A C=C double bond is two bonds between the same two carbon atoms. Hydrocarbons with a C=C double bond are called alkenes. Ethene, which you met early on, is the smallest. In an addition reaction, one of the two bonds opens, and each carbon uses its free hand to hold a new atom.
Two atoms add across the double bond, one on each carbon, and the C=C becomes a single bond. Every atom of both reactants ends up in one product.
Some of the ethanol used in industry is made this way, as a solvent and to make other chemicals. Ethene and steam pass over a phosphoric acid catalyst at about 300 °C and high pressure. A catalyst speeds up a reaction and comes out unchanged, so it works again and again. The ethanol in drinks is made by yeast, from sugar.
Every carbon in an alkane already holds four partners with single bonds. An alkene has a double bond to open, so alkenes take part in addition reactions and react far more readily than alkanes. Ethene molecules can even add to each other, opening their double bonds to join into the long chains of poly(ethene). That is the next lesson.
The bromine water test
Bromine, Br₂, adds across a C=C in the same way. Ethene and bromine make 1,2-dibromoethane, C₂H₄Br₂, with one bromine atom on each carbon.
Bromine dissolved in water, called bromine water, is orange. Shake it with an alkene and the orange colour disappears, because the bromine is used up adding across the double bonds. Shaken with an alkane, it stays orange. Chemists use this as a quick test for a C=C double bond.