What is an alkene and how might one be identified?

Alkenes are a family of hydrocarbons (compounds made up of only hydrogen and carbon atoms) that all share the same chemical formula, similar chemical properties (due to same functional group) and trends in physical properties. The functional group they all share is at least one or more carbon to carbon double bond hence they are unsaturated (Don't have the maximal number of hydrogen atoms possible due to presence of double bonds. 

They have the chemical formula CnH2n. An example of an alkene is ethene (see structure). The series can be rememebered by the acronym, Eat Peanut Butter - Ethene (2C) , Propene (3C) , Butene (4C). Alkenes decolourise bromine water from orange.