 So the first one we want to have a look at is hydrogenation. Now this is the addition of hydrogen across a double bond. So I guess the first thing we want to do is make sure that we understand what we're talking about when we talk about addition reactions. So if I take a simple unsaturated hydrocarbon like ethene, then what I have here is a concentration of electrons, a high density of electrons that are associated with that double bond for electrons in this space. Now these four electrons make this region quite reactive and therefore we can readily add substances across that double bond, break that double bond and form a new substance. When we do that, we only form one product. So one of the ways of doing that is simply to add hydrogen. Now if we had hydrogen, then the hydrogen is going to add across where the double bond is. It'll break the double bond. So we have only a single bond and we have the original hydrogens that were part of the ethene molecule. But then we have two more hydrogens which have added across that double bond. Generally speaking, a catalyst is used to help facilitate this process, usually something like platinum or palladium, and we can identify that by just putting it above the arrow in our reaction. So this gives us the structural formulae and it's often easy to show these sorts of addition reactions with the structural formulae because you can show exactly where these substances are adding to. So this would be an example of an addition reaction. Note there's only one product. This is going to be important for later. There's only one product. There are two reactants and the two reactants have added together to form a single product. That hydrogen has added across the double bond and it has broken that double bond open. And so those two hydrogens have gone one to each of the carbons on either side of the double bond. This is one example of an addition reaction involving the addition of hydrogen.