Remember alkenes contain the functional group C=C. As soon as you see this functional group in any molecule as a chemist you should think of only one thing- addition reactions! This C=C group in a molecule is vulnerable to attack by other substances which simply add across it e.g.
Hopefully you will be able to see a pattern in these reactions. A small molecule approaches the C=C, this bond then breaks to leave a C-C and the bonds holding the approaching small molecule break, the small molecule then adds to the two carbon atoms that were in the carbon carbon double bond. The last reaction above where an alkene is turned into a saturated alkane is a particularly useful reaction industrially. It is called hydrogenation, simply because hydrogen is added to the C=C, it is used to "harden" vegetable oils and to turn them into margarines. It requires a catalyst of nickel and gentle heat to get it going.
note the numbers 1,2- in the names of the compounds above is simply used to indicate which carbon atoms the small molecule has added to.
It is also possible to add steam across the C=C bond in alkenes to make alcohols This method of making alcohols is called direct hydration. It requires a fairly high temperature of around 3000C and a pressure of 65 atmospheres as well as a phosphoric acid catalyst. The reaction is similar to the examples above, in the end the elements of water are added across the C=C bond, this is shown below:
In fact the addition of bromine across a carbon carbon double bond, C=C, is used as a test for unsaturation. Bromine is dissolved in water to form a red-brown solution called bromine water. When bromine water is added to a suspected unsaturated substance in a boiling tube and shaken then, if the substance is unsaturated the bromine water will decolourise almost immediately. If the substance in the boiling tube is saturated then the bromine water will decolourise very slowly. This is shown in the drawing below: