Chapter 2: Is Matter Around Us Pure? PDF

Chapter 2 IS MATTER AROUND US PURE? How do we judge whether milk, ghee, butter, evaporation. However, sodium chloride is itself salt, spices, mineral water or juice that we a pure substance and cannot be separated by buy from the market are pure?...

Chapter 2 IS MATTER AROUND US PURE? How do we judge whether milk, ghee, butter, evaporation. However, sodium chloride is itself salt, spices, mineral water or juice that we a pure substance and cannot be separated by buy from the market are pure? 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. Fig. 2.1: Some consumable items 2.1.1 TYPES OF MIXTURES Have you ever noticed the word ‘pure’ Depending upon the nature of the components written on the packs of these consumables? that form a mixture, we can have different For a common person pure means having no types of mixtures. adulteration. But, for a scientist all these things are actually mixtures of different substances Activity ______________ 2.1 and hence not pure. For example, milk is Let us divide the class into groups A, actually a mixture of water, fat, proteins etc. B, C and D. When a scientist says that something is pure, Group A takes a beaker containing it means that all the constituent particles of 50 mL of water and one spatula full of that substance are the same in their chemical copper sulphate powder. Group B takes nature. A pure substance consists of a single 50 mL of water and two spatula full of copper sulphate powder in a beaker. type of particles. In other words, a substance Groups C and D can take different is a pure single form of matter. amounts of copper sulphate and As we look around, we can see that most potassium permanganate or common of the matter around us exist as mixtures of salt (sodium chloride) and mix the given two or more pure components, for example, components to form a mixture. sea water, minerals, soil etc. are all mixtures. Report the observations on the uniformity in colour and texture. 2.1 What is a Mixture? Groups A and B have obtained a mixtur e which has a unifor m Mixtures are constituted by more than one composition throughout. Such kind of pure form of matter. We know that mixtures are called homogeneous dissolved sodium chloride can be separated mixtures or solutions. Some other from water by the physical process of examples of such mixtures are: (i) salt dissolved in water and (ii) sugar 2020-21  dissolved in water. Compar e 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. Gr oups C and D have obtained mixtures, which contain physically distinct parts and have non-uniform compositions. Such mixtures are called Fig. 2.2: Filtration heterogeneous mixtures. Mixtures of sodium chloride and iron filings, salt Now, we shall learn about solutions, and sulphur, and oil and water are suspensions and colloidal solutions in the examples of heterogeneous mixtures. following sections. Activity ______________ 2.2 uestions Let us again divide the class into four groups – A, B, C and D. Distribute the following samples to each group: − Few crystals of copper sulphate to group A. − One spatula full of copper sulphate to group B. − Chalk powder or wheat flour to Q 1. What is meant by a substance? 2. List the points of differences between homogeneous and heterogeneous mixtures. 2.2 What is a Solution? group C. A solution is a homogeneous mixture of two − Few dr ops of milk or ink to or more substances. You come across various group D. types of solutions in your daily life. Lemonade, 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 a glass rod. Are the particles in the contains either a solid, liquid or a gas 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). through the beaker containing the In a solution there is homogeneity at 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 visible? sugar or salt are evenly distributed in the 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 for m 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. IS MATTER AROUND US PURE? 15 2020-21  A solution has a solvent and a solute as its proportion of the solute and solvent can be 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 A (usually present in lesser quantity) is called is dilute as compared to that obtained by the solute. group B. Examples: Activity ______________ 2.3 (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 Add salt in one beaker and sugar or solvent. barium chloride in the second beaker with continuous stirring. (ii) A solution of iodine in alcohol known When no more solute can be dissolved, as ‘tincture of iodine’, has iodine (solid) heat the contents of the beaker to raise as the solute and alcohol (liquid) as the temperature by about 5°C. the solvent. Start adding the solute again. (iii) Aerated drinks like soda water etc., are gas in liquid solutions. These contain Is the amount of salt and sugar or barium 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 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 other gases are present in very small can be dissolved in a solution at a given