Chemical process that Watney uses to make water?

In what will be a recurring process (every twenty hours), Watney plans to add ten liters of carbon dioxide from the fuel plant into the Hab, and in turn, the oxygenator will turn the carbon dioxide into oxygen. Afterward, Watney will separate the hydrazine into nitrogen and hydrogen, and slowly burn the hydrogen. The combination of hydrogen and oxygen in the air will make water, which will take the form of humidity in the Hab’s atmosphere. The water reclaimer will then suck the water out of the air, where it will be stored. This is dangerous because there is a good chance that the chemicals can explode.