Strength of solution vs concentration

- [Instructor] If we have two solutions which have the same amount of H plus ions in them, do you think that their acidic strength is going to be the same? Well, that will also depend on the volume of the solutions in which the H plus ions are dissolved. And that's what we are going to talk about in this video. How does the strength of a solution depend on the concentration of the solution? And by concentration I mean the amount of H plus ion in a given volume of solution. So let's begin with an example. Let's start with an example of acidic solution and then we'll come to basic solution. So you will remember that acidic solutions, they have more number of H plus ions. But see, if I have different acidic solutions, which have the same amount of H plus ion, that doesn't mean that their acidic strength is going to be the same. Let's do an experiment for that. So here I have a glass of water, a bottle of water and a tank of water. And I'm going to mix equal amount of H plus ions in all of them. So I will mix 100 H plus in the glass, 100 H plus in this bottle of water and 100 H plus in this tank of water. Now, probably you can guess that the acidic strength is going to be different in all three of them. You might have an intuition that the acidic strength in this glass is going to be the highest, this is going to be most acidic whereas this solution is going to be least acidic. But let's look at in detail. See over here, I had 100 H plus ions but in a glass of water, the volume of water is very less, right? So the volume of solution is very less. So I can say that the H plus ions are most concentrated in this solution. So this solution is going to be most concentrated solution. If you take a small sample, if you take one spoon of the solution, you will find many H plus ions in it and that's what makes it most acidic. Most acidic solution, whereas if you look at this solution, this also has 100 H plus ions but those ions are distributed in a large volume, right? So this solution is slightly diluted or in other words, I can say that this is less concentrated in comparison to this one. The bottle of water is less concentrated, so this is going to be less acidic in comparison to this one whereas if you come here, this also has the same amount of H plus ions but they are distributed in a very large volume of water, such a huge volume over here, right? So this is very highly diluted solution, in other words, I will say that this is the least concentrated solution. The least concentrated. If you take a sample of this solution, a spoon of this solution, you will find very little amount of H plus ions in it and that's why this is going to be least acidic. Well, if you did not understand this, I have another analogy for you. See, if I take a spoon of sugar and I mix this in the small glass of water, you can understand that this sugar solution is going to be highly concentrated, most concentrated and if you taste this, it is going to be very sweet, right? But if I take the same spoon of sugar and I mix that in this bottle of water, you will have an idea that this sugar solution is going to be less concentrated. It will be diluted compared to this one, right? And this will definitely taste less sweet in comparison to this one. Now, if I take the same spoon of sugar, and I mix this in this tank of water, so you'll definitely tell me that this solution is very dilute, most dilute, it is least concentrated and if you taste this, this might not even taste any sweet at all, right? So what we notice over here, the lesser the concentration of sugar solution, the less sweet it tastes and something similar is happening over here also. The lesser the concentration of H plus ions, the less acidic it is and more the concentration of H plus ion, the more acidic it is. So let's write down the relationship between H plus ion concentration and the acidic nature of the solution. So we are saying, if the H plus ion concentration is more, the more acidic the solution, and if the H plus ion concentration is less, the less acidic the solution. So so far we have seen that the acidic strength of a solution does not only depend on the amount of H plus present in the solution but also the volume of solution in which the H plus ions are dissolved, okay? And this ratio, this is called the concentration of H plus ions. So we have seen that if the concentration increases, meaning this ratio increase, then the acidic strength is going to be more, it is going to be more acidic and if the concentration of H plus is less, meaning this ratio is less, then the acidic strength is going to be less, it's going to be less acidic. By the way, you don't have to worry about this ratio right now. We will talk about this in detail in a separate video, okay? For now, let's take another example and check our understanding. So here I have two glasses of water and they have the same volume, okay? And I have dissolved 100 H plus ions in the first one and 50 H plus ions in the second one. Can you pause the video and think about which solution is going to be more concentrated? Now, if you've tried it, let's see. So both of these solutions have the same volume but this one has more number of H plus ions in the same volume of solution. That means this is going to be more concentrated solution. This will have a higher concentration of H plus ions. And that's why you'll see this is going to be more acidic in comparison to this one. Now let's talk about basic solution. See, they will also follow the same logic. The only difference will be that basic solutions, they have more amount of OH minus ion, so the strength of basic solution, this will not only depend on the amount of OH minus ion but also the volume of the solution in which the OH minus ions are dissolved in. That means the strength of basic solution will depend on the concentration of OH minus ions. So if the concentration of OH minus is more, that means it's more basic and if the concentration of OH minus is less, that means the solution is less basic in nature. Now with that, let's do one problem. So I've been given that a basic solution becomes more basic after a chemical reaction. Comment on the concentration of OH minus ion in the solution. So can you pause the video and try this problem by yourself first? Now if you have tried it, let's see. So over here, I'm given that I had a basic solution to begin with. That means it already had OH minus ions, okay? Now the solution is becoming more basic. That means the basic strength is increasing. The solution is becoming more basic. Now, can we comment anything about the concentration of OH minus ion, okay? So see, we have seen that basic strength is directly related to concentration of OH minus ion. The more the concentration, the higher the concentration, the more basic the solution will be, right? So over here, our solution is becoming more basic. That means the concentration of OH minus, the concentration of OH minus, this should be increasing. The OH minus ions should be getting more concentrated, then only the solution will become more basic. Now, let's do one last problem. So this time I'm given that in an acidic solution, the concentration of H plus ions decrease. Comment on the strength of the solution. Now, can you pause the video and try this by yourself first? Now, if you have tried it, let's see. So here I'm given the concentration of H plus ions is decreasing. The concentration of H plus ions is decreasing and we have seen earlier that concentration of H plus ions, this only defines the acidic strength of the solution, meaning higher the concentration of H plus ions, more acidic the solution and lower the concentration of H plus ions, the less acidic the solution, right? So over here, since the concentration of H plus ions is decreasing, meaning the solution is becoming less acidic. The solution is becoming less acidic, right? So that is going to be the answer. Now, let's summarize the video. In this video, we saw that the acidic strength of a solution, it not only depends on the amount of H plus ions, but also on the volume of the solution. That means it depends on the concentration of H plus ions. The higher the concentration of H plus ions, the more acidic the solution and the lower the concentration of H plus ions, the less acidic the solution. And similarly, the basic strength of a solution, it depends on the concentration of OH minus ions. The more the concentration of OH minus ion, the more basic the solution. And the less concentration of OH minus ion, means less basic the solution.