quantities. temperature, it is called a saturated solution. The amount of the solute present in the Properties of a solution saturated solution at this temperature is called its solubility. A solution is a homogeneous mixture. If the amount of solute contained in a The particles of a solution are smaller solution is less than the saturation level, it is than 1 nm (10-9 metre) in diameter. So, called an unsaturated solution. they cannot be seen by naked eyes. What would happen if you were to take a Because of very small particle size, they saturated solution at a certain temperature do not scatter a beam of light passing and cool it slowly. through the solution. So, the path of We can infer from the above activity that light is not visible in a solution. different substances in a given solvent have The solute particles cannot be different solubilities at the same temperature. separated from the mixture by the The concentration of a solution is the amount process of filtration. The solute particles (mass or volume) of solute present in a given do not settle down when left undisturbed, amount (mass or volume) of solution. that is, a solution is stable. There are various ways of expressing the concentration of a solution, but here we will 2.2.1 CONCENTRATION OF A SOLUTION learn only three methods. In activity 2.2, we observed that groups A and (i) Mass by mass percentage of a solution B obtained different shades of solutions. So, Mass of solute = ×100 we understand that in a solution the relative Mass of solution 16 SCIENCE 2020-21  (ii) Mass by volume percentage of a solution The particles of a suspension can be seen by the naked eye. Mass of solute = ×100 The particles of a suspension scatter a Volume of solution beam of light passing through it and (iii) Volume by volume percentage of a make its path visible. solution The solute particles settle down when a suspension is left undisturbed, that is, Volume of solute = ×100 a suspension is unstable. They can be Volume of solution separated from the mixture by the process of filtration. When the particles Example 2.1 A solution contains 40 g of settle down, the suspension breaks and common salt in 320 g of water. it does not scatter light any more. Calculate the concentration in terms of mass by mass percentage of the solution. 2.2.3 WHAT IS A COLLOIDAL SOLUTION? Solution: The mixture obtained by group D in activity 2.2 is called a colloid or a colloidal solution. Mass of solute (salt) = 40 g Mass of solvent (water) = 320 g The particles of a colloid are uniformly spread We know, throughout the solution. Due to the relatively Mass of solution = Mass of solute + smaller size of particles, as compared to that of Mass of solvent a suspension, the mixture appears to be = 40 g + 320 g homogeneous. But actually, a colloidal solution = 360 g is a heterogeneous mixture, for example, milk. Mass percentage of solution Because of the small size of colloidal particles, we cannot see them with naked eyes. Mass of solute But, these particles can easily scatter a beam = ×100 Mass of solution of visible light as observed in activity 2.2. This scattering of a beam of light is called the 40 Tyndall effect after the name of the scientist = × 100 =11.1% 360 who discovered this effect. Tyndall effect can also be observed when a fine beam of light enters a room through a small 2.2.2 What is a suspension? hole. This happens due to the scattering of light by the particles of dust and smoke in the air. Non-homogeneous systems, like those 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 Tyndall effect, (b) mixture of water and milk Suspension is a heterogeneous shows Tyndall effect. mixture. IS MATTER AROUND US PURE? 17 2020-21  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 dispersed phase, and the component in which the dispersed phase is suspended is known 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 given in Table 2.1. From this table you can see that they are very common everyday life. Fig. 2.4: The Tyndall effect uestions Q Properties of a colloid 1. Differentiate between homogen- 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 eyes. suspension different from each other? Colloids are big enough to scatter a 3. To make a saturated solution, 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 undisturbed, that is, a colloid is quite temperature. stable. Table 2.1: Common examples of colloids Dispersed Dispersing Type Example 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 2020-21 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 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 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 HOW CAN WE SEPARATE CREAM FROM MILK? 2.3.1 HOW 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? packs or tetra packs in the market. These varieties of milk contain different amounts Activity ______________ 2.4 of fat. Fill half a beaker with water. Put a watch glass on the mouth of the Activity ______________ 2.5 beaker (Fig. 2.5). 