 If I give you a salt, can you guess what acid and base would have reacted to give us this salt? See, we have seen in earlier videos that an acid and a base, they react to form salt and water. And I'm pretty sure if I give you an example of an acid and a base reacting, you will be able to guess what salt is going to be formed. But in this video, we are going to do the opposite. Given a salt, we will try to find out what acid and base would have reacted to give us this salt. So, let's begin with an example. First of all, let's recall how acids and bases react to give us salt and water. So, here I have taken HCl acid and NaOH base. Now, when I mix them in water, HCl is going to dissociate and give me H plus ion and chlorine minus ion. Similarly, NaOH, when we put this in water, this is going to dissociate and give us Na plus ion, NaOH minus ion. Now, the ions are going to exchange position. A double displacement reaction is going to happen. The H plus ion will now get attracted to the OH minus ion. And this will form water, HOH or H2O. Let me write it down like this. And sodium ion will in turn now get attracted to chlorine minus ion and form the salt, NaCl. NaCl, salt. So, this is how we are getting salt and water as the product. The ions are exchanging position. So, if the ions of acid and base, they exchange position, we get a salt and water. So, that means if we exchange the ions of salt and water, we can get back our acid and base. Let's try that out. So, here the ions in salt NaCl will be Na plus and chlorine minus. And similarly, I can write HOH water as H plus and OH minus ion. Now, let's exchange the ions. Now, sodium will get attracted towards OH minus and we will get NaOH. And we know that NaOH is a base because it has OH in it. It can increase the concentration of OH ions in an aqua solution. And similarly, now H plus will get attracted to chlorine minus. And this will form HCl, which is an acid. This has hydrogen in it and it can increase the H plus ion concentration in water. So, yes, we have got our acid and base that can react and form the given salt NaCl. By the way, I would like to clarify here that this is just a technique to get the parent acid and base of a given salt. It's not that if you put the salt in water, it will give you acid and base and you can use it. That is not going to happen. Now, let's look at one more example. So, this time I have the salt, copper sulfate. And we need to find out which acid and base could have reacted to give us the salt. So, we have just seen that acid and bases, they undergo double displacement or the ions between them, they exchange position to give us salt and water. So, we will exchange the ions of salt and water to get back the acid and base. So, let's do that. So, water formula is H2O or I'm going to write it as HOH. Now, let's write down the ions that are present over here. So, copper sulfate will have copper plus 2 ion and sulfate minus 2 ion. And water I can write as H plus ion and OH minus ion. Now, let's exchange the ions. So, copper will now get attracted to OH minus ion. We will get the ions of a base, copper plus 2 and OH minus 1. And the reason why these are the ions of a base, because the bases, they increase the OH minus ion concentration. And this has OH minus ion in it. Okay, now COOH, this is not going to be the final formula. Because see, copper has a charge of plus 2 and OH has a charge of minus 1. This molecule is not going to be electrically neutral. For that, copper plus 2 will have to attract one more OH minus. Because see, then only we will have plus 2 and minus 2, electrically neutral. So, our final formula of the base is going to be COOH whole twice. You can also get this by exchanging of the charges, like 2 comes over here, 1 comes over here. COOH whole twice. Now, let's go and exchange the other ions. So, now H plus will get attracted to sulphate ion over here. And we will get the ions of an acid. H plus and sulphate ions, SO4 minus 2. And why these are the ions of an acid? Because acid, they dissociate to increase the concentration of H plus. And this has H plus in it. So, this can only be the ions of an acid. Okay, now this is not going to be the final formula. H SO4 is not the final formula. Because this is not electrically neutral. See, this has a charge of plus 1 and this has a charge of minus 2. So, for this to be electrically neutral, there has to be 2H plus. Then only we will have plus 2 and minus 2. So, that means the final formula of the acid will be H2SO4. H2SO4 is our acid. Now, see, we can also get this by exchanging of the charges. 2 comes over here, 1 comes over here. H2SO4. Now, this is the acid and this is the base. And when they both react, we will get copper sulphate salt and water will be formed. So, yes, we have solved our problem. Now, let's do one last problem. So, this time I am given with the salt KNO3, potassium nitrate. And we have to find out which acid and base could react to give us this salt. So, I want you to pause the video and try this by yourself first. Give it a try. Now, if you have tried it, let's see. So, we know that the ions of acid and bases, they exchange position to give us a salt and water. So, if we exchange the ions of salt and water, we can get back the acid and base. So, let's do that. So, let me write down water. Water is H2O or HOH. Let me use same color. HOH. Now, let's break the salt and water into their ions. So, KNO3 will have potassium ion K plus and nitrate ion NO3 minus. From water, I will get H plus and OH minus. Now, let's rearrange. Let's exchange the ions. So, potassium will get attracted to OH minus and this will give us the ions of a base. K plus and OH minus. And why these are the ions of the base? Because it has OH minus, right? Bases increase the OH minus ion concentration. And similarly, H plus will now get attracted to NO3, nitrate ion. And we will get the ions of an acid. H plus and NO3 minus. And these are the ions of an acid because they have H plus. Acids, they increase the H plus ion concentration, right? Now, with this, let's try to find out the formula of the acid and the base. See, over here, if we exchange the charges, HNO3, we will get the acid as HNO3. Similarly, we exchange the ions, the charges of KOH. So, one comes here, one comes here, we'll get KOH. So, we have found out the acid and the base that will react to give us the salt, KNO3. So, if you are given a salt and you have to find out the acid and base that can make it, then break the salt into its ions and exchange these ions with the ions of water. You will get the ions of the acid and base. And from there, you can build the chemical formula of the acid and the base by exchanging the charges.