 In this tutorial you will learn how to carry out a test to show the presence of an alkene. In our previous video on the alkanes and alkanes we discussed how the alkanes have a carbon-carbon double covalent bond and are known as unsaturated hydrocarbons. They are unsaturated because the double bond can break and allow two new atoms to join the molecule in an addition reaction. The word unsaturated suggests the fact that an alkene molecule is not bonded to the maximum number of atoms that it could be if the double bond was to break. Alkanes however are saturated. There are no double bonds and so no addition reaction can take place therefore no more atoms can be added to the molecule. Both the alkanes and alkanes are colourless. For example, hexane and hexene are both colourless liquids. Their displayed formula looks like this. Let us now consider bromine water. Bromine is a diatomic molecule and when dissolved in a solvent it forms an orange or yellowy brown solution. Using bromine water provides us with a test for the alkanes. Look at the displayed formula below. Pause this video and take a moment to consider what reactions could happen. Since the alkene, hexane is unsaturated, an addition reaction will occur. The double bond will break and bromine will split and the atoms added across the double bond. The product formed is called a dibromoalkane and is also colourless. Provided only a small amount of bromine water is added, all the brown bromine will react with the excess alkene to form a colourless solution. Let us try the same with the alkane. Pause the video and have a think about what reaction, if any, will occur this time. Since alkanes are already saturated, there is no room for any bromine atoms to be added to the molecule. Consequently, no reaction takes place and the solution remains brown. So in summary, alkene are unsaturated. The double bond can break and new atoms can be added to the molecule. Bromine water is a brown or yellowy orange solution. When you add an alkene to bromine water, the solution goes from brown to colourless. When an alkane is added to bromine water, no reaction occurs and the solution remains brown.