 You might have seen these erupting volcanoes in a science fair or you might have made one for your science project. In this video, we are going to talk about the type of chemical reaction that makes this happen. Double displacement reaction. And towards the end of the video, I will also show you by making one volcano. So let's begin. So if I have to explain you in short, in double displacement reactions, the ions of the reactant, they exchange position, something like this. Now let's see how to write this in a chemical equation. So here I have two random reactants, AB and CD. Now if double displacement reaction is happening here, then the ions are going to exchange position. So here in this reaction, A will go and displace C and form AD. Whereas C will go and displace A in turn and form CB. And finally our products are going to be AD and CB. Now you can see that why this is called a double displacement reaction. Because in a way, we can see that A is displacing C and C is in turn displacing A. And since there are two displacement, so we can call it a double displacement reaction. Okay, now let's look at an actual reaction. So here I have hydrogen chloride and sodium hydroxide. And when they react, it is going to be a double displacement reaction. Meaning the ions are going to exchange position here. So here hydrogen ion is going to exchange position with sodium ion, let's say. So here hydrogen will displace or take the position of sodium and form HOH. Or I can say H2O, which is water. And sodium will in turn take the position, displace hydrogen and form sodium chloride, NaCl. And this is going to be our product. HOH or H2O and sodium chloride. Now I've got two reactions and both of these are double displacement reactions. Now I want you to pause the video and try to find out the product of these by yourself first. Now if you have tried it, let's see. So here in this reaction, since this is double displacement, ions are going to exchange position. Here sodium will exchange position with barium. So okay, sodium displaces barium in one sense and forms sodium chloride. And in turn barium takes the place of sodium and forms barium sulphate. See the product is going to be sodium chloride and barium sulphate. Now let's look at this reaction. So here also double displacement, so ions exchange. So sodium takes in the place of hydrogen and forms sodium chloride. And hydrogen in turn takes the place of sodium and forms hydrogen sulphide. So the product is going to be sodium chloride and hydrogen sulphide. So okay, with this you understand the basic structure of double displacement reactions. But you might be having many questions. Like over here, why sodium only exchanging position with hydrogen? Why can't it exchange position with let's say chlorine? Or can sulphur exchange position with chlorine? Or what is happening at the core? Like how are these reactions even taking place? Now all of these questions are amazing questions. But to understand these, let's look at a little bit in detail. So here I have sodium sulphide, Na2S. Now if I put this in water, meaning I am taking an aqueous solution of sodium sulphide. Then in that case this molecule is going to dissociate or break down into its ions. I will get sodium ion and sulphur ion. Something like this. Here I will get sodium ion and sulphur minus 2 ion. Now this is nothing to be worried about. Many molecules when put in water they will dissociate into its ions. Similarly over here HCl when put in water. This will also dissociate into hydrogen ion and chlorine ion. Now see why certain molecules dissociate in water is something that we will talk about in a separate video. Let's focus over here now. So if I only have a solution of sodium sulphide. Then in that case I will only have sodium plus ions and sulphur minus 2 ions. And they will be attracting each other. Because we know that opposite charges they attract each other. But if to that solution I also add a solution of HCl. Meaning now I also have H plus ions and chlorine minus ions. Now see sodium has no reason just to be attracted towards sulphur. Now it also sees that there is chlorine minus ions. So sum of sodium will also get attracted to chlorine minus ions. And we will have sodium chloride being formed. Which will look something like this. Sodium chloride. And similarly over here hydrogen that was earlier only attracted to chlorine. Now sees that ok there is sulphur minus 2 ions also present in solution. So sum of the hydrogen ions they will get attracted to sulphur minus 2 ions. And they will form hydrogen sulphide. Something like this. Now this is something that is happening in every double displacement reaction. First of all the reactants will break down into their ions. And then the ions will exchange position to form new new products. Now this model can help us answer all our questions. So the first question is can sodium exchange its position with chlorine instead of hydrogen. I want you to pause the video and think about this by yourself first. Now if you have tried let's see. So see if sodium exchanges position with chlorine. That means that a molecule of sodium and hydrogen should be formed right. But see sodium and hydrogen both are positively charged. And we know that like charges they do not attract but they repel each other. And if they are repelling there is no way that they can form a molecule right. So that's why sodium cannot exchange position with chlorine. In fact no positive ion can exchange position with a negative ion. Positive ion can only exchange position with another positive ion. Now another question is can sulphur exchange position with chlorine. So yes that is definitely possible. See if sulphur comes here