 Let's solve some problems on how change in concentration of H plus and OH minus ion changes the pH of the solution and even vice versa. So here is my first problem. When acid rain mixes with the river water, the concentration of H plus ion increases. So it's given to us that the concentration of H plus ion, this is increasing. When the concentration of H plus ion increases, pH of the river water dash, meaning what is happening to the pH of the river water if the concentration of H plus ions is increasing. Okay, we know that if a solution has excess of H plus ions, that means it's acidic. And if the concentration of H plus ions is increasing, meaning the amount of H plus ions in the solution is increasing, that means the solution is becoming more acidic. See, the acidic nature of the solution depends on the concentration of H plus ions. If more concentration of H plus ions, then more acidic is the solution. Just like if in a sugar solution, the concentration of sugar that increased, that means it's more sweet now, right? So here also I will say, since the concentration of H plus ion is increasing, it is becoming more acidic, more acidic. Now let's see that if a solution becomes more acidic, how does its pH change? So can you pause the video and think about this by yourself first? Okay, now that you have thought about it, let's see. So for this, I'm going to bring back my pH scale. Okay, over here I can see that a pH of 7 means the solution is neutral. And a pH less than 7 means it's acidic. And a pH of 0, this means the most acidic solution. So from this, I can understand that the lesser the pH number, the more acidic is the solution, right? And here in our example, the solution is becoming more acidic. That means its pH number should have reduced. So my answer will be that pH is reducing or pH is decreasing, decreasing. Okay, now let's solve one more problem. So this time I'm given in the lab, Harry prepares an energy drink by mixing two unknown solutions. During the reaction, he notices an increase of OH minus ion concentration. Okay, that means that concentration of OH minus ion, this is increasing. When the concentration of OH minus ion increases, pH value. So we have to find out if the concentration of OH minus ion increases, what will happen to the value of pH of the solution? Will it increase or will it decrease? Now, can you pause this video and try to solve this by yourself first? Now, if you have tried it, let's see. So I know that if there are excess of OH minus ion present, that means the solution is basic in nature. And if the concentration of OH minus ion, this increases, meaning there are more and more amount of OH minus ion present in the same amount of liquid. That means it is even more basic. It is becoming more and more basic, right? So concentration of OH minus ion increases. The solution becomes more basic, more basic. Now, what do you think will happen to the pH of the solution, which is becoming more basic? Will it increase or will it decrease? So for this, let's bring back our pH scale. So this shows that pH of 7 is neutral and a pH greater than 7 means basic solution. And a pH of 14, this means it is most basic solution. So from this, I can understand that if the pH value this increases, that means the solution is becoming more and more basic. So here our solution is becoming more and more basic. That means its pH value should be increasing. So my answer is going to be here, the pH value increases. Okay, with this, let's solve one last problem. So we are given that for a class project, Sita collects a sample of soil from a backyard. On adding some solution to it, she finds that the pH of the soil increases from 8 to 10. That means pH of soil is increasing from 8 to 10. When pH increases from 8 to 10, the concentration of OH minus ion. So we have to tell what is happening to the concentration of OH minus ion. Okay, so can you pause the video and try to solve this by yourself first? Now we have tried it, let's see. So we're given that initially original pH was 8, right? What does this mean? So let me bring back my pH scale. So here we can see that 7 means a neutral pH and any pH greater than 7 means a basic solution. So over here, our initial solution had a pH of 8. That means our solution was basic in nature, right? And we see that pH is increasing. What does this mean? So we can see that here pH of 14 is most basic. That means I can say that as the pH increases, as the pH value increases, the solution becomes more and more basic. So that means our solution of soil is becoming more basic over here. So that means the solution is becoming more basic, right? pH is increasing and solution is becoming more basic. Now based on this, what can we say about the concentration of OH minus ion? So see, we know that basic solutions, they have excess of OH minus ion. But if a solution is becoming more basic, that means the amount of OH minus ion is increasing in the solution. The concentration of OH minus ion should be increasing in the solution. This should be increasing in the solution because the basic nature is dependent on the concentration. The more the concentration of OH minus, the more basic the solution is. So my answer over here is going to be increasing, increasing. So as pH increases, the solution becomes more basic and the concentration of OH minus ion increases.