Is Matter Around Us Pure?

Namaste students, welcome to today's science class. I'm so happy to see you all ready to learn something new and interesting about the world around us. Today we are going to study Chapter 2 from your NCERT Science book, and the title is "Is Matter Around Us Pure?" This is a very important chapter that will help you understand the nature of matter and how we classify different substances. So let's begin our journey into the fascinating world of chemistry.

Now students, think about this. When you go to the market with your mother or father to buy groceries, you often see labels that say "pure" on things like milk, ghee, butter, salt, spices, mineral water, or juice. But have you ever wondered what the word "pure" really means? For a common person like you and me, pure means that there is no adulteration, no mixing of anything unwanted. If we buy pure ghee, we expect it to be only ghee and nothing else mixed in it. But for a scientist, the meaning of pure is completely different. Let me explain this to you.

When a scientist says that something is pure, it means that all the constituent particles of that substance are the same in their chemical nature. A pure substance consists of a single type of particle. In other words, a substance is a pure single form of matter. So students, if we take the example of milk, which we all drink, is milk pure from a scientific point of view? No, absolutely not! Milk is actually a mixture of water, fat, proteins, lactose, and various other substances. Similarly, when we say sugar is pure, it is because sugar contains only one kind of pure matter and its composition is the same throughout, whether you take sugar from the top of the container or the bottom. On the other hand, soft drink and soil are not single pure substances because they contain many different types of particles mixed together.

So students, let me make this very clear. A pure substance is something that cannot be separated by physical processes into its chemical constituents. For example, if you have pure water, you cannot separate it into different chemicals using physical methods like filtering or evaporation - water will always remain water. But if you have a mixture of salt and water, you can separate the salt from the water by evaporation, and that is why it is a mixture, not a pure substance.

Now, as we look around us, we can see that most of the matter around us exists as mixtures of two or more pure components. Sea water is a mixture, minerals in the earth are mixtures, soil is a mixture. So basically, most things we see around us are not pure in the scientific sense. This is a very important point to remember.

Now let us move to the next section, which is about mixtures. What exactly is a mixture? Students, a mixture is constituted by more than one kind of pure form of matter. Let me give you a simple example. When you dissolve sodium chloride which is common salt in water, you get a mixture. But sodium chloride itself is a pure substance - it is just one type of matter. However, when we mix it with water, we get a mixture because now we have two different substances mixed together. And this mixture can be separated back into salt and water by the physical process of evaporation. But sodium chloride by itself, when we have it in its pure form, cannot be broken down into simpler substances by physical processes.

Now students, let us understand the different types of mixtures. This is very important for your exams. Mixtures can be of two main types - homogeneous mixtures and heterogeneous mixtures. Let me explain each one with examples.

To understand this better, let us look at Activity 2.1 from your textbook. Imagine we divide the class into four groups - A, B, C, and D. Group A takes a beaker containing 50 millilitres of water and adds one spatula full of copper sulphate powder to it. Group B takes 50 millilitres of water and adds two spatula full of copper sulphate powder to it. Groups C and D can take different amounts of copper sulphate and potassium permanganate or common salt and mix these components to form a mixture. Now, what do they observe? They need to report their observations on the uniformity in colour and texture.

Students, groups A and B have obtained a mixture which has a uniform composition throughout. This means that if you take a drop from the top of the solution or a drop from the bottom, they will look exactly the same - same colour, same taste, everything uniform. Such mixtures are called homogeneous mixtures or solutions. Some other examples of homogeneous mixtures are salt dissolved in water and sugar dissolved in water. Now, compare the colour of the solutions of the two groups. Though both the groups have obtained copper sulphate solution, the intensity of colour of the solutions is different. Group A's solution is lighter in colour because they added less copper sulphate, while group B's solution is darker because they added more copper sulphate. This shows that a homogeneous mixture can have a variable composition - you can have more or less of the solute and still have a homogeneous mixture.

Now, what about groups C and D? They have obtained mixtures which contain physically distinct parts and have non-uniform compositions. For example, if you mix salt and iron filings, you can see the salt crystals and the iron filings separately - they are not uniformly distributed. Similarly, if you mix oil and water, you can see separate layers. Such mixtures are called heterogeneous mixtures. Other examples of heterogeneous mixtures are sodium chloride and iron filings, salt and sulphur, and oil and water. In all these cases, you can see different parts with your eyes, and the composition is not the same throughout.

So students, let me recap what we have learned so far. A mixture is something that contains more than one pure substance. Mixtures can be homogeneous, where the composition is uniform throughout, or heterogeneous, where the composition is not uniform and you can see different parts. This is a very important distinction that you must remember.

