Physical and chemical changes | Chemical reactions | AP Chemistry | Khan Academy

So what we have are three different pictures of substances undergoing some type of change. What we're going to focus on in this video is classifying things as either being physical changes or chemical changes. You might have already thought about this or seen this in a previous science class, but when we talk about a physical change, we're talking about where there could be a change in properties, but we're not having a change in the actual composition of what we're talking about.

While in a chemical change, you actually do have a change in composition. How the different constituent atoms and elements match up, or connect or bond to each other might be different. So my first question to you is: pause this video. We have some ice melting here, we have some propane combusting or burning here, and we have some iron rusting here. I want you to think about which of these are physical changes and which of these are chemical changes and why.

Alright, now let's first think about this water, this ice melting. If we wanted to write it in fancy chemical language or chemistry language, we could write this as H2O going from its solid form to H2O going into its liquid form. Now, we don't have a change in composition in either state. Whether you're looking at this liquid water here, whether you're looking at the solid water there, you'll see a bunch of water molecules. Each oxygen is still bonded to two hydrogens, so you're not having a change in composition. This over here is a physical change.

If we kept heating that water up and it started to vaporize, that would also be a physical change. Whereas it turns into water vapor, you have your intermolecular forces being overcome, but the covalent bonds between the oxygens and the hydrogens, those aren't breaking or forming in some way. So once again, when you go from ice to water, that's a physical change. From water to vapor, or you could say from liquid to gas, that is also going to be a physical change.

One general rule of thumb, when you think about what's going on at a microscopic level, and this is a general rule of thumb, it doesn't always apply, and we'll think about an edge case in a little bit, is when you're overcoming intermolecular forces, that tends to be a physical change. But if you have chemical bonds forming or breaking, that would be a chemical change.

Now let's think about what's going on here with the propane. If you were to write the chemical reaction here, it would be propane (C3H8) in gas form. It needs oxygen to combust, so for every mole of propane, we have five moles of molecular oxygen in gas form. When it combusts, you're going to produce three moles of carbon dioxide gas and four moles of water in vapor form as well. So what you actually have is the bonds in those molecules are actually breaking and then reforming.

You don't just have a physical change going on here; you have a chemical change. One way to think about it: you had propane here (C3H8) before. After the reaction, you no longer have the propane here; what you actually see is fire, which is fascinating. This is just very hot gas, and that very hot air that you're seeing, and there's going to be some carbon dioxide in there, and there's going to be some water vapor in there. The reason why it's getting so hot is because this releases a lot of energy.

Now let's think about what's going on here with this iron. If I were to write this as a chemical reaction, for every four moles of iron in solid form plus three moles of molecular oxygen in gas form—and that would just be the ambient oxygen around this iron—it is going to produce two moles of iron oxide as a solid. That's what you see there in the orange; that is the iron oxide.

Notice this reaction is forming new ionic bonds in that ferrous oxide. To undergo the reaction, we had to break the metallic bonds of the solid iron and the covalent bonds in the molecular oxygen. So anytime we are breaking and making these chemical bonds, we have a chemical change.

Now let me give you an interesting question: what about the dissolving of sodium chloride or table salt? You have NaCl in its solid form. When you place it into water, you get sodium chloride in aqueous form. How does that look when this happens? Well, sodium chloride in solid form forms this lattice because they have these ionic bonds.

I'll try to draw a few of them so it looks something like this, where we've seen this multiple times. Chlorine nabs electrons, becomes chloride, becomes negative; sodium loses an electron, becomes a positive ion. They get attracted to each other, and then they can form this three-dimensional lattice structure when it is in solid form.

I'm trying my best to draw it in three dimensions over here. Actually, I could put one right over here like this, but when you dissolve it into water, the ions get split apart. You get the ions that all get dissolved, split apart in the water, and so the ionic bonds themselves are being broken.

Now you could argue that, hey, you know, the constituent ions are just being separated, so maybe this is a physical change, or you could say, no, ionic bonds, those are legit bonds, those are strong bonds, and those are getting broken. So you're also getting a chemical change.

It actually turns out that this is a gray area when you're talking about dissolving a salt like this into water—whether you're talking about a physical change or you're talking about a chemical change. Actually, just this past weekend, I was at the beach, and we ran out of salt. I did it in reverse; I went to the ocean and got a pot full of salt water. It actually took me about 30 minutes to boil it, and I was able to evaporate all of the water out of it. Essentially, the salt in that water was able to reform these ionic bonds.

One could debate whether I was creating a physical or a chemical change. I was definitely doing a physical change with the water; I was evaporating it. But I was actually forming ionic bonds between the sodium and chloride, so one could argue that that was maybe both a physical and a chemical change.