When 2 hydrogen molecules (H₂) react with 1 oxygen molecule (O₂), the product is 2 molecules of water (H₂O).

This is a classic example of a chemical reaction, specifically a combustion reaction, where hydrogen and oxygen combine to form water and release energy in the form of heat and light.

Here's the balanced chemical equation:

2H₂ + O₂ → 2H₂O