Now let's move to Activity 2.2 to understand more about different types of mixtures. Let us again divide the class into four groups - A, B, C, and D. We 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 group C, and few drops of milk or ink to group D. Each group should add the given sample in water and stir properly using a glass rod. Now, the first question is: are the particles in the mixture visible? Groups A and B will find that the particles are not visible - the copper sulphate seems to have dissolved completely. But group C will see that the chalk powder or wheat flour is floating in the water and the particles are visible. Group D will also see that the milk or ink is distributed in the water but the individual particles are not clearly visible like in group C.

Now, let's do another test. Direct a beam of light from a torch through the beaker containing the mixture and observe from the front. Was the path of the beam of light visible? For groups A and B, the path of light will not be visible because the particles are too small to scatter light. But for groups C and D, the path of light will be visible. For group C, because the particles are large, they will scatter light strongly. For group D, which has milk or ink, the medium-sized particles will also scatter light.

Now, let's leave the mixtures undisturbed for a few minutes. Is the mixture stable or do the particles begin to settle after some time? For groups A and B, the mixture remains stable - nothing settles down. For group C, the particles will settle down at the bottom after some time because they are heavy. For group D, the mixture may remain stable for some time but eventually the particles may settle.

Now, let's filter the mixture. Is there any residue on the filter paper? For groups A and B, there will be no residue because the copper sulphate has completely dissolved. For group C, there will be a lot of residue - the chalk powder or wheat flour will remain on the filter paper. For group D, there may or may not be a residue depending on whether the milk or ink particles are small enough to pass through the filter paper.

Based on these observations, we can say that groups A and B have got a solution, group C has got a suspension, and group D has got a colloidal solution. Now let us study each of these in detail.

First, let us understand what a solution is. A solution is a homogeneous mixture of two or more substances. You come across various types of solutions in your daily life. Lemonade, soda water, etc., are all examples of solutions. Usually we think of a solution as a liquid that contains either a solid, liquid or a gas dissolved in it. But students, we can also have solid solutions which are called alloys, and gaseous solutions which is basically air. In a solution there is homogeneity at the particle level. For example, if you taste lemonade from any part of the glass, it tastes the same. This shows that particles of sugar or salt are evenly distributed in the solution.

Now, let me tell you about alloys. Alloys are mixtures of two or more metals or a metal and a non-metal and cannot be separated into their components by physical methods. But still, an alloy is considered as a mixture because it shows the properties of its constituents and can have variable composition. For example, brass is a mixture of approximately 30% zinc and 70% copper. Similarly, steel is a mixture of iron and carbon. Alloys are very important in our daily life because they have properties that are different from the pure metals. For instance, pure iron is soft and rusts easily, but steel which is an alloy of iron and carbon is hard and does not rust easily.

Every solution has two components - the solvent and the solute. The component of the solution that dissolves the other component in it, and is usually present in larger amount, is called the solvent. The component of the solution that is dissolved in the solvent, and is usually present in lesser quantity, is called the solute. Let me give you some examples to make this clear.

Example (i): A solution of sugar in water is a solid in liquid solution. In this solution, sugar is the solute and water is the solvent. The sugar dissolves in the water, so sugar is the substance that gets dissolved, and water is the substance that does the dissolving.

Example (ii): A solution of iodine in alcohol known as 'tincture of iodine', has iodine (solid) as the solute and alcohol (liquid) as the solvent. Tincture of iodine is commonly used as an antiseptic, and it is made by dissolving iodine in alcohol.

Example (iii): Aerated drinks like soda water, etc., are gas in liquid solutions. These contain carbon dioxide (gas) as solute and water (liquid) as solvent. When you open a soda bottle, you see bubbles - those are carbon dioxide gas that was dissolved in water under pressure.

Example (iv): Air is a mixture of gas in gas. Air is a homogeneous mixture of a number of gases. Its two main constituents are: oxygen (21%) and nitrogen (78%). The other gases like argon, carbon dioxide, etc., are present in very small quantities. So when we breathe air, we are breathing a solution of many gases.

Now students, let us understand the properties of a solution. This is very important for your exams.

Property 1: A solution is a homogeneous mixture. This means that the composition is uniform throughout.

Property 2: The particles of a solution are smaller than 1 nanometre (10⁻⁹ metre) in diameter. So, they cannot be seen by naked eyes. 1 nanometre is extremely small - you cannot see such tiny particles even with a powerful microscope sometimes.

Property 3: Because of very small particle size, they do not scatter a beam of light passing through the solution. So, the path of light is not visible in a solution. This is why when you shine a torch through a solution, you don't see a beam of light passing through.

Property 4: The solute particles cannot be separated from the mixture by the process of filtration. This is because the particles are so small that they pass through the filter paper along with the solvent.

Property 5: The solute particles do not settle down when left undisturbed, that is, a solution is stable. If you keep a solution for days, the solute will not settle down at the bottom.

