Alkenes
Alkenes have double bonds, making them unsaturated hydrocarbons.
Alkenes have the general formula of CnH2n – the first four being ethene, propene, butene and pentene.
Combustion
Like alkanes, alkenes burn in oxygen or air to give carbon dioxide and water.
Reaction with Bromine
Alkenes undergo addition reactions, in which part of the double bond breaks and is used to join other atoms onto the two carbon atoms. When added to alkenes, and the test tube is shook, the brown of the bromine would be decolourised, making it suitable as a test for alkenes.
The product of reacting ethene to bromine gives 1, 2-dibromoethane and is a colourless liquid.
CH2=CH2(g) + Br2(aq) -> CH2BrCH2Br(l)