 So let's have a quick look at some of the ways that it's possible to add things to an alkene molecule. I'm going to show you the four most common kinds of addition reaction. In the first one here, one of the two bonds in the carbon-carbon double bond breaks and the two freed up bonds, one on each carbon, join on to halogen atoms from a diatomic halogen molecule. I've shown bromine here but it can happen with any of the halogens, fluorine, chlorine or iodine. This reaction is called halogenation because you're adding halogen atoms to the alkene and this reaction happens fairly easily and at room temperature. The second reaction is hydrogenation. The process is the same but this time two hydrogen atoms are added to the two new bonds which turns the alkene into an alkane. This reaction has a much higher activation energy and a catalyst is often used to make the reaction go faster. The platinum shown here is an example of a catalyst that can be used. Another possibility is hydrohalogenation, sort of a cross between halogenation and hydrogenation. In this reaction, a hydrogen halide or in effect an acid, one of these will add across the double bond so that one carbon gets an extra hydrogen and the other carbon gets a halogen atom of some kind. And finally there's hydration. The name suggests water, you drink water to keep hydrated, right? Well in the hydration reaction a water molecule is split into two parts and it adds across the double bond. One carbon atom gets an extra hydrogen atom and the other carbon atom gets the rest of the water molecule, an OH, an oxygen and a hydrogen. And in organic chemistry when a molecule has an OH group we call it an alcohol. So this reaction is one way of turning an alkene into an alcohol. This difference in the chemical properties of alkanes and alkanes, this specific ability of alkanes to add things across the double bond, gives us a really convenient way of distinguishing them despite the fact that visually they look identical. As we've just said, alkanes will react with halogens easily at room temperature. Alkanes on the other hand are quite unreactive and to make them react with halogens you would need to expose them to UV light or high temperatures. This difference gives us the bromine water test. Bromine water is an aqueous solution of bromine which has the characteristic brown color of bromine. When bromine water is added to an alkene the less dense alkene floats on top of the bromine water and it doesn't mix with it. But if you shake the test tube to temporarily mix the two liquids the brown color completely disappears from the bromine water. This is because the bromine that was dissolved in the water has reacted with the alkene molecules in one of these addition reactions. In contrast if you add bromine water to an alkane at room temperature no color change occurs at all because there's insufficient activation energy to cause a reaction and the water remains brown. So the essence of the test is if you have an unknown organic liquid and you want to know if it's an alkane or an alkene you add bromine water and you shake it. If the bromine water loses its color it's an alkene and if it stays brown it's an alkane.