Chapter 2: Is Matter Around Us Pure? (Science PDF)

C hapter 2 IS MATTER AROUND US PURE? How do we judge whether milk, ghee, butter, a pure substance and cannot be separated by salt, spices, mineral water or juice that we buy physical process into its chemical constituents. from the market are pure?...

C hapter 2 IS MATTER AROUND US PURE? How do we judge whether milk, ghee, butter, a pure substance and cannot be separated by salt, spices, mineral water or juice that we buy physical process into its chemical constituents. from the market are pure? 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. 2.1.1 TYPES OF MIXTURES Fig. 2.1: Some consumable items Depending upon the nature of the components Have you ever noticed the word ‘pure’ that form a mixture, we can have different written on the packs of these consumables? types of mixtures. For a common person pure means having no adulteration. But, for a scientist all these things Activity ______________ 2.1 are actually mixtures of different substances Let us divide the class into groups A, and hence not pure. For example, milk is B, C and D. actually a mixture of water, fat, proteins, etc. Group A takes a beaker containing When a scientist says that something is pure, 50 mL of water and one spatula full of copper sulphate powder. Group B it means that all the constituent particles of takes 50 mL of water and two spatula that substance are the same in their chemical full of copper sulphate powder in a nature. A pure substance consists of a single beaker. type of particle. In other words, a substance is Groups C and D can take different a pure single form of matter. amounts of copper sulphate and As we look around, we can see that most potassium permanganate or common salt (sodium chloride) and mix the of the matter around us exists as mixtures of given components to form a mixture. two or more pure components, for example, Report the observations on the sea water, minerals, soil, etc., are all mixtures. uniformity in colour and texture. Groups A and B have obtained a 2.1 What is a Mixture? mixture which has a uniform Mixtures are constituted by more than one composition throughout. Such mixtures are called homogeneous kind of pure form of matter. We know that mixtures or solutions. Some other dissolved sodium chloride can be separated examples of such mixtures are: (i) salt from water by the physical process of dissolved in water and (ii) sugar evaporation. However, sodium chloride is itself dissolved in water. Compare the 2024-25  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 heterogeneous mixtures. Mixtures of Fig. 2.2: Filtration 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 Q uestions 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 each group: heterogeneous mixtures. − Few crystals of copper sulphate to group A. − One spatula full of copper sulphate to group B. 2.2 What is a Solution? − Chalk powder or wheat flour to group C. A solution is a homogeneous mixture of two − Few drops 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 Usually we think of a solution as a liquid that using a glass rod. Are the particles contains either a solid, liquid or a gas dissolved in the mixture visible? in it. But, we can also have solid solutions Direct a beam of light from a torch (alloys) and gaseous solutions (air). In a through the beaker containing the solution there is homogeneity at the particle mixture and observe from the front. Was level. For example, lemonade tastes the same the path of the beam of light visible? throughout. This shows that particles of sugar Leave the mixtures undisturbed for or salt are evenly distributed in a few minutes (and set up the the solution. filtration apparatus in the meantime). Is the mixture stable or do the particles begin to settle after some Alloys: Alloys are mixtures of two or more metals or a metal and a non-metal More to know time? Filter the mixture. Is there any and cannot be separated into their residue on the filter paper? components by physical methods. But 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. IS MATTER AROUND US PURE? 15 2024-25  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 dilute, (usually the component present in larger concentrated or 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 (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 Add salt in one beaker and sugar or the 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 the temperature by about 5oC.° 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 water chloride, that can be dissolved in water at a (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 can be dissolved in a solution at a given small 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 separated different solubilities at the same temperature. from the mixture by the process of The concentration of a solution is the amount filtration. The solute particles do not (mass or volume) of solute present in a given settle down when left undisturbed, that amount (mass or volume) of solution. 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 2024-25  (ii) Mass by volume percentage of a solution The particles of a suspension scatter a beam of light passing through it and Mass of solute = ×100 make its path visible. Volume of solution The solute particles settle down when a (iii) Volume by volume percentage of a suspension is left undisturbed, that is, solution a suspension is unstable. They can be separated from the mixture by the Volume of solute = ×100 process of filtration. When the particles Volume of solution settle down, the suspension breaks and it does not scatter light any more. Example 2.1 A solution contains 40 g of common salt in 320 g of water. Calculate the concentration in terms of mass by 2.2.3 WHAT IS A COLLOIDAL SOLUTION? mass percentage of the solution. The mixture obtained by group D in activity Solution: 2.2 is called a colloid or a colloidal solution. The 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 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 = 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 eyes. Mass percentage of solution But, these particles can easily scatter a beam Mass of solute of visible light as observed in activity 2.2. This = ×100 scattering of a beam of light is called the Mass of solution Tyndall effect after the name of the scientist 40 who discovered this effect. = ×100 =11.1% Tyndall effect can also be observed when a 360 fine beam of light enters a room through a small 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 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. Properties of a Suspension (a) (b) Suspension is a heterogeneous mixture. Fig. 2.3: (a) Solution of copper sulphate does not The particles of a suspension can be seen show Tyndall effect, (b) mixture of water by the naked eye. and milk shows Tyndall effect. IS MATTER AROUND US PURE? 17 2024-25  Tyndall effect can be observed when Colloids are big enough to scatter a sunlight passes through the canopy of a dense beam of light passing through it and forest. In the forest, mist contains tiny droplets make its path visible. of water, which act as particles of colloid They do not settle down when left dispersed in air. 