 In this video, we'll be talking about displacement reactions and displacement reactions can be represented in a simple form like this. So let's say if you have a compound AB reacting with C, what happens is C will displace B to form a product AC and a separate product B. So this is the representation of a displacement reaction. Now sometimes these are also called replacement reactions because as we just saw here C is replacing B. And looking at this itself, we know that for this reaction to happen, we could say that C would be more reactive than B, which is why C reacts to form a product with A. So because these reactions involve some very reactive species, the examples of displacement reactions that we're going to see are some of the coolest reactions that you'll see. Like for instance, if you take this example, which is of sodium reacting with water, you can see how this oxygen along with the hydrogen has formed any OH in the product and hydrogen is released. So you can see here how this reaction takes place. Here, let's say if you have a small glass dish in which you've taken some water and you drop a piece of sodium metal into it, the sodium that is dropped into the water reacts to form any OH and hydrogen is released. Now the hydrogen gas that is released gets ignited when the energy released in this reaction is very high, which is why it catches fire like this. And this image here is of the same experiment, but this time done with a larger amount of sodium. So this was the case where three pounds of sodium metal was dropped into water. And you can see how when the amount of sodium is increased, the energy released here is also high, which is why the hydrogen here ignites into something like a small explosion here. So this was the reaction of sodium with water. Another example of a displacement reaction is the thermite reaction, which is when hematite, which is Fe2O3, and aluminium react to give iron and Al2O3. Now this reaction is very exothermic and it releases a lot of heat. This reaction would occur in a process which is called thermite welding in which the heat released from this reaction would be used to fuse together two pieces of metal. And that was how railway tracks were joined for a long, long time. And this reaction too, as you can see here, releases a lot of heat and leads to this combustion. Now yet another example is that of silver reacting with hydrogen sulfide to give Ag2S and hydrogen. Now this reaction may seem very unfamiliar to you right now, but you may have seen this reaction take place if you've seen old silverware that gets tarnished when exposed to atmosphere. So this tarnish that you see here, this is this Ag2S, which is forming when this silverware is exposed to the H2S in air. So all of these are examples of displacement reactions. Now if you see here these reactions are also called single displacement reactions because there is only one displacement taking place. But displacement reactions can also be double displacement reactions where two displacements take place. Let's see what they look like. So double displacement reactions look something like this where let's say if we have AB and CD as the reactants, D replaces B to form AD and C replaces A to form BC. So let's go through an example of such a reaction. So in this reaction we have potassium iodide reacting with lead nitrate to form potassium nitrate and this lead iodate is formed as a precipitate and this lead iodide precipitate has a bright yellow color like this. So this is one of the identifications of our double displacement reactions because these reactions typically lead to precipitation, which is why another place where you will see double displacement reactions is the treatment of hard water. So hard water usually contains sulphates and chlorides of calcium and magnesium and on reaction with constituents of soap, they usually precipitate and form the insoluble compounds which can be removed out to treat hard water. So such reactions which are involved in the treatment of hard water are also usually double displacement reactions. So it's not that only precipitation is a feature of these reactions. Double displacement reactions can also be accompanied by the release of gases like for example in this reaction we have sodium sulphide reacting with HCl to give salt and H2S. So when you perform this reaction, you see bubbles of H2S rising through the water. And so this is an example of a double displacement reaction in which no precipitation occurs but a gas is released. And another form of double displacement reactions are usually seen in this neutralization type reactions where you have an acid and a base reacting to give salt and water. So these type of reactions are also double displacement reactions.