Now students, let us understand about the concentration of a solution. In Activity 2.2, we observed that groups A and B obtained different shades of solutions. Group A had a lighter blue colour because they added less copper sulphate, while group B had a darker blue colour because they added more copper sulphate. So, we understand that in a solution the relative proportion of the solute and solvent can be varied. Depending upon the amount of solute present in a solution, it can be called dilute, concentrated or saturated solution. Dilute and concentrated are comparative terms. In Activity 2.2, the solution obtained by group A is dilute as compared to that obtained by group B because group A had less solute.

Now, let us understand what a saturated solution is. This is a very important concept. Look at Activity 2.3. Take approximately 50 millilitres of water each in two separate beakers. Add salt in one beaker and sugar or barium chloride in the second beaker with continuous stirring. When no more solute can be dissolved, heat the contents of the beaker to raise the temperature by about 5°C. Then start adding the solute again. What do you observe? Is the amount of salt and sugar or barium chloride, that can be dissolved in water at a given temperature, the same? No, it is different. Different substances have different solubilities.

At any particular temperature, a solution that has dissolved as much solute as it is capable of dissolving, is said to be a saturated solution. In other words, when no more solute can be dissolved in a solution at a given temperature, it is called a saturated solution. The amount of the solute present in the saturated solution at this temperature is called its solubility. So students, solubility is the maximum amount of solute that can be dissolved in 100 grams of solvent at a particular temperature to form a saturated solution.

If the amount of solute contained in a solution is less than the saturation level, it is called an unsaturated solution. This means you can still dissolve more solute in it.

What would happen if you were to take a saturated solution at a certain temperature and cool it slowly? Students, when you cool a saturated solution, some of the dissolved solute may crystallize out because the solubility decreases at lower temperatures. This is how crystals are formed, and you might have seen this happening when making sugar crystals or salt crystals at home.

We can infer from the above activity that different substances in a given solvent have different solubilities at the same temperature. For example, at room temperature, you can dissolve more sugar in water than you can dissolve salt in water.

Now, the concentration of a solution is the amount (mass or volume) of solute present in a given amount (mass or volume) of solution. There are various ways of expressing the concentration of a solution, but here we will learn only three methods.

Method (i): Mass by mass percentage of a solution = (Mass of solute / Mass of solution) × 100

Method (ii): Mass by volume percentage of a solution = (Mass of solute / Volume of solution) × 100

Method (iii): Volume by volume percentage of a solution = (Volume of solute / Volume of solution) × 100

Now students, let me solve Example 2.1 from your textbook for you. The question is: A solution contains 40 g of common salt in 320 g of water. Calculate the concentration in terms of mass by mass percentage of the solution.

Let me solve this step by step.

First, we need to find the mass of solute and mass of solvent. Mass of solute (salt) = 40 g Mass of solvent (water) = 320 g

Now, we know that: Mass of solution = Mass of solute + Mass of solvent = 40 g + 320 g = 360 g

Now, mass percentage of solution = (Mass of solute / Mass of solution) × 100 = (40 / 360) × 100 = 0.1111 × 100 = 11.1%

So the concentration is 11.1%. This means that in this solution, 11.1% of the total mass is salt.

Now students, let us move to the next topic which is suspension. What is a suspension? 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. The particles of a suspension are visible to the naked eye. For example, when you mix chalk powder in water, you get a suspension. The chalk particles do not dissolve; they just float in the water and eventually settle down.

Now let us understand the properties of a suspension.

Property 1: Suspension is a heterogeneous mixture. This means you can see different parts in it.

Property 2: The particles of a suspension can be seen by the naked eye. Unlike solution particles which are too small to see, suspension particles are big enough to see.

Property 3: The particles of a suspension scatter a beam of light passing through it and make its path visible. This is because the particles are big enough to scatter light.

Property 4: The solute particles settle down when a suspension is left undisturbed, that is, a suspension is unstable. If you keep a suspension for some time, the solid particles will settle down at the bottom.

Property 5: They can be separated from the mixture by the process of filtration. When you filter a suspension, the solid particles remain on the filter paper as residue, and the clear liquid passes through as filtrate.

When the particles settle down, the suspension breaks and it does not scatter light any more. This is an important property.

Now students, let us move to colloids. The mixture obtained by group D in activity 2.2 is called a colloid or a colloidal solution. The particles of a colloid are uniformly spread throughout the solution. Due to the relatively smaller size of particles, as compared to that of a suspension, the mixture appears to be homogeneous. But actually, a colloidal solution is a heterogeneous mixture, for example, milk. Milk looks uniform to the naked eye, but if you look at it under a powerful microscope, you will see that it contains tiny fat droplets dispersed in water. So it is actually heterogeneous.