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 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 Fig. 2.4: The Tyndall effect as the dispersing medium. Colloids are classified according to the state (solid, liquid Properties of a Colloid or gas) of the dispersing medium and the A colloid is a heterogeneous mixture. dispersed phase. A few common examples are The size of particles of a colloid is too given in Table 2.1. From this table you can small to be individually seen with see that they are very common everyday life. naked eyes. Q uestions 1. Differentiate between homogen eous and heterogeneous mixtures with examples. 2. How are sol, solution and suspension different from each other? 3. To make a saturated solution, 36 g of sodium chloride is dissolved in 100 g of water at 293 K. Find its concentration at this temperature. 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 S CIENCE 2024-25 2.3 Physical and Chemical burning of paper and wood. 2. T ry segregating the things Changes around you as pure substances or mixtures. In the previous chapter, we have learnt about a few physical properties of matter. The properties that can be observed and specified 2.4 What are the Types of Pure like colour, hardness, rigidity, fluidity, Substances? density, melting point, boiling point etc. are the physical properites. On the basis of their chemical composition, The interconversion of states is a physical substances can be classified either as elements change because these changes occur without or compounds. a change in composition and no change in the chemical nature of the substance. Although 2.4.1 ELEMENTS ice, water and water vapour all look different and display different physical properties, they Robert Boyle was the first scientist to use the are chemically the same. term element in 1661. Antoine Laurent Both water and cooking oil are liquid but Lavoisier (1743–94), a French chemist, was the their chemical characteristics are different. first to establish an experimentally useful They differ in odour and inflammability. We definition of an element. He defined an element know that oil burns in air whereas water as a basic form of matter that cannot be broken extinguishes fire. It is this chemical property down into simpler substances by chemical of oil that makes it different from water. reactions. Burning is a chemical change. During this Elements can be normally divided into process one substance reacts with another to metals, non-metals and metalloids. undergo a change in chemical composition. Metals usually show some or all of the Chemical change brings change in the following properties: chemical properties of matter and we get new They have a lustre (shine). substances. A chemical change is also called They have silvery-grey or golden-yellow a chemical reaction. colour. During burning of a candle, both physical They conduct heat and electricity. and chemical changes take place. Can you They are ductile (can be drawn into distinguish these? wires). They are malleable (can be hammered Q into thin sheets). uestions They are sonorous (make a ringing sound when hit). 1. Classify the following as chemical or physical changes: Examples of metals are gold, silver, cutting of trees, copper, iron, sodium, potassium etc. Mercury melting of butter in a pan, is the only metal that is liquid at room rusting of almirah, temperature. boiling of water to form steam, Non-metals usually show some or all of the passing of electric current, following properties: through water and the water They display a variety of colours. breaking down into hydrogen They are poor conductors of heat and and oxygen gases, electricity. dissolving common salt in They are not lustrous, sonorous water, or malleable. making a fruit salad with raw Examples of non-metals are hydrogen, fruits, and oxygen, iodine, carbon (coal, coke), bromine, IS MATTER AROUND US PURE? 19 2024-25 chlorine etc. Some elements have intermediate Compare the texture and colour of the properties between those of metals and non- material obtained by the groups. metals, they are called metalloids; examples Add carbon disulphide to one part of are boron, silicon, germanium, etc. the material obtained. Stir well and filter. Add dilute sulphuric acid or dilute The number of elements hydrochloric acid to the other part of known at present are more the material obtained.(Note: teacher than 100. Ninety-two elements supervision is necessary for are naturally occurring and the rest are man-made. this activity). More to know Perform all the above steps with both Majority of the elements are the elements (iron and sulphur) solid. separately. Eleven elements are in gaseous state at room temperature. Two elements are liquid at Now answer room temperature—mercury and bromine. Did the material obtained by the two Elements, gallium and cesium groups look the same? become liquid at a temperature Which group has obtained a material slightly above room with magnetic properties? temperature (303 K). Can we separate the components of the material obtained? 2.4.2 COMPOUNDS On adding dilute sulphuric acid or A compound is a substance composed of two dilute hydrochloric acid, did both the or more elements, chemically combined with groups obtain a gas? Did the gas in one another in a fixed proportion. both the cases smell the same What do we get when two or more elements or different? are combined? The gas obtained by Group I is hydrogen, Activity ______________ 2.4 it is colourless, odourless and combustible– it is not advised to do the combustion test for Divide the class into two groups. Give hydrogen in the class. The gas obtained by 5 g of iron filings and 3 g of sulphur Group II is hydrogen sulphide. It is a colourless powder in a china dish to both the groups. gas with the smell of rotten eggs. You must have observed that the products Group I Mix and crush iron filings and obtained by both the groups show different sulphur powder. properties, though the starting materials were the same. Group I has carried out the activity Group II Mix and crush iron filings and involving a physical change whereas in case sulphur powder. Heat this mixture of Group II, a chemical change (a chemical strongly till red hot. Remove from reaction) has taken place. flame and let the mixture cool. The material obtained by group I is a Groups I and II mixture of the two substances. The Check for magnetism in the material substances given are the elements—iron obtained. Bring a magnet near the and sulphur. material and check if the material is The properties of the mixture are the attracted towards the magnet. same as that of its constituents. 20 SCIENCE 2024-25  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 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 by group II is a The composition of a compound is the compound. same throughout. We can also observe On heating the two elements strongly we that the texture and the colour of the get a compound, which has totally compound are the same throughout. different properties compared to the Thus, we can summarise the physical combining elements. and chemical nature of matter in the following graphical organiser: What you have learnt A mixture contains more than one substance (element and/ or compound) mixed in any proportion. IS MATTER AROUND US PURE? 21 2024-25  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. 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 22 SCIENCE 2024-25  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? 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? IS MATTER AROUND US PURE? 23 2024-25  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 (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 24 SCIENCE 2024-25  (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. IS MATTER AROUND US PURE? 25 2024-25