One skill that can be pretty useful when we're studying chemical reactions is learning to identify different types of chemical reactions so that we can classify them in groups rather than thinking about everything individually. Two of the easier types to spot are called combination and decomposition reactions. Combination and decomposition reactions really involve making things and breaking things. If we've got a number of reactants that all come together to form a single product, that is a combination reaction or sometimes called a composition reaction. If we have a single reactant that's breaking down to form two or more products, that's a decomposition reaction. And you can see that's why these are sometimes called compositions because the opposite of a composition is a decomposition. And these two reaction types are really just the opposite process of one another, either multiple things combining to form one product or one thing breaking apart to form multiple products. We can look at a couple of different examples here, but first of all, some sources, depending where you look, will be very, very specific and say that a combination reaction has to involve elements. I don't know that that always has to be true. It turns out that for most of these reaction types that we come across regularly, functionally it ends up being true because a lot of them really are elements reacting to form substances. But I don't think that needs to be an absolute truth. So we've got one like that here. Sodium solid, the element sodium reacts with chlorine gas, the element to form sodium chloride solid. Make sure you're remember to look at your polyatomics. Chlorine is one of those polyatomic elements. So this always comes in as chlorine molecules, CL2. Because of that, we need a couple of twos to balance out our reaction. This is just as much a combination reaction as that is. But here we're starting with a molecule, we're starting with H2S gas. And that can react with oxygen gas, another one of those diatomic elements to form H2S04 liquid. Decomposition similarly only have the one reactant and more than one product and those can be broken down to elements or two molecules and other formulas. So a couple of quick examples down here. Sodium carbonate decomposes on heating to form magnesium oxide and carbon dioxide gas. That's actually a very important reaction in a number of settings. We could also think about something like sodium phosphate breaking down to form sodium phosphate. So one of those oxygens decomposing off of there to form oxygen gas and sodium phosphate. Again, this one we needed a two to balance out because our oxygen gas product now is a diatomic molecule. One thing that I want to point out just real quick to help you avoid some problems or avoid some misidentifications is when we're talking about combinations and decompositions, we're talking about the full molecular type equation. One of the things that we often do is we write what's called a net ionic equation where all we do is show the ions that are reacting. So if we've got a reaction like this led plus two ions in solution react with sulfate ions in solution to make lead sulfate solid. That might look like a combination reaction because we only have one product. But because these are ions, this is a net ionic equation. This is really just a different way of writing this larger reaction led nitrate in solution plus sodium phosphate. Excuse me. Plus sodium sulfate in solution gives us lead sulfate and sodium nitrate. So when we write out the full molecular style equation, it sure doesn't look like a combination ionic equation anymore because we've got two products, we've got multiple reactants. So watch out for those as you're trying to identify combinations and decompositions. As with all these reaction types, the more of them you look at, the more practice you get, the easier it'll be to recognize those reactions when you come across them. So always take advantage of having any opportunity you have to look at different reactions and try to identify them and as always keep practicing.