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. not want to heat the ink directly. You Centrifuge it by using a centrifuging 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 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 Observe carefully and record your and ask (i) how they separate cream observations. from milk and (ii) how they make cheese (paneer) from milk. 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 IS MATTER AROUND US PURE? 19 2020-21 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. Used in washing machines to squeeze out water from wet clothes. 2.3.4 HOW CAN WE SEPARATE A MIXTURE OF SALT AND CAMPHOR? 2.3.3 HOW CAN WE SEPARATE A MIXTURE We have learnt in chapter 1 that camphor OF TWO IMMISCIBLE LIQUIDS? changes directly from solid to gaseous state on heating. So, to separate such mixtures that Activity ______________ 2.6 contain a sublimable volatile component from a non-sublimable impurity, the sublimation Let us try to separate kerosene oil process is used (Fig. 2.7). Some examples of from water using a separating funnel. Pour the mixture of kerosene oil and solids which sublime are ammonium chloride, water in a separating funnel (Fig. 2.6). naphthalene and anthracene. Let it stand undisturbed for sometime so that separate layers of oil and water are formed. Open the stopcock of the separating funnel and pour out the lower layer of water carefully. Close the stopcock of the separating funnel as the oil reaches the stop-cock. Fig. 2.7: Separation of camphor and salt by Fig. 2.6: Separation of immiscible liquids sublimation 20 SCIENCE 2020-21 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. 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 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 drugs from blood. on the filter paper. Record your observations. 2.3.6 HOW CAN WE SEPARATE A MIXTURE OF TWO MISCIBLE LIQUIDS? Activity ______________ 2.8 Let us try to separate acetone and water from their mixture. Take the mixture in a distillation flask. Fit it with a thermometer. Arrange the apparatus as shown in Fig. 2.9. 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 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 as the water rises on it? flask. Do you obtain different colours on the 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 IS MATTER AROUND US PURE? 21 2020-21 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 ther mometer 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? This method is called distillation. It is used for the separation of components of a mixture 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 boiling points is less than 25 K, fractional distillation process is used, for example, for the separation of different gases from air, different factions from petroleum products etc. The apparatus is similar to that for simple distillation, except that a fractionating column is fitted in between the distillation flask and the condenser. A simple fractionating column is a tube packed with glass beads. The beads provide surface for the vapours to cool and condense repeatedly, as shown in Fig. 2.10. Fig. 2.11: Flow diagram shows the process of obtaining gases from air 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 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 2020-21  Fig. 2.12: Separation of components of air 2.3.8 HOW CAN WE OBTAIN PURE COPPER it. To remove these impurities, the process of 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 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 sugar, may get charred on heating to dish. dryness. Dissolve it in minimum amount of some impurities may remain dissolved water. in the solution even after filtration. On Filter the impurities out. evaporation these contaminate the Evaporate water from the copper solid. sulphate solution so as to get a saturated solution. Applications Cover the solution with a filter paper and leave it undisturbed at room Purification of salt that we get from sea temperature to cool slowly for a day. water. You will obtain the crystals of copper Separation of crystals of alum (phitkari) sulphate in the china dish. from impure samples. 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. IS MATTER AROUND US PURE? 23 2020-21  Fig. 2.13: Water purification system in water works They differ in odour and inflammability. We uestions Q know that oil burns in air whereas water 1. How will you separate a mixture extinguishes fire. It is this chemical property containing kerosene and petrol of oil that makes it different from water. (difference in their boiling points Burning is a chemical change. During this is more than 25ºC), which are process one substance reacts with another miscible with each other? to undergo a change in chemical composition. 2. Name the technique to separate Chemical change brings change in the (i) butter from curd, chemical properties of matter and we get new (ii) salt from sea-water, substances. A chemical change is also called (iii) camphor from salt. a chemical reaction. 