Dehydration of ethanol => ethylene

Ethanol is dehydrated by heating with a catalyst of excess phosphoric or sulfuric acid at about 170oC and high pressure.
C2H5OH(g) => C2H4(g)+H2O(g)

one of the lone pairs of electrons on the oxygen picks up a hydrogen ion from the sulphuric acid. The alcohol is said to be protonated.
The protonated ethanol loses a water molecule to give a carbocation (a carbonium ion).
a hydrogensulphate ion (from the sulphuric acid) pulls off a hydrogen ion from the carbocation.

simplified version – sulfuric acid represented by H+ ion; arrows represent movement of electrons
Jim Clark. (2000). dehydration of ethanol to give ethene. Available: Last accessed 26/1/11. [Bibliography item label not found.] - for citations