Intro to acids and bases | Chemistry | Khan Academy

Check out this cool experiment we did a while back. I take some red color solution, put it in a transparent solution, and it becomes blue. What's going on? That's not it. Now I take a blue solution, put it in a transparent solution, and it turns red again! It looks like magic. It's not magic; it's the chemistry of acids and bases, and that's what we're going to explore in this video. So let's begin.

So what exactly are acids and bases? Well, if you look at their most primitive definition, it was based on their tastes. Well, acids are usually sour tasting, and they can be sticky. In fact, the word "acid" actually comes from the Latin "acidas," which means sour. Some of the most common examples you can think of are lemon juice, which contains citric acid, vinegar, coffee, soft drinks with soda in them. Yeah, they're all acids.

Okay, what about bases? Well, it turns out bases are bitter tasting, and they're quite slippery. Again, common examples would be soaps. If you've had soap water gone into your mouth, you probably know you've tasted bases, and you probably know that they are bitter. If you've tasted stuff that's too much baking soda in them, well, they taste bitter. Well, because bases taste bitter, other examples include detergents, antacids, and so on. But of course, this is not a great definition of acids and bases. There could be so many other things that are sour or bitter which may not be acids or bases. So can we come up with a more concrete definition?

Well, it turns out there is more than one way to define acids and bases. But we're going to start simple. We're going to use what we call the Arrhenius definition, or the Arrhenius model of thinking about acids or bases. In this particular definition, we basically say acids are things that give you H⁺ ions in aqueous solutions. Similarly, bases are things that give you OH⁻ ions in aqueous solutions. In other words, if you mix something in water and it starts giving you H⁺ ions, these are acids, and if you mix things in water that give you OH⁻ ions, those are bases.

So let's take an example. If you take HCl in an aqueous medium, what you will find is that HCl will dissociate into H⁺ ions and Cl⁻ ions. Now, since we got H⁺ ions from mixing HCl in water, H is an acid, and you probably know this as hydrochloric acid. Another example could be acetic acid, and if you mix it in water, again you'll get some H⁺ ions. So this is also an acid.

Similarly, let's take some examples of bases. If you take sodium hydroxide or potassium hydroxide and mix them in water, or take an aqueous solution of them, you will find sodium ions, potassium ions, and OH⁻ ions over here. So since they give you OH⁻ ions, they are bases. Now when we look at this, it might be reasonable to think that all bases should have OH in their formula, right? Otherwise, how else will you get OH⁻ ions when you put them in water, right? Well, it turns out that's not true. For example, if you take ammonia and put it in water, that will also increase the amount of OH⁻ ions, which means technically ammonia is also a base, but it's not so straightforward to identify it as a base just by looking at Arrhenius' definition.

This is the reason why we have other definitions of acids and bases as well. But for the purpose of this video, let's not worry too much about it. Let's just stick to ours because guess what? Even with this Arrhenius definition, it has a lot of explanatory power. We can understand a lot about acids and bases. For example, you probably know that acids are corrosive; they can corrode metals and they can also erode stones. This is why acid rain that you get due to pollution can also damage monuments. But why does it happen?

Well, a simplified explanation could be that the hydrogen ion that is formed from the acids can suck electrons from the metals. In doing so, hydrogen gas is formed and that gets released, causing that hissing sound. Now, the metal ion that is formed, a positive ion that is formed because it has lost an electron, can actually bond with this anion over here, and that molecule can actually dissolve in solution. That's how corroding metals can get corroded. Something similar will happen with your stones as well. So the culprit is the H⁺ ions.

Okay, what about bases? Well, bases have the ability to break oil or grease, and that's the reason why they're used in soaps. But again, how does that happen? Again, an oversimplified explanation over here would be that usually oil or grease do not like water, so they do not dissolve in water. That's why you cannot use water to wash them off. But OH⁻ ions, when they attach to oil and grease, make them water-soluble. So in the presence of a base, oils and grease can dissolve in water, and you can wash them away. Amazing, isn't it?

But wait! There's more that we can explain. What would happen if you were to mix an acid and a base? Well, again, let's look at it. If you mix HCl with, say, NaOH, the H⁺ can attach to OH⁻, and you get H₂O (water). Cl⁻ will attach with Na⁺, forming NaCl, which means when you mix an acid and a base, you will get a salt, an ionic salt, and water. This is what we call the neutralization reaction, and this is why we say acids and bases neutralize each other.

So in a neutralization reaction, when you mix an acid and a base, you'll always get an ionic salt and water. Just like how when you know when magnets stick to each other, they release energy, when these ions and these molecules stick to each other, they release energy, meaning neutralization reactions are always exothermic reactions.

