 Chemical equations are used to describe what's going on during a chemical reaction. The form of a chemical equation is always the same. On the left you have the reactants, then you have an arrow that says in which direction the reaction is going, and then you have the products on the end. Products is what you have at the end of the reaction and the reactants is what you have at the beginning of the reaction. As an example, if hydrogen reacts with oxygen, we're going to end up with water and the complete reaction looks as follows. We have two hydrogen molecules as gas and one oxygen molecule as gas, which will create two water molecules in the liquid form. As you can see, there is always the molecule written in chemical formulas. So here is the molecule formula. Then you have the number of molecules that is involved. Of course, if you actually do this reaction, there are billions of hydrogen molecules involved. But what the reaction equation is saying is that for two hydrogen molecules, you're going to get two water molecules. And then here in brackets, we have the state. Was it gas, G, liquid, L, or was it solid, S? Or in solution, Aq? In a chemical reaction, the total mass is conserved. That means if you would measure all the mass you have at the beginning and all the mass you have at the end, you would get exactly the same mass. We cannot destroy or create mass with a chemical reaction. It's a bit different with a nuclear reaction, which we're going to get later on. Also, the number of atoms of each element is conserved. If we're looking on the left side, how many hydrogen atoms I have? So I have two molecules of hydrogen, which each molecule has two atoms of hydrogen. So two times two is four hydrogen. How many hydrogen do I have on the right? I have two molecules of water in each water molecule. They are two hydrogen atoms, so four hydrogen. So you have the same amount of atoms of each element on the left, on the right. Again, this is true for a chemical reaction. In a nuclear reaction, you could actually change the element. If you look for oxygen, I have one times two atoms of oxygen, so total of two on the left. And I have two times one. If I write nothing here, that means one equals two oxygen atoms on the right.