Because of the small size of colloidal particles, we cannot see them with naked eyes. But, these particles can easily scatter a beam of visible light as observed in activity 2.2. This scattering of a beam of light is called the Tyndall effect after the name of the scientist who discovered this effect, John Tyndall.

Students, Tyndall effect can also be observed in many everyday situations. For example, when a fine beam of light enters a room through a small hole, you can see the path of the light. This happens due to the scattering of light by the particles of dust and smoke in the air. Another example is when sunlight passes through the canopy of a dense forest. In the forest, mist contains tiny droplets of water, which act as particles of colloid dispersed in air. When sunlight passes through, you can see the path of light because of the Tyndall effect. This is why the forest looks so beautiful when sunlight filters through the trees.

Now let us understand the properties of a colloid.

Property 1: A colloid is a heterogeneous mixture. Even though it looks uniform, it actually has two phases - the dispersed phase and the dispersion medium.

Property 2: The size of particles of a colloid is too small to be individually seen with naked eyes, but larger than the particles in a solution.

Property 3: Colloids are big enough to scatter a beam of light passing through it and make its path visible. This is the Tyndall effect, which distinguishes colloids from solutions.

Property 4: They do not settle down when left undisturbed, that is, a colloid is quite stable. Unlike suspensions, colloidal particles do not settle down on their own.

Property 5: They cannot be separated from the mixture by the process of filtration because the particles are too small to be caught by filter paper. But, a special technique of separation known as centrifugation can be used to separate the colloidal particles.

Now students, let me explain the components of a colloidal solution. 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. For example, in milk, the fat droplets are the dispersed phase, and water is the dispersion medium.

Colloids are classified according to the state (solid, liquid or gas) of the dispersing medium and the dispersed phase. Let me give you some examples from Table 2.1 in your textbook.

When the dispersed phase is liquid and the dispersing medium is gas, we call it an aerosol. Examples are fog, clouds, and mist. When the dispersed phase is solid and the dispersing medium is gas, it is also an aerosol, like smoke and automobile exhaust.

When the dispersed phase is gas and the dispersing medium is liquid, we call it a foam. Examples are shaving cream. When the dispersed phase is liquid and the dispersing medium is liquid, we call it an emulsion. Examples are milk and face cream.

When the dispersed phase is solid and the dispersing medium is liquid, we call it a sol. Examples are milk of magnesia and mud. When the dispersed phase is gas and the dispersing medium is solid, we call it a foam. Examples are foam, rubber, sponge, and pumice stone.

When the dispersed phase is liquid and the dispersing medium is solid, we call it a gel. Examples are jelly, cheese, and butter. When the dispersed phase is solid and the dispersing medium is solid, we call it a solid sol. Examples are coloured gemstone and milky glass.

Students, as you can see, colloids are very common in everyday life. Milk, butter, cheese, jelly, fog, clouds, smoke, shaving cream, face cream - all these are colloids. This shows how important colloids are in our daily life.

Now, before we move to the next section, let me quickly recap what we have learned about solutions, suspensions, and colloids. Solutions are homogeneous mixtures where the solute particles are very small (less than 1 nanometre) and cannot be seen. They do not scatter light and do not settle down. Suspensions are heterogeneous mixtures where the particles are large enough to be seen with naked eyes. They scatter light, settle down when left undisturbed, and can be filtered. Colloids are heterogeneous mixtures where the particles are intermediate in size - too small to see with naked eyes but large enough to scatter light (Tyndall effect). They do not settle down and cannot be filtered normally but can be separated by centrifugation.

Now students, let us answer some questions from the textbook to check our understanding.

Question 1: Differentiate between homogeneous and heterogeneous mixtures with examples.

Let me answer this. A homogeneous mixture has uniform composition throughout, meaning all parts look the same and have the same properties. Examples are salt dissolved in water, sugar dissolved in water, and air. A heterogeneous mixture has non-uniform composition, meaning you can see different parts with different properties. Examples are sand and salt, oil and water, and soil.

Question 2: How are sol, solution and suspension different from each other?

Let me explain this. A solution is a homogeneous mixture with particle size less than 1 nanometre. The particles cannot be seen, do not scatter light, and do not settle. A suspension is a heterogeneous mixture with particle size larger than 1000 nanometres. The particles can be seen, scatter light, and settle down when left undisturbed. A colloid or sol is a heterogeneous mixture with particle size between 1 and 1000 nanometres. The particles cannot be seen with naked eyes but scatter light (Tyndall effect), and do not settle down easily.

Question 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.

To find the concentration, we need to calculate the mass by mass percentage. Mass of solute (sodium chloride) = 36 g Mass of solvent (water) = 100 g Mass of solution = mass of solute + mass of solvent = 36 g + 100 g = 136 g Mass percentage = (mass of solute / mass of solution) × 100 = (36 / 136) × 100 = 26.47%

So the concentration is approximately 26.47%.

