When sodium oxide reacts with water, it undergoes a chemical reaction to form sodium hydroxide, which is a strong base. The reaction can be represented as follows:

Na2O + H2O → 2NaOH

In this reaction, sodium oxide combines with water to produce two molecules of sodium hydroxide. Sodium hydroxide is a highly soluble compound and dissociates in water to release sodium (Na+) and hydroxide (OH-) ions. These hydroxide ions contribute to the basic properties of the solution.

Therefore, when sodium oxide reacts with water, the base formed is sodium hydroxide (NaOH).