The pH scale

0:00- [Instructor] I have five beakers with some clear liquid and I'm gonna pour a red-colored liquid into all of them. What do you think is gonna happen to their colors? Well, let's see. Whoa! What did you notice? Oh, wow! They all had this different color changes.

0:13But why? To answer that question, we need to introduce something called the pH and that's what we're gonna do in this video. Now, before we do that, let's quickly recap what we already know about acids and bases. We start thinking of acids as stuff that are sour and sticky, but we got a little bit more rigorous.

0:27Then, we realized that they're actually stuff that produce electrically charged hydrogen atoms in aqueous solutions. And as a result, they conduct electricity, because there are charged particles in aqueous solutions. And we also learned about their properties, like they can corrode stones and metals.

0:41What about bases? We started thinking of them as some things, stuff that is bitter and slippery. But then, we realized, hey, they're the stuff that produce hydroxide in aqueous solution. And again, because there are charged particles in aqueous solution, they're electrolytes, they too conduct electricity.

0:56And they have cool properties, like they can break down oil and grease. And that's one of the reasons why we use them in soaps and detergents. But there's another cool property that we wanna talk about and that is their pH. So, what exactly is pH? Well, we can think of pH as a measure of how acidic or basic a given solution is.

1:15To be more concrete, it's actually a measure of the relative amount of the H+ that we find in a given solution, or you can also say it's a measure of the concentration of this positively charged hydrogen atoms in a given solution, okay? So, if it's highly concentrated, we know that it's more acidic, because acids tend to produce more H+ atoms, isn't it?

1:40And if it's less concentrated, we know it's more basic. And that's how measuring the pH will tell us how acidic or how basic a solution is. And in fact, the word P, the letter P over here stands for power of hydrogen or potential of hydrogen. So, with this measure, we can now build a pH scale and that scale runs from zero to 14.

2:03A pH of seven is neutral. So, you can imagine distilled water, which is neither acidic nor basic. But if the pH is below seven, you are going towards increasing acidity. So, acids have a pH that is less than seven. But wait a second, more acidic means a higher concentration of H+, right?

2:19So, shouldn't that mean a higher pH? This used to confuse me a lot, because it turns out pH is an inverse scale. More acidic, more H+ means a lower pH. That's just how we have built it, okay? So, pH is an inverse scale. And that's why lower the pH, more acidic it is.

2:37So, acids have pH less than seven. On the other hand, if the pH is more than seven, we are going towards increasing alkalinity. Alkaline means basic, okay? So, it's becoming more basic. And therefore, bases have a pH between seven and 14. So, here are our common examples on the pH scale.

2:55Another interesting thing over here is we know now something that has a pH of two, for example, is more acidic than something that has a pH three, isn't it? But how much more? Well, this is actually a logarithmic scale, so, it's 10 times more. Something that has pH of two is 10 times more acidic than something that has pH three.

3:12And similarly, something that has pH one is 10 times more acidic than something that has pH two and so on and so forth. Okay, we understand what pH is, but how do we measure it? Well, one of the ways to do that, is by using something called the pH meters, which are very similar to connectivity testers.

3:27But instead of just testing for the presence of any ion, pH meters specifically measure the relative amount of H+ ions by directly measuring the voltage. So, these are the most accurate devices. You'll actually get a number, you'll know exactly what the pH is.

3:40But the problem is they're quite expensive. So, in our labs, we tend to use something else, which we call pH indicators. Indicators change color based on, again, the relative amount of H+, or the concentration of H+ that's there in a solution. A famous example is the litmus test.

3:57We get something called litmus papers. They're either red or blue. These litmus papers are one of the simplest and the most accessible tool for pH testing. It is made from naturally occurring compounds that are in living organisms known as lichens. These compounds change color based on the H+ concentration, but they're very simple.

4:14So, the color is either red or blue. Red for acidic, blue for basic. For example, the compounds in the red litmus paper are already in their acidic form. So, once you dip it in a solution, if it stays red, then, the solution is either acidic or neutral. But if it turns blue, we know that the solution must be basic.

4:34So, let's see what happens when I dip over here. Hey, it turned blue! Because it turned blue, I know that this solution must be basic. So, look, bases usually change colors to blue. And similarly, the compounds in a blue litmus paper, are already in their basic form.

4:51So, after dipping in the solution, if it stays blue, we know that the solution must be basic, or, of course, it can be neutral as well. But after dipping, if it turns red, we know the solution must be acidic. And let's see what's gonna happen now. And there you have it!

5:05It turned red! So, we know that this solution over here must be an acid, because acids turn blue litmus red. In general, we can think that acids turn things to red in color. We also have litmus solutions with us. So, if you put red litmus and it turns blue, look, I know that that is a base.

5:25And similarly, if blue litmus gets turned to red, look, I know that that is an acid. But the litmus test can only tell you, whether something is acidic or basic. It doesn't tell you how acidic or how basic it is, right? And for that, we have something called the pH paper and universal indicators.

5:42These are more complex than a litmus paper, because they are composed of many human-made components that each change color over a specific pH range. So, there'll be a lot more color changes. And based on the colors, we can not only tell whether they're acidic or basic, but we can also tell how acidic or how basic they are.

5:59So, let's look at a demo. Here is our pH paper. Here it comes, there you go. And now, let's look at the color changes. Ooh! What do we notice? This is very reddish. So, I know this is very acidic. This is orange-ish. It's still acidic, but I know that it is less acidic than this one.

6:18This is greenish, so, it's somewhere close to neutral. And then, these are basic. And similar to the pH papers, we also have universal indicators. And this is what we saw at the beginning of the video. Again, let's have a look and let's see the color changes. Ooh-hoo!

6:33Beautiful, isn't it? Again, we can see that these are very acidic. This one is yellowish, which means it's also acidic, but it's less acidic than these two. And then, we have very basic solutions over here. Finally, we can go back and complete our definition of acids and bases.

6:49Acids are substances that produce H+ in aqueous solution and have a pH, which is less than seven and turn blue litmus paper red. What about bases? Well, they are the substances that produce OH-, hydroxide in aqueous solution. And they have a pH, which is more than seven and turn red litmus paper blue.

7:06All right, putting it all together. We learned that pH is a measure of the relative amount of H+ in a solution, or you can think of it as a measure of the concentration of H+ in a solution. When the pH is less than seven, it is acidic. And if the pH is more than seven, it is basic.

7:22And how do we measure the pH? Well, we can use pH meters, but they're quite expensive. But a much more cost-effective and fun way is by using pH indicators. We have litmus test, which are quite limited, because they only tell you, whether something is acidic or basic.

7:34And then, we have pH papers and universal indicators, which also tell you how acidic or how basic something is.