Now students, let us move to the next section which is about physical and chemical changes. In the previous chapter, you have learnt about a few physical properties of matter. The properties that can be observed and specified like colour, hardness, rigidity, fluidity, density, melting point, boiling point, etc., are the physical properties.

Now, what is a physical change? The interconversion of states is a physical change because these changes occur without a change in composition and no change in the chemical nature of the substance. Although ice, water and water vapour all look different and display different physical properties, they are chemically the same. They are all H₂O - just in different states. So when ice melts to form water, or water boils to form steam, it is a physical change because the substance remains water chemically.

Now students, both water and cooking oil are liquid but their chemical characteristics are different. They differ in odour and inflammability. We know that oil burns in air whereas water extinguishes fire. It is this chemical property of oil that makes it different from water.

What is a chemical change? Burning is a chemical change. During this process one substance reacts with another to undergo a change in chemical composition. Chemical change brings change in the chemical properties of matter and we get new substances. A chemical change is also called a chemical reaction.

During burning of a candle, both physical and chemical changes take place. Can you distinguish these? The wax melting and flowing down the candle is a physical change, but the wax burning and producing carbon dioxide and water vapour is a chemical change.

Now students, let me give you some examples of physical and chemical changes. Physical changes include cutting of trees, melting of butter in a pan, boiling of water to form steam, and dissolving common salt in water. Chemical changes include rusting of almirah, burning of paper and wood, and passing of electric current through water which breaks it down into hydrogen and oxygen gases.

Now, let us move to the next important section which is about types of pure substances. On the basis of their chemical composition, substances can be classified either as elements or compounds.

First, let us understand what an element is. Robert Boyle was the first scientist to use the term element in 1661. Antoine Laurent Lavoisier (1743–94), a French chemist, was the first to establish an experimentally useful definition of an element. He defined an element as a basic form of matter that cannot be broken down into simpler substances by chemical reactions.

Elements can be normally divided into three categories: metals, non-metals, and metalloids.

Metals usually show some or all of the following properties: - They have a lustre (shine). For example, gold and silver shine. - They have silvery-grey or golden-yellow colour. - They conduct heat and electricity very well. This is why cooking utensils are made of metal. - They are ductile, which means they can be drawn into wires. For example, copper can be drawn into thin wires. - They are malleable, which means they can be hammered into thin sheets. For example, gold can be hammered into very thin sheets called gold leaf. - They are sonorous, which means they make a ringing sound when hit. For example, when you hit a metal plate, it makes a ringing sound.

Examples of metals are gold, silver, copper, iron, sodium, potassium, etc. Mercury is the only metal that is liquid at room temperature. This is very interesting - most metals are solid at room temperature, but mercury is liquid. Students, you must have seen thermometers that contain mercury.

Non-metals usually show some or all of the following properties: - They display a variety of colours. For example, iodine is purple, chlorine is greenish-yellow, and carbon (in its different forms) can be black or grey. - They are poor conductors of heat and electricity. This is why we use plastic or rubber to cover electrical wires - they are non-metals and do not conduct electricity. - They are not lustrous, sonorous or malleable. Most non-metals are brittle - they break when hammered.

Examples of non-metals are hydrogen, oxygen, iodine, carbon (coal, coke), bromine, chlorine, etc. Bromine is the only non-metal that is liquid at room temperature (besides mercury which is a metal).

Now students, some elements have intermediate properties between those of metals and non-metals. They are called metalloids. Examples are boron, silicon, germanium, arsenic, antimony, and tellurium. These elements have properties of both metals and non-metals. For example, silicon is a semiconductor - it conducts electricity better than non-metals but not as well as metals. This property makes silicon very important in making computer chips and electronic devices.

Now, let me tell you some interesting facts about elements. The number of elements known at present are more than 100. Ninety-two elements are naturally occurring and the rest are man-made. Majority of the elements are solid. Eleven elements are in gaseous state at room temperature. Two elements are liquid at room temperature - mercury and bromine. Elements gallium and cesium become liquid at a temperature slightly above room temperature (303 K). So if you hold gallium in your hand, it will melt because your body temperature is enough to melt it!

Now students, let us understand the difference between mixtures and compounds. This is a very important topic. Look at Activity 2.4 from your textbook. Divide the class into two groups. Give 5 g of iron filings and 3 g of sulphur powder in a china dish to both the groups.

Group I: Mix and crush iron filings and sulphur powder together.

Group II: Mix and crush iron filings and sulphur powder together. Then heat this mixture strongly till red hot. Remove from flame and let the mixture cool.

Now, both groups should check for magnetism in the material obtained. Bring a magnet near the material and check if the material is attracted towards the magnet.

