water at 353 K and leaves the solution to cool at room temperature. What would she observe as the solution cools? Explain. (c) Find the solubility of each salt at 293 K. Which salt has the highest solubility at this temperature? (d) What is the effect of change of temperature on the solubility of a salt? 28 SCIENCE  4. Explain the following giving examples. (a) saturated solution (b) pure substance (c) colloid (d) suspension 5. Classify each of the following as a homogeneous or heterogeneous mixture. soda water, wood, air, soil, vinegar, filtered tea. 6. How would you confirm that a colourless liquid given to you is d pure water? 7. Which of the following materials fall in the category of a “pure he substance”? (a) Ice (b) Milk is (c) Iron (d) Hydrochloric acid (e) Calcium oxide bl (f) Mercury (g) Brick pu (h) Wood (i) Air. be T 8. Identify the solutions among the following mixtures. (a) Soil re o R (b) Sea water (c) Air (d) Coal tt E (e) Soda water. 9. Which of the following will show “Tyndall effect”? C (a) Salt solution (b) Milk no N (c) Copper sulphate solution (d) Starch solution. 10. Classify the following into elements, compounds and mixtures. © (a) Sodium (b) Soil (c) Sugar solution (d) Silver (e) Calcium carbonate (f) Tin (g) Silicon I S MATTER A ROUND U S P URE 29  (h) Coal (i) Air (j) Soap (k) Methane (l) Carbon dioxide (m) Blood 11. Which of the following are chemical changes? (a) Growth of a plant (b) Rusting of iron d (c) Mixing of iron filings and sand (d) Cooking of food he (e) Digestion of food (f) Freezing of water (g) Burning of a candle. is Group Activity bl Take an earthen pot (mutka), some pebbles and sand. Design a small-scale filtration plant that you could use to clean muddy pu water. be T re o R tt E C no N © 30 SCIENCE