The chemical reaction is:

Na2O (sodium oxide) + H2O (water) → 2 NaOH (sodium hydroxide) + H2 (hydrogen gas)

This reaction is highly exothermic, meaning it releases a large amount of heat. This reaction is commonly used in the laboratory to prepare sodium hydroxide. Sodium hydroxide is a very strong base, making this reaction a very dangerous one and proper safety precautions should be taken.