Students, what do you think will happen? Group I has just mixed iron and sulphur physically. The iron filings can still be attracted by a magnet because they have not changed chemically. But group II has heated the mixture strongly, and a chemical reaction has taken place. The iron and sulphur have combined to form a new compound called iron sulphide. This compound does not have magnetic properties like iron does.

Now, let us add carbon disulphide to one part of the material obtained. Stir well and filter. Carbon disulphide dissolves sulphur but not iron sulphide. So in group I, the sulphur will dissolve, and iron filings will remain. But in group II, since iron and sulphur have combined to form iron sulphide, the carbon disulphide will not dissolve the iron sulphide in the same way.

Now, let us add dilute sulphuric acid or dilute hydrochloric acid to the other part of the material obtained. (Note: teacher supervision is necessary for this activity). In group I, iron will react with the acid to produce hydrogen gas, which is colourless and odourless. In group II, iron sulphide will react with the acid to produce hydrogen sulphide gas, which has the smell of rotten eggs.

So students, what did we learn from this activity? The material obtained by group I is a mixture of the two substances. The properties of the mixture are the same as that of its constituents - you can still attract the iron with a magnet, and you can separate the sulphur using carbon disulphide. But the material obtained by group II is a compound. On heating the two elements strongly, we get a compound which has totally different properties compared to the combining elements. The composition of a compound is the same throughout. The texture and the colour of the compound are the same throughout.

Now students, let me summarize the differences between mixtures and compounds in a table format that you should remember for your exams.

Mixtures: 1. Elements or compounds just mix together to form a mixture and no new compound is formed. 2. A mixture has a variable composition. You can have any proportion of the components. 3. A mixture shows the properties of the constituent substances. For example, a mixture of iron and sulphur will still be attracted by a magnet. 4. The constituents can be separated fairly easily by physical methods like filtration, evaporation, etc.

Compounds: 1. Elements react to form new compounds. 2. The composition of each new substance is always fixed. For example, water is always H₂O - two hydrogen atoms for every oxygen atom. 3. The new substance has totally different properties. For example, sodium (a reactive metal) and chlorine (a poisonous gas) combine to form sodium chloride (common table salt), which is safe to eat. 4. The constituents can be separated only by chemical or electrochemical reactions. You cannot separate water into hydrogen and oxygen by simple physical methods; you need electrolysis.

Now students, we have covered the main concepts of the chapter. Let me now solve all the exercises from the textbook for you.

Exercise Question 1: Which separation techniques will you apply for the separation of the following?

(a) Sodium chloride from its solution in water For this, we use the technique of evaporation. When we heat the solution, water evaporates and sodium chloride remains behind as residue.

(b) Ammonium chloride from a mixture containing sodium chloride and ammonium chloride We can use the technique of sublimation. Ammonium chloride sublimes - it directly changes from solid to gas when heated, while sodium chloride does not. So if we heat the mixture, ammonium chloride will vaporize and can be collected, leaving sodium chloride behind.

(c) Small pieces of metal in the engine oil of a car We can use the technique of filtration. The metal pieces are solid and can be filtered out from the oil.

(d) Different pigments from an extract of flower petals We can use the technique of chromatography. This separates different pigments based on their solubility and movement through a filter paper.

(e) Butter from curd We can use the technique of churning or centrifugation. Butter separates from curd because it is less dense.

(f) Oil from water We can use the technique of separating funnel. Oil and water are immiscible liquids, so they form separate layers and can be separated using a separating funnel.

(g) Tea leaves from tea We can use the technique of filtration. Tea leaves remain on the filter paper as residue, and the tea (filtrate) passes through.

(h) Iron pins from sand We can use the technique of magnetic separation. Iron is magnetic and will be attracted to a magnet, while sand is not.

(i) Wheat grains from husk We can use the technique of winnowing. The lighter husk is blown away by wind, while the heavier wheat grains fall down.

(j) Fine mud particles suspended in water We can use the technique of filtration or sedimentation. The mud particles will settle down at the bottom (sedimentation) or can be filtered out.

Exercise Question 2: Write the steps you would use for making tea. Use the words solution, solvent, solute, dissolve, soluble, insoluble, filtrate and residue.

Let me write the steps for making tea:

First, we take water in a pan. Water is the solvent. Now we add tea leaves and sugar to the boiling water. Tea leaves and sugar are solutes. Sugar is soluble in water, so it dissolves completely to form a solution. Tea leaves are insoluble in water, so they do not dissolve. We boil the mixture for some time. The colour of the water changes as the tea leaves release their pigments. This is because the soluble components of tea leaves dissolve in the water. Now we strain the tea using a strainer or filter. The tea leaves remain on the strainer as residue, and the liquid that passes through is the filtrate, which is our tea. The tea is a solution of water, sugar, and tea extracts. We can add milk to make it more delicious. Milk mixes uniformly with the tea to form a homogeneous mixture.