3. What type of mixtures are During burning of a candle, both physical separated by the technique of and chemical changes take place. Can you crystallisation? distinguish these? uestions Q 2.4 Physical and Chemical Changes 1. Classify the following as chemical or physical changes: In the previous chapter, we have learnt about cutting of trees, a few physical properties of matter. The melting of butter in a pan, properties that can be observed and specified rusting of almirah, like colour, hardness, rigidity, fluidity, boiling of water to form steam, density, melting point, boiling point etc. are passing of electric current, the physical properites. through water and the water The interconversion of states is a physical breaking down into hydrogen change because these changes occur without and oxygen gases, a change in composition and no change in dissolving common salt in water, the chemical nature of the substance. making a fruit salad with raw Although ice, water and water vapour all look fruits, and different and display different physical burning of paper and wood. properties, they are chemically the same. 2. T ry segregating the things Both water and cooking oil are liquid but around you as pure substances their chemical characteristics are different. or mixtures. 24 SCIENCE 2020-21 2.5 What are the Types of Pure The number of elements known at present are more than 100. Substances? Ninety-two elements are naturally On the basis of their chemical composition, occurring and the rest are man- More to know substances can be classified either as made. 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. Lavoisier (1743-94), a French chemist, was Elements, gallium and cesium the first to establish an experimentally useful become liquid at a temperature 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 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 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? colour. They conduct heat and electricity. Activity _____________ 2.10 They are ductile (can be drawn into Divide the class into two groups. Give wires). 5 g of iron filings and 3 g of sulphur They are malleable (can be hammered powder in a china dish to both the into thin sheets). groups. They are sonorous (make a ringing Group I sound when hit). Mix and crush iron filings and sulphur 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 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. 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 malleable. obtained. Bring a magnet near the 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 IS MATTER AROUND US PURE? 25 2020-21  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 is always fixed. 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 reactions. 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). 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) involving a physical change whereas in case separately. of Group II, a chemical change (a chemical reaction) has taken place. Now answer 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 and sulphur. Which group has obtained a material The properties of the mixture are the with magnetic properties? same as that of its constituents. Can we separate the components of the The material obtained by group II is a material obtained? compound. 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 2020-21  What you have learnt A mixture contains more than one substance (element and/ or compound) 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. IS MATTER AROUND US PURE? 27 2020-21  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. (e) Butter from curd. (f) Oil from water. (g) Tea leaves from tea. (h) Iron pins from sand. (i) Wheat grains from husk. (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 and residue. 3. Pragya tested the solubility of three different substances at different temperatures and collected the data as given below (results are given in the following table, as grams of substance dissolved in 100 grams of water to form a saturated solution). Substance Dissolved Temperature in K 283 293 313 333 353 Solubility Potassium nitrate 21 32 62 106 167 Sodium chloride 36 36 36 37 37 Potassium chloride 35 35 40 46 54 Ammonium chloride 24 37 41 55 66 (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 2020-21  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 pure water? 7. Which of the following materials fall in the category of a “pure substance”? (a) Ice (b) Milk (c) Iron (d) Hydrochloric acid (e) Calcium oxide (f) Mercury (g) Brick (h) Wood (i) Air. 8. Identify the solutions among the following mixtures. (a) Soil (b) Sea water (c) Air (d) Coal (e) Soda water. 9. Which of the following will show “Tyndall effect”? (a) Salt solution (b) Milk (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 IS MATTER AROUND US PURE? 29 2020-21  (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 (c) Mixing of iron filings and sand (d) Cooking of food (e) Digestion of food (f) Freezing of water (g) Burning of a candle. Group Activity Take an earthen pot (mutka), some pebbles and sand. Design a small-scale filtration plant that you could use to clean muddy water. 30 SCIENCE 2020-21