Ethanol (\(C_2H_5OH\)) forms ethene when heated at 443 K in the presence of excess concentrated sulphuric acid.
Concentrated sulphuric acid acts as a dehydrating agent in this reaction. It removes a molecule of water from ethanol to form ethene.
\[ C_2H_5OH \xrightarrow{\text{conc. } H_2SO_4, \, 443 K} C_2H_4 + H_2O \]