Physical and chemical changes | Chemical reactions | High school chemistry | Khan Academy

So what we have are three different pictures of substances undergoing some type of change, and 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.

All right, 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 or 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, and 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, it'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 on a microscopic level—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. For every one mole of propane and five moles of molecular oxygen, you're going to produce this mix. What you actually have is the bonds in those molecules are breaking and then reforming. So 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 (C3H8) here before. After the reaction, you no longer have the propane here. What you see as fire, which is fascinating, is just very hot gas. 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—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.

So 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.