Contrast the mechanism of the two preceding reactions, the dehydration and condensation of ethanol.

There are numerous dehydration reactions. Some of them are nitrile formation and ketene formation. The dehydration of primary amides generates a nitrile.

An alcohol molecule may strike a protonated primary alcohol to get subjected to an S_N2 displacement reaction. This results in bimolecular dehydration forming an ether. This is an example of a condensation reaction.

Both the mechanisms start with the protonation of oxygen.

The protonation of the oxygen atom

Another ethanol molecule functions as a base in one mechanism leading to elimination reaction.

Elimination reaction where a molecule of ethanol serves as a base

Another molecule of ethanol serves as a nucleophile in the other mechanism leading to a substitution reaction.

Substitution reaction where a molecule of ethanol serves as a nucleophile