Exercise Question 3: Let me solve this question about solubility.

(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?

From the table, at 313 K, the solubility of potassium nitrate is 62 grams per 100 grams of water. This means 62 grams of potassium nitrate dissolves in 100 grams of water to form a saturated solution.

For 50 grams of water, the amount of potassium nitrate needed would be: (62 / 100) × 50 = 31 grams

So, 31 grams of potassium nitrate would be needed.

(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.

At 353 K, the solubility of potassium chloride is 54 grams per 100 grams of water. As the solution cools, the solubility decreases. This means that at lower temperatures, the water can hold less potassium chloride. So as the solution cools, some of the potassium chloride will crystallize out and settle at the bottom of the container. This is because the amount of solute exceeds the new saturation point at the lower temperature.

(c) Find the solubility of each salt at 293 K. Which salt has the highest solubility at this temperature?

From the table, at 293 K: - Potassium nitrate: 32 g per 100 g water - Sodium chloride: 36 g per 100 g water - Potassium chloride: 35 g per 100 g water - Ammonium chloride: 37 g per 100 g water

Ammonium chloride has the highest solubility at 293 K with 37 grams per 100 grams of water.

(d) What is the effect of change of temperature on the solubility of a salt?

The solubility of most salts increases with increase in temperature. This means that at higher temperatures, more solute can be dissolved in the same amount of solvent. However, for some salts like sodium chloride, the solubility does not change much with temperature.

Exercise Question 4: Explain the following giving examples.

(a) Saturated solution: A saturated solution is one that contains the maximum amount of solute that can be dissolved at a given temperature. For example, if you keep adding sugar to water while stirring, eventually the sugar will stop dissolving and settle at the bottom. At that point, the solution is saturated.

(b) Pure substance: A pure substance is one that consists of a single type of particle and cannot be separated into different substances by physical methods. Examples are water (H₂O), gold (Au), oxygen (O₂), etc.

(c) Colloid: A colloid is a heterogeneous mixture in which the particle size is intermediate between solution and suspension. The particles are small enough to not be seen with naked eyes but large enough to scatter light (Tyndall effect). Examples are milk, fog, jelly, etc.

(d) Suspension: A suspension is a heterogeneous mixture in which the solute particles do not dissolve but remain suspended throughout the medium. The particles are large enough to be seen with naked eyes and will settle down if left undisturbed. Examples are chalk powder in water, sand in water, etc.

Exercise Question 5: Classify each of the following as homogeneous or heterogeneous mixture. soda water, wood, air, soil, vinegar, filtered tea.

Let me classify each: - Soda water: Homogeneous (the carbon dioxide is dissolved uniformly in water) - Wood: Heterogeneous (wood has different components like fibers, etc.) - Air: Homogeneous (gases are uniformly mixed) - Soil: Heterogeneous (soil has sand, clay, organic matter, etc.) - Vinegar: Homogeneous (acetic acid is uniformly dissolved in water) - Filtered tea: Homogeneous (after filtering, the tea is uniform)

Exercise Question 6: How would you confirm that a colourless liquid given to you is pure water?

We can confirm that a colourless liquid is pure water by: 1. Checking its boiling point - pure water boils at 100°C (373 K) at sea level. 2. Checking its melting point - pure water freezes at 0°C (273 K). 3. Checking its density - pure water has a density of 1 g/mL at 4°C. 4. Using a simple test - if we add a few drops of pure water to anhydrous copper sulphate, it will turn blue, indicating the presence of water. 5. Another test is to check if it conducts electricity - pure water is a poor conductor, but impurities make it conduct electricity.

Exercise Question 7: Which of the following materials fall in the category of a "pure substance"? (a) Ice - Yes, ice is pure water in solid state. (b) Milk - No, milk is a mixture of water, fats, proteins, etc. (c) Iron - Yes, iron is an element, so it is a pure substance. (d) Hydrochloric acid - No, hydrochloric acid is a solution of hydrogen chloride gas in water, so it is a mixture. (e) Calcium oxide - Yes, calcium oxide is a compound, so it is a pure substance. (f) Mercury - Yes, mercury is an element, so it is a pure substance. (g) Brick - No, brick is made of clay and other materials, so it is a mixture. (h) Wood - No, wood is a complex mixture of cellulose, lignin, etc. (i) Air - No, air is a mixture of many gases.

So the pure substances are: (a) Ice, (c) Iron, (e) Calcium oxide, (f) Mercury.

Exercise Question 8: Identify the solutions among the following mixtures. (a) Soil - No, soil is a heterogeneous mixture. (b) Sea water - Yes, sea water is a homogeneous mixture of water, salt, and other minerals. (c) Air - Yes, air is a homogeneous mixture of gases. (d) Coal - No, coal is a heterogeneous mixture of carbon and other substances. (e) Soda water - Yes, soda water is a solution of carbon dioxide in water.

