Final answer:
In the case of hydration of an alkene, two water molecules can be added across the double bond in a syn addition manner.
Step-by-step explanation:
Syn addition is the addition of two substituents from the same side (same face) of the double bond. In the case of hydration of an alkene, two water molecules can be added across the double bond in a syn addition manner. The reaction requires a strong acid catalyst, such as sulfuric acid, and proceeds through the formation of a carbocation intermediate.