But wait! There are more properties that we can explain. For example, if you look at these solutions, acidic or basic solutions, do you think they conduct electricity? What do you think? Well, guess what! We have ions. When you have them in solutions, this means we have electrolytes, which means they do conduct electricity. So both acids and bases can conduct electricity. And guess what? It's the H⁺ ions that actually causes acids to taste sour, and it's the OH⁻ ions that actually causes the bases to be bitter. It's got something to do with how our taste receptors work and everything.

But what I find incredible is that just by knowing, just by looking at this one definition, there's so much we can understand and so much we can explain. And by the way, now that we know what acids and bases are, instead of having to taste them—which we should never try doing in a lab, that's a very dangerous way to identify acids and bases—we can create solutions that change colors depending on whether they are in an acidic medium or basic medium.

One such indicator is what we call a litmus solution, and that's what we saw at the beginning of the video. So this is the litmus solutions. These are made by extracting dyes from lichens, which are interesting types of organisms. Anyways, they usually come in two colors: we have the blue litmus solution and the red litmus solution. The reason we have these two colors is because acids turn blue litmus to red. Again, this has something to do with the H⁺ ions; they react with the blue litmus solution, change their properties, and as a result, the kind of color they absorb from the light changes. That's the reason why it starts showing red in color. It's amazing if you think about it!

And bases do the other way around; bases turn red litmus to blue. If you find it hard to remember, I used to find it hard to remember. Well, a simpler way to think about it is: acids turn litmus to red; bases turn litmus to blue. That's how I remember. So acids will turn things to red.

Okay, so now let's add the litmus solution and see what we get. So here we're adding red litmus, and look! The color is changing to blue, so that means this is a basic solution. So this is a base. So that's what we saw earlier. Similarly, if you now take blue litmus solution and add it over here, look! It's changing color to red. What changes color to red? Well, it's an acid. So this is an acid.

So this is how we no longer need to use our tongue. We can use litmus solutions to identify whether we're dealing with bases or acids. But, of course, it does not tell you how basic or how acidic it is. Is it very acidic? Is it very basic? It doesn't tell you the pH. For that, we have better indicators, like we have pH indicators, we have pH sensors, and all that good stuff.

This brings us to the last part, which is my favorite part. That is thinking about the strength of the acids. You probably know certain acids, like say citric acid or vinegar, they're not very harmful. You can taste them, and we probably taste them on a daily basis. But what about hydrochloric acid? Oh, that can be very harmful! Same is the case with bases; certain bases can be very harmful. Why is that the case? Well, that's got something to do with the strength.

What does that mean? Well, let's see. If you look at an acid, a generic acid can be written as HA. Well, in aqueous medium, it gives you H⁺ ions—that's what makes it an acid—and some anions, which are A⁻ ions over here.

Now, when I dissolve this in water, there are two kinds of cases that we can see, and I want you to think about what the difference between them is. Okay, this is case number one. Sorry, here’s case number one, and here is case number two. What difference do you find between them? Well, in the first one, you find 100% of all the H molecules have dissociated. There are four I've shown over here, and all four are dissociated. Such acids are called strong acids. When you have 100% dissociation, almost all of them get dissociated, we call them strong acids. Your hydrochloric acid, sulfuric acid, nitric acid, they're all strong acids.

The same is the case with bases; when bases have complete dissociation, we call them very strong bases. But what's happening over here? Well, here also I have four acidic molecules, but only one of them has dissociated. In other words, partial dissociation. Whenever acids undergo partial dissociation, we call them weak acids. Your citric acid, carbonic acid, phosphoric acid, they're all weak acids. And by the way, these are the acids that you find in your soft drinks.

Now here's the cool thing. I can increase the concentration of it. See, currently, the concentration is the same, right? You can imagine this is one liter; I have four molecules per liter here and four molecules per liter over here, just taking simple numbers. But what if I added more molecules over here? I add more molecules over here. Look, I have made this more concentrated, so this now has become a more concentrated solution compared to this one.

This is compared to this one, which is more diluted. So notice I can have a concentrated weak acid, but even in that case, because it's very partially dissociating—the dissociation percentage is usually around 5-6% or something—I won't have a lot of H⁺ ions over there, so nothing much is going to happen. It's not that harmful. But in strong acids, even if it's diluted, I can have a lot of H⁺ ions because 100% dissociation happens.

Now imagine if I had concentrated strong acids. You have a lot of H⁺ ions, and you probably want to stay away from them. So this is why these acids can be super dangerous if they are very concentrated. You can see the strength is a measure of how much they dissociate: 100% dissociation—strong; partial dissociation—weak. But remember, concentration is how much amount of acidic molecules you have. You know, if you have a lot of them per liter, it's very concentrated; if you have a little bit of them, it's less concentrated or diluted.

Finally, if you're thinking about weak bases, bases are slightly more complicated, and so we'll not talk too much about them.