 We have a bunch of substances here, so to start off with we have helium gas in this in these balloons. This is coal. This right here is, this is salt or NaCl, sodium chloride. This over here is soil. This is a box of tin. This is a box of tin. Pure tin. This right here is water, H2O. And this is sand mixed with water. And you can think about which of these substances can be called as a pure substance and which of these can be called as a mixture. By the end of this video, we will be able to answer this question. We will be able to categorize many more substances under either pure or mixtures and to do that we should clearly understand the difference between these two. So let's begin. Now a pure substance can be either, can be either an element, can be an element or a compound. We know that elements can be metals, non-metals, even metalloids and compounds are formed when two or more elements join together. So for example, let's say we take, let's say we take helium gas. So this helium, it's a single element and because it's a single element, it can be called as a pure substance. It's a pure substance. This is helium right here. Now if we take water, H2O. Now over here, we can see that it is formed with two atoms, hydrogen and oxygen. So it's not really an element. When two or more elements combine, they form a compound. So H2O, H2O is really a compound. But still it's a pure substance. We can take more examples. So let's say we take salt, NaCl. Again, this is made up of two atoms, sodium and chlorine. Now this is also a pure substance. Not an element, but it's a compound, but still it's a pure substance. So NaCl comes right over here. Now a mixture is formed when elements are compounds or let's call them pure substances and pure substances mix with each other. So let's say we have, we have, let's say we have helium plus water. Let's say we have it in a beaker. Now this will be a mixture. This right here, it will be a mixture. Or if we look at the air around us. Now the air around us is made up of many gases. It has, it has nitrogen. It has oxygen, nitrogen, oxygen, some helium, some carbon dioxide. All of these gases, they make up the air around us. And therefore air is a mixture. These elements, these compounds, they are mixing with each other to form a mixture. Now let's take, let's take salt water. Let's take salt water. So salt water is really, it's really NaCl. It's sodium chloride, salt mixed with water. Now individually sodium chloride is a compound. It's a pure substance. Individually water is a pure substance. But when you, when you mix them, when two or more than two pure substances mix with each other, they form a mixture. So salt water that is NaCl plus H2O. This is a mixture. Now there is a major point of difference between pure substances and mixtures. That is pure substances, they have a fixed or a definite, fixed or a definite composition. And mixtures, mixtures, they have variable, variable composition. Composition. All right. What, what does that mean though? So let's take the example of salt water and understand what fixed and variable composition might mean. Now we can take five grams of salt. You can take five grams of salt and we can mix it with, you can mix it with 95 grams of water. So that will give us, that will give us really 5% salt solution. Or we can take, we can take 20 grams of salt and mix it with 80 grams of water. This will give us 20% salt solution. Now we can see that a mixture like salt water can have different, different compositions. There can be slightly less concentrated, like a 5% salt solution. There can be a highly concentrated salt solution, like a 20%. It can even go like 50%. So mixtures can have different or variable compositions. But within this particular solution, that is 5% salt solution, the salt is distributed uniformly. So we can call it uniform distribution, that is salt is distributed uniformly in this 5% salt solution. Similarly, salt is uniformly distributed in this 20% salt solution. So that really means if you have a salt solution, salt solution like this, you will have salt spread out throughout the body of the water, throughout the body of the water and showing salt with black, throughout the body of the water. NaCl is distributed evenly throughout the body of the, of the solvent, whatever the solvent is in this case, water. That is a uniform distribution. And mixtures can have uniform distribution. They can also have non-uniform distribution. So think of the image that you saw in the beginning, that was sand mixed with water, Now all the sand was settled at the bottom. It was not evenly distributed throughout the body of the water. That was non-uniform distribution. And mixtures can have two types, two types of distributions. Like I said, there is a name to that. Those mixtures that have a uniform distribution, they are called homogeneous mixtures. Homo gen OUS, I guess it's right, homogeneous mixtures. And then there is heterogeneous mixtures, heterogeneous mixtures, H-E-T-E-R-O heterogeneous mixtures. So homogeneous have uniform distribution, heterogeneous have non-uniform distribution, like chalk in water or sand in water. Homogeneous would be salt water or sugar in water, but both of them, they have variable composition. They have different composition, therefore they are mixtures. And you can see how they can have variable composition. Like you can have 10 grams sand in some water and you can add more sand in the same amount of water. So you have a variable composition, even for a heterogeneous mixture. Now if we come to pure substances, let's say we take a beaker and we take 10 ml of water. And we take one more beaker. We can take one more beaker. And then take, in that beaker we can have 50 ml of water. The only thing that we can change of pure substances is the amount that we take. We can take 10 ml, 50 ml, 1 liter. But in each case, the composition of water remains the same. This right here is 100% pure water. This right here is 100% pure water. Composition remains fixed no matter how much you take of that substance. Alright, so this is a major point that we can understand that is fixed and variable composition. Let's move on and talk more about how these two pure substances and mixtures, how they differ. So if we think about mixtures, if let's say we take salt water again, if we take salt water, we can separate it, we can separate it by physical means. We can separate it by physical means or physical methods. And pure substances can be separated by chemical methods. So what are some examples of these? Well for physical methods, you can think about evaporation, you can think about filtration, chromatography, crystallization, all of these are physical methods of separation. So let's say if you have, if you have some salt water in a crucible, you have some salt water and you can try and heat it, you can try and heat it from below. You can have a flame, you can heat it from below. After some time, all the water will really just get evaporated and all that will be left will be pure, pure salt, pure salt. So this is one way of separating a mixture and there can be, there can be filtration as well. So for example, if you have sand water, then you can filter out all the sand from that water. Again, that's a physical means of separation. Now for chemical, let's say if we have, if we have water, if you want to separate hydrogen and oxygen from water, we can't really have any