So the solutions are: (b) Sea water, (c) Air, (e) Soda water.

Exercise Question 9: Which of the following will show "Tyndall effect"? (a) Salt solution - No, salt solution is a true solution and does not show Tyndall effect. (b) Milk - Yes, milk is a colloid and shows Tyndall effect. (c) Copper sulphate solution - No, copper sulphate solution is a true solution and does not show Tyndall effect. (d) Starch solution - Yes, starch solution is a colloid and shows Tyndall effect.

So the answers are (b) Milk and (d) Starch solution.

Exercise Question 10: Classify the following into elements, compounds and mixtures. (a) Sodium - Element (b) Soil - Mixture (c) Sugar solution - Mixture (sugar is a compound, but sugar solution is a mixture of sugar and water) (d) Silver - Element (e) Calcium carbonate - Compound (contains calcium, carbon, and oxygen) (f) Tin - Element (g) Silicon - Element (h) Coal - Mixture (i) Air - Mixture (j) Soap - Compound (or mixture, depending on the type) (k) Methane - Compound (contains carbon and hydrogen) (l) Carbon dioxide - Compound (contains carbon and oxygen) (m) Blood - Mixture (contains plasma, cells, etc.)

Exercise Question 11: Which of the following are chemical changes? (a) Growth of a plant - Chemical change (photosynthesis and other metabolic processes) (b) Rusting of iron - Chemical change (iron reacts with oxygen and moisture to form iron oxide) (c) Mixing of iron filings and sand - Physical change (no new substance is formed) (d) Cooking of food - Chemical change (new substances are formed during cooking) (e) Digestion of food - Chemical change (food is broken down into simpler substances) (f) Freezing of water - Physical change (water changes from liquid to solid, but remains water) (g) Burning of a candle - Chemical change (wax burns to produce carbon dioxide and water vapour)

So the chemical changes are: (a) Growth of a plant, (b) Rusting of iron, (d) Cooking of food, (e) Digestion of food, (g) Burning of a candle.

Now students, there is also a group activity at the end of the chapter. Let me explain it briefly. The activity asks you to take an earthen pot (matka), some pebbles and sand, and design a small-scale filtration plant that you could use to clean muddy water. This is a simple water filtration experiment. You can layer the earthen pot with sand at the bottom, then pebbles, and then more sand. When you pour muddy water through this setup, the sand and pebbles will trap the mud particles, and cleaner water will collect at the bottom. This is similar to how water is filtered in nature and in water treatment plants.

Now students, let me give you a complete summary of everything we have learned in this chapter.

In this chapter, we learned about:

1. Pure substances and mixtures: A pure substance has only one type of particle and cannot be separated by physical methods. A mixture contains two or more pure substances that can be separated by physical methods.

2. Types of mixtures: Mixtures can be homogeneous (uniform composition throughout) or heterogeneous (non-uniform composition).

3. Solutions: A solution is a homogeneous mixture of two or more substances. It has a solvent (the component that dissolves) and a solute (the component that is dissolved). Properties of solutions include that the particles are smaller than 1 nanometre, they do not scatter light, and they do not settle down.

4. Concentration of a solution: It is the amount of solute present in a given amount of solution. It can be expressed as mass by mass percentage, mass by volume percentage, or volume by volume percentage.

5. Saturated solution: A solution that contains the maximum amount of solute that can be dissolved at a given temperature. The amount of solute in a saturated solution is called its solubility.

6. Suspension: A heterogeneous mixture in which solid particles are dispersed in a liquid but do not dissolve. The particles are visible to the naked eye, scatter light, and settle down when left undisturbed.

7. Colloids: A heterogeneous mixture with intermediate particle size. The particles are too small to see with naked eyes but large enough to scatter light (Tyndall effect). Examples include milk, fog, jelly, etc.

8. Physical and chemical changes: Physical changes do not form new substances, while chemical changes result in the formation of new substances.

9. Elements: Pure substances that cannot be broken down into simpler substances by chemical reactions. They can be metals, non-metals, or metalloids.

10. Compounds: Pure substances composed of two or more elements chemically combined in a fixed proportion. They have properties different from their constituent elements.

11. Differences between mixtures and compounds: Mixtures have variable composition, show properties of constituents, and can be separated by physical methods. Compounds have fixed composition, have different properties from constituents, and can only be separated by chemical methods.

Students, this is a very important chapter that forms the foundation for understanding chemistry. I hope you all have understood the concepts clearly. Remember to practice the exercises and revise the definitions. Thank you for your attention, and I'll see you in the next class. Goodbye and keep studying!