 It gets very cold in winter and that's why I like to use my heater. It gives out a lot of heat, whereas in summers it gets very hot and that's when I like to use my AC. It takes in a lot of heat. So just like these devices, we also have certain chemical reactions that give out a lot of heat or take in a lot of heat. For example, if you burn coal, coal is mostly made of carbon, so when you burn it, you get a lot of heat and light energy. And another example is if you burn a magnesium ribbon, it will burn with a dazzling white flame giving out a lot of heat and light energy. So these kind of reactions that give out a lot of energy are called exothermic reactions and you can guess the meaning of it by knowing the meaning of the words. See, exo, it means external or out and thermic, this means something to do with heat, meaning the reactions that give out heat, they will be called exothermic reaction. Whereas we have some other kind of reactions, like burning of iron sulphate. Now when you burn this, there will be a color change happening. There will be some gases that will be released. So definitely this is a chemical reaction. But in this reaction, we are not getting a lot of heat or light energy. In fact, we had to supply a lot of heat energy to initiate this chemical reaction. Now these kind of chemical reactions are called endothermic chemical reactions. Now the meaning of this is endo means internal or in and thermic means heat, something to do with heat. So the reactions in which heat is taken in, they are called as endothermic reactions. Now before we look at other examples, let me first tell you why certain reactions give out heat and why certain other reactions take in heat. Now let's look at the atomic picture. So if you have some atoms that are sticking together in a molecule and you want to pull them apart, so for this you need to provide some energy. Energy is required to pull away the atoms in a molecule. Now we can understand this from a magnet analogy. If there are two magnets that are attracting and are stuck together. So to pull them apart, we need to provide some energy. Now on the contrary, if there are two atoms that are attracting each other and if we let go of them, then they will come together and in this process energy will be released. Now we can understand this also from a magnets analogy. If there are two magnets that are attracting each other and we let go of them, then they will come together and stick. They will be sound generated, right? And sound is also a form of energy. So in this example, if two magnets are attracting and we let go, they come and stick together and sound energy is released. Similarly over here, when two atoms are attracting each other and we let go of them, they come together and energy is released. Now see in any chemical reaction, there will be rearrangement of atoms happening. Few atoms will be going away from each other and few atoms will be coming together. Now whenever two atoms are going away, we will need to provide energy for this. And if two atoms are attracting and they are coming together, then in this process energy will be released. Now if in total, a lot of energy is being released in the chemical reaction, we call it exothermic reaction. Whereas if in total, a lot of energy was required, then we call that reaction as endothermic reaction. You can understand this more clearly from an example of buying and selling. Imagine that you are buying a phone and you bought it for rupees 100. Rupees 100. So you invested 100 rupees in buying it. But you sold this phone and you were able to sell it for rupees 200. 200. So in total, you made profit in this transaction. This transaction gave out more money. So you can call that this transaction was an exo money process. It gave out more money. Whereas imagine you had bought this phone for the same price, rupees 100, but you were able to sell it for a lower price, say rupees 50. In this case, you lost money in this transaction. This transaction took away money from you. So we'll call this process as endo money process. So similarly, you can see that processes that give out in total more energy are called exothermic. Whereas the chemical reactions that take away more energy are called endothermic. Now that you know a lot about exo and endothermic reactions, let's look at some examples of that. So here I have a magnesium ribbon and I'm going to burn it. Now you can see that it's burning with a dazzling white flame. It's giving a lot of light and heat energy. So clearly this is an exothermic reaction. It gives out a lot of heat energy. Now let's write down its chemical equation. So first we took magnesium ribbon and we burnt it in air. When something burns in air, it actually reacts with the oxygen in the air because oxygen gas is very reactive. Now this, these two reactants, they are going to combine to give me one single product. They are going to give me magnesium oxide, MgO. And this is the white residue that we see in the end. Now let me quickly balance this equation. Here we have 2Mg and we'll get here 2MgO. Now this reaction is balanced. Now see, in this reaction, we are getting a lot of heat. This is an exothermic reaction. So a lot of heat is being generated along with the product. So that's why I will write plus heat on the product side, okay? To represent that a lot of heat is being