Eli5: How is hydrogen, a potent source of fuel with high energy release, combined with oxygen, a facilitator for combustion and energy release, mix together to make water which we drink like it’s no big deal?

Question

bonus points if you can explain why water has no caloric value, but hydrogen can power huge machinery.

In: 3

Answer 1

Water is stable for exactly the same reason that hydrogen and oxygen mixed together (which is *not* the same thing) aren’t.

In a water molecule, the bond between hydrogen and oxygen is very strong. That means it takes a lot of energy to break it. But since chemical processes are reversible, that also means it *releases* a lot of energy to *form* it. This isn’t quite an exact analogy, but you can think of it as being sort of like putting two magnets on a smooth table: they’ll slide together and collide with considerable force, and then it *takes* a lot of energy to pull them back apart.

That released energy is what makes hydrogen a good fuel. But in water, the bonds are already formed, so you’re not able to release extra energy by forming what’s already there.

Water has no caloric value for the same reason. Your body extracts energy from foods by reacting them with oxygen and capturing the energy released by the formation of strong carbon-oxygen and hydrogen-oxygen bonds. In water, those bonds are already formed, so there’s no energy to extract.