of these physical methods do the work because if you vapor, if you heat water, it will just evaporate, right? So for separating water into hydrogen and oxygen, what we do is, we can take, we take, we take an electrolytic, electrolytic cell. So this one has, it has two electrodes. You can have both of the electrodes of graphite and they are connected to a cell or a battery. And then you have an electrolyte. You have an electrolyte. It's mostly water. So it's mostly water and also some sodium chloride is also some, some sodium chloride is also present in it and it also has water. Now, if when the current is passed through this, through the circuit, what starts happening is water starts breaking down into hydrogen and oxygen. So you start seeing hydrogen gas bubbles in one of the electrodes and oxygen gas bubbles in the other electrode. Now this is a chemical means of separation and that's how compounds can be separated or they can be broken down into their elements. But for doing that, you need to have a chemical means of separation. So this is one more difference between pure substances and mixtures that is the constituents or things that make up a compound. They can only be separated by chemical means and the constituents or the things that make up a mixture can be separated by physical means like evaporation, crystallization, filtration, sedimentation. All right, let's look at one last difference. So one last difference that really is that pure substances and we can think of compounds like water, sodium chloride or carbon dioxide. We can write compounds have different properties, different properties from their constituents. So this is really a fancy word but what it really means is anything that makes up a compound. So hydrogen and oxygen makes up water, right? So hydrogen and oxygen would be constituents of water. Compounds have different properties from their constituents but mixtures have the same properties as their constituents. All right, what does this mean now? Let's take the example of air in this case for mixtures. So if we take the example of air, we know that air is made up of nitrogen, there is oxygen, there is carbon dioxide, there is helium and if we look at the properties of air, all the properties that air shows are the same properties that these gases have. So for example, you can light up a matchstick in air and that is because oxygen supports combustion. If you remove oxygen and just look at it individually, even then it will support combustion. Each gas in this mixture really retains its own properties. We can take one more example, we can think about salt water. Again, if you look at this mixture of salt water, salt and water, they retain their properties even when they are mixed. And what are the properties of salt really? Well, you can think about how it tastes or when it starts melting. So if you look at salt apart from water, it will melt at a certain temperature. It's very high. If you mix it with water, it will still melt at that temperature, whatever that temperature is. So salt and water, both of them, they really retain their properties when they are mixed. So mixtures, they have the same properties as their constituents. But if we look at compounds, for example, let's say we look at water. Now, water has very, very different properties from its constituents, that is hydrogen and oxygen. Hydrogen and oxygen are gases at room temperature, but water is a liquid. Also, oxygen supports combustion. It helps things burn. But water, you know, it's the opposite effect, right? So compounds, they have different properties from their constituents. We can take one more example, we can think about carbon dioxide. Carbon dioxide is made up of carbon. It says oxygen. Carbon is solid at room temperature. It can exist in the form of graphite, diamond. And oxygen is a gas at room temperature. So again, the properties of constituents are different from the properties of the compound, that is carbon dioxide. Carbon dioxide is used in fire extinguishers, prevents fire, it extinguishes fire. And oxygen, it helps burning, it supports combustion. So drastically different properties. And that is why we say that compounds have different properties from the constituents. All right, so let's summarize the differences now. Let me, let me write them over here. So this is pure substances and mixtures. Now the first difference is that pure substances are, they are really elements and compounds, single atom or atoms combined to form a compound. Mixtures are really formed when elements and compounds mix together. So formed due to mixing of elements and compounds. And when they mix, no new substances formed really, right? Air is not a new substance. It's just a mixture of many different things. But when elements like hydrogen and oxygen combine, they form something new. They form water. And that's a compound. Now the second difference is that they have a fixed composition. And mixtures, they have a variable composition. It doesn't matter how much of an element or a compound you take, the composition will be fixed. It doesn't change. But a mixture like, for example, salt water can have different compositions. It can be a 5% salt solution, 20%, 30% salt solution. It can have different composition. But within that one solution for salt water, it will be uniformly distributed. So such mixtures are formed, are called homogenous mixtures. So that is when the distribution is uniform. But that is not true for all types of mixtures. If we have sand water, then they're not really uniformly distributed, right? It's a non-uniform distribution. So even mixtures can have some types that is homogenous and heterogeneous. But both of them, both of them, they have variable composition. And you can understand that and you can understand that by looking at a salt water example in this video. All right. The third difference, the third difference is that pure substances show different properties from its constituents. And mixtures, they show the same properties. The last difference is that mixtures can be separated by physical means, evaporation, crystallization, chromatography. And for pure substances, you need to have chemical means of separation. To break down compounds, you need to have chemical means of separation. But to separate air into its constituent particles, you can have physical means. To separate salt water into salt and water individually, you can have physical means. So these are the differences between pure substances and mixtures. Now if we go back to this list of substances, we can try and categorize them under pure and mixtures, pure substances and mixtures. So helium, that's a pure substance. Coal is really a mixture, mixture of many, many hydrocarbons, compounds that are formed from hydrogen and carbon. And you will learn more about coal later in your courses in chemistry. Sodium chloride is a compound. Soil again is a mixture of many different things, many different types of compounds and elements. Pure tin, it's a metal, it's a pure substance. Water is a pure substance, it's a compound. Sand mixed in water, this is a mixture, right? It has a variable composition. You can have many, you can have a lot of sand in the same amount of water. You can have very little sand in the same amount of water. It has a variable composition. You can also separate it by filtration or sedimentation, physical means of separation. So sand water is a mixture.