Water forms from its elements according to the reaction: 2 H2 + O2 - -> 2 H2O. In this reaction, which element is reduced?

Oxygen is reduced while hydrogen is oxidized.

Explanation:

Chemical reaction:

2H₂ + O₂ → 2H₂O

The oxidation state of hydrogen and oxygen in left side is zero while on right side the charge on oxygen is - 2 and hydrogen is H₂⁺². So hydrogen is oxidized and oxygen is reduced.

oxygen is oxidizing agent while hydrogen is reducing agent.

Oxidation:

Oxidation involve the removal of electrons and oxidation state of atom of an element is increased.

Reduction:

Reduction involve the gain of electron and oxidation number is decreased.

Oxidizing agents:

Oxidizing agents oxidize the other elements and itself gets reduced.

Reducing agents:

Reducing agents reduced the other element are it self gets oxidized.