If fire needs oxygen of burn, how come it is used in H2O to put out fire?
Fire occurs when free oxygen combines with (oxidizes) another element (mostly Carbon), creating new molecules (like carbon dioxide) and energy in the process. The oxygen in water has already been 'burned' after it combines with hydrogen, creating energy and molecules of hydrogen dioxide, better known as water. The fuel cells used in Hydrogen fueled cars translates that energy into motion, sending water vapor out the exhaust. So the oxygen in H2O has already 'oxidized' and is not free to contribute to further burning.
Another way of thinking about this is to understand that atoms lose their individual identities when they become parts of molecules. Sodium by itself reacts violently in contact with water. Chlorine is a poisonous gas. When a sodium atom and chlorine atom combine, however, they form a molecule of sodium chloride, also known as table salt.
Paul Walorski, B.A., Part-time Physics/Astronomy Instructor
'If one wishes to obtain a definite answer from Nature one must attack the question from a more general and less selfish point of view.'