instead of chlorine it will form a molecule with hydrogen. Hydrogen sulphide and that is what we are actually getting over here. Hydrogen sulphide. Similarly if chlorine exchanges position with sulphur it will have to form a molecule with sodium. And we will get sodium chloride and that's what we are getting over here right. So that's what happens in double displacement. Either the positively charged ions they exchange position or the negatively charged ions they exchange position. They both basically mean the same thing as long as you do not exchange position between a positively charged ion and a negatively charged ion. Now there is one more question to answer. You might be wondering Ram when we mixed the reactants some of the sodium ions they got attracted to chlorine ions and that's how sodium chloride was formed and we wrote that on the product side. But rest of the sodium was still attracted to sulphur right. And we should still have some sodium sulphide ions and we should also write that on the product side. Why aren't we mentioning that? Similarly some of the hydrogen ions they were attracted to sulphur ions and we got hydrogen sulphide and we wrote that on the product side. Whereas rest of the hydrogen should still be with chlorine right and we should still have some hydrogen chloride left and we should also write that on the product side. Why aren't we writing that? So that's a very good question. See hydrogen sulphide this is a gas and this cannot be present in a liquid. This will escape out of the liquid. What this means is the moment hydrogen ions and sulphur ions they come close together and combine they will form hydrogen sulphide which is a gas and this gas will escape out of the solution. Now what this means is as time passes by more and more hydrogen ions and sulphur ions are getting lost out of the solution. So we are no longer having sulphur ions in the solution. That means we can no longer form sodium sulphide and also we are no longer having hydrogen ions in the solution and that means we can no longer form hydrogen chloride. The hydrogen and the sulphide ions they will combine together to form hydrogen sulphide gas which will escape out of the solution. And similarly if we take this example barium chloride plus sodium sulphate now this is also a double displacement reaction. When we mix these in an aqua solution or that means in water then barium chloride this will dissociate into its ions and its ions are going to be barium plus 2 ions and chlorine minus 1 ion. Similarly sodium sulphate this will also dissociate into its ion. Sodium and sulphate ion. Na plus 1 and SO4 minus 2. Now see now barium plus 2 ion this can get attracted to even sulphate ion now and this will form barium sulphate also. Similarly sodium ion now this can get attracted to chlorine ion also and this will form sodium chloride. And so the products are going to be barium sulphate and sodium chloride. Now see the moment barium ion and sulphate ion come together they form barium sulphate which is insoluble in water. Meaning it will settle down out of the solution. It will precipitate out of the solution. Just like when you mix sand and water the sand after sometime it settles down right. Something very similar is going to happen with barium sulphate. That means after some times barium and sulphate ions they won't be available in the solution anymore. The only ions available in the solution are of sodium and chloride which will form sodium chloride and barium sulphate will settle out of the solution. And since we don't have any barium ion left we can't have any barium chloride being formed. And similarly since we don't have any sulphate ion left we can't have any sodium sulphate being formed. So just two products. Here I have an experiment to show the same thing. Here I have taken barium chloride solution and sodium sulphate solution both of which are colorless. But the moment I mix them I am getting a white compound being formed. Now that is barium sulphate and after some time this barium sulphate is going to settle down at the bottom of the flask. Now such reactions in which a precipitate is formed something that settles down these reactions are also called as precipitation reactions. Now let's make our volcano. For this you will require baking soda and vinegar. Now the moment you add vinegar to baking soda wow! You can see the amount of froth being generated. A lot of gases evolving over here. And if you put this reaction in a small volcano you will have a lot of bubbles coming up. And this will look amazing. In fact if I zoom in you can still see the bubbles coming out. Now let's look at the chemical equation and understand what's happening. So the chemical formula of baking soda is NaHCO3 and vinegar is CH3COOH. Now when they both are mixed in water they dissociate into their ions. We get Na plus ion and HCO3 minus ion. Similarly here we get CH3COO minus ion and H plus ion. Now the ions will exchange position to have double displacement reaction, right? So the ions will exchange position let's say. So here Na plus will exchange with H and we will get CH3COONA. Similarly H will exchange with Na and we will get HHCO3 or H2CO3. So the products are these CH3COONA and H2CO3. Now this does not stop here itself. See H2CO3 this further dissociates, this further breaks down into water and carbon dioxide and this carbon dioxide is the gas that is erupting out that is making the bubbles form. You can see the bubbles right? Those were carbon dioxide molecules coming out of the solution. Now with this let's summarize the video. In this video we spoke about double displacement reaction and in these reactions the ions of the reactants they exchange position.