Hydrolysis of Salts - The demonstration that I'm going to perform today is called hydrolysis of salts. And I use this in an acid base unit. And what I have lined up here on the light box are seven beakers, each filled with some distilled water to which universal indicator has been added. Now, I will mention that not everybody's distilled water comes out to be right at this green pH of seven, so if necessary, you might have to tweak the pH by either adding a little bit of acid or a base to get it to the color that you want. The important thing is that we want to start out with a neutral pH of green. Now, what I'm going to do is add seven different salts. And by salt here, I'm talking in the generic term. I'm talking about ionic solids where the cation is not a hydrogen ion and the anion is not a hydroxide ion. We're going to start by just looking at the reactions. And then we'll go back and talk about what's happening to give us the results. So my first one that I'm going to add here is aluminum chloride. And I'm just going to add solid right into the beaker and stir that up. My second one is potassium carbonate. The third one is ammonium chloride. The fourth one is sodium bicarbonate. And the last three are different phosphates. This is sodium phosphate. This is sodium monohydrogen phosphate. And the last one is sodium dihydrogen phosphate. Now, I think you have to admit that this is a pretty colorful display. And my students really enjoy color changes as much as I do. But it's not sufficient just to look at these color changes. What we have to do is figure out what is the chemistry that's going on here. So what I'm going to do is go over to the easel, and we're going to discuss these reactions. Now, we're using universal indicator, so we're talking about the Roy G. Biv scale here. And so what I want to do is focus on those that actually gave us a basic solution. So that would be the Biv side of the Roy G. Biv side, so hydrolysis that yields a basic solution. Now, when does this happen? When the water, when it reacts with the salt, donates a hydrogen to the anion of the salt and produces a hydroxide ion. Hydroxide ions or would make basic solutions. So in beaker number 2, the carbonate ion produces hydroxide ion. The carbonate ion from the potassium carbonate is taking a hydrogen from that water or the water is donating a hydrogen, if you like, to produce hydroxide ion. Here's our bicarbonate ion forming carbonic acid and also producing a hydroxide ion. Here's the phosphate ion, taking that hydrogen and making HPO4 2 minus. And then we also have the sodium monohydrogen phosphate in beaker number 6, also taking that hydrogen and forming hydroxide. Now in general you can talk about a salt that forms a basic solution as being a salt that's formed from a weak acid and a strong base. And in a first year chemistry class, that's a very simpler way to refer to it. But with my second year chemistry class, we would be writing these equations. Now those were the hydrolysis that produced basic solutions. Now let's look at the ones that produced acidic solutions. In that case, the water is accepting the hydrogen from the cation to make a hydronium ion. Now in the first beaker, remember I used aluminum chloride. We're not as accustomed to seeing the aluminum ion written this way. But, actually, it is a aluminum hexahydrate ion with a 3 plus charge. And what's happening is that the water is taking a hydrogen from this, and this is a rather exotic looking ion. But, again, with my second year class, they can handle that, making the hydrogen ion. Here's the ammonium ion reacting with the water to making ammonia and the hydrogen ion. And so in general, you can talk about a salt reacting with water, forming a strong acid in a weak base actually to give us an acidic solution. Now what about number 7? That one's a little bit tricky there because why did that turn up acidic? Well, in that case, the water is actually accepting the hydrogen from the anion. And that's why that H2PO4 minus ends up forming a hydronium ion. So these are the examples that you can use to show hydrolysis. The first five would be simpler for a first year class. Now you might say, what happens if you put together a strong acid and a strong base? Well, then it's going to be a neutral solution. And our green color wouldn't change. Personally, I find that pretty boring, so I don't do that with my students. Now what about a weak acid and a weak base? Well, that'll depend on the Ka and the Kb, so whichever one predominates. Then you can go into a greater discussion. Again, that would make it a little bit more technical as far as the mathematics go. But colorful reaction, hydrolysis of salts, and a very good place in your acid base curriculum to place this demonstration.