Ethene hydration as an addition reaction

Steps:

• Identify the reaction: ethene (C₂H₄) + steam (H₂O) → ethanol (C₂H₅OH) with phosphoric acid catalyst.
• Recognize ethene as an alkene with a C=C double bond.
• Note that the double bond breaks as H and OH from water add across it.
• Classify as addition since an unsaturated molecule gains atoms to become saturated.

Why A is correct:

• Addition reactions involve small molecules adding to the double bond of alkenes, as in the hydration formula C₂H₄ + H₂O → C₂H₅OH, saturating the alkene.

Why the others are wrong:

• B: Fermentation produces ethanol biologically from sugars like glucose, not from alkenes.
• C: Polymerisation joins many alkene monomers into long chains, not forming a single alcohol molecule.
• D: Reduction adds hydrogen or electrons to decrease oxidation state, but hydration adds water without net hydrogen gain.

Final answer: A