generated. Or instead of heat, you can write plus delta. Delta is a Greek letter that is used to represent heat in short form, okay? So this is how you write an exothermic reaction. Let's look at another example. If we burn coal, something very similar happens. This coal has carbon and when it burns in air, it reacts with the oxygen to form carbon dioxide. And in the process, a lot of heat is being released. And a chemical reaction looks something like this. Carbon plus oxygen gives carbon dioxide plus heat on the product side. Let's look at another example. So here I have a white powder, calcium oxide. Now if I add water to it, we will get a white solution, milky white solution. Now the fun part over here is that if you touch the container in which this reaction is happening, that container will feel warm to you. That means in this reaction, a lot of heat is being generated. That's strange, right? You're adding water to something and instead of getting cold, it's getting hotter. So let's write down its chemical reaction. So here we took calcium oxide and we combine this with water H2O. And in the product side, we are getting calcium hydroxide. Calcium CaOH2. And along with this, also a lot of heat is being generated. Heat plus delta. So these are some examples of exothermic reactions. And you also saw how do we represent them in chemical equations. We write plus heat on the product side because along with the product, we get a lot of heat energy. Now one more thing I want you to focus on is that, see, in all of these three reactions, there are reactants that are combining to give me one single product. Reactants are combining to give me one single product. Same thing is happening here. Now, see these kind of reactions are called combination reactions and we will talk about them in detail in a separate video. But over here, we can see that in general, combination reactions are exothermic in nature. They release heat. So combination reactions in general are exothermic. Not always. There are some exceptions to it. But we can say that in general, they are exothermic. Now with this, let's move on to examples of endothermic reactions. So here I have light green crystals of iron sulfate. And I'm going to heat them. Now, when I do that, I notice a color change happening. In fact, there's also some smelly gas that is evolving. So definitely there is some chemical reaction that is going on. But see, in this process, I'm not getting any light or heat energy. In fact, I had to provide a lot of heat energy for this reaction to start. That means this reaction is an endothermic reaction. It requires a lot of heat energy. It takes in a lot of heat energy. Let's write down its chemical reaction. So iron sulfate, when heated, it breaks down. It decomposes into iron oxide, sulfur dioxide and sulfur trioxide. And in this process, not a lot of heat is being generated. In fact, a lot of heat is being required to initiate this process. Now, to represent that, we write plus heat on top of this arrow, plus heat, or we can even write delta, the Greek letter delta. Now, you might be wondering why on top of this arrow and why not on the reactant side? Well, I don't know. Maybe this is a convention that we follow. So this represents that heat was required to initiate this reaction. Now, let's look at another example. I have a white powder of silver chloride. And I'm going to leave it outside under the sun or any light for that matter. After some time, I see that there is a color change over here. The white powder is turning grayish in color. So what's happening over here is that silver chloride, it's taking in energy from sunlight and it's breaking down into silver and chlorine gas. So this is also an example of endothermic reaction. Here, the compound takes in energy from in the form of sunlight. So to represent that, I will write sunlight on top of this arrow. Now, let's look at another example. Here, when I pass electricity through water, I can see that some gases are evolving. Now, what's happening over here in this case is that when we pass electricity, the water takes in energy from the electricity and breaks down decomposes into hydrogen gas and oxygen gas. And this is also an example of endothermic reaction. Here, the energy is taken in in the form of electricity. So I will write that on top of the arrow, electricity, yes. So these were some examples of endothermic reaction. Here in all of these examples, energy is being taken in. Now, you can see one common thing that is happening in all of these three reactions. There's one single reactant that is breaking down or decomposing into multiple products. Now, such kind of reactions are called decomposition reactions. And we have talked about that in detail in a separate video. But over here, I want to point out that in general, decomposition reactions are endothermic in nature. They require more energy for their reactions. So we can say decomposition reactions in general. Not always, there are some exceptions, but in general are endothermic in nature. Okay, now let's summarize the video. We saw that in exothermic reactions, heat is given out. Whereas in endothermic reaction, heat is taken in. We also saw that to represent exothermic reaction, we write plus heat or delta on the product side. Whereas to represent endothermic reactions, we write heat or energy on top of the arrow.