Published: 15 August, 2002

Hydrogen, oxygen burn, but water…

If hydrogen and oxygen are both flammable why doesn't water burn?

Let's rephrase the question. If wood burns, why doesn't wood ash burn? Water is the ash left by the burning of hydrogen and oxygen. When things burn, they rearrange their atoms into a lower energy form, and give up the energy difference in light and heat. You see this as the flame. Generally the lower energy molecules have given up all their excess energy, and cannot burn.

There are a few exceptions. Wood is mostly a combination of molecules containing carbon, hydrogen and oxygen. When wood burns, it combines with the oxygen in the air to make water and carbon dioxide, neither of which burn. If wood is burned in air without enough oxygen, some of carbon forms carbon monoxide, which is a very poisonous gas that can be burned.

Most of the natural fuels also contain carbon and form carbon dioxide when burned in air. Carbon dioxide is a "greenhouse" gas which traps heat in the atmosphere, and is leading to global warming. Hydrogen only forms water when it burns with oxygen, and is the cleanest fuel known. The least polluting energy cycle is to use solar energy to separate water into hydrogen and oxygen, and then burn the hydrogen when you need a fuel.

Under conditions found on Earth, water doesn't burn, but if a water molecule were put into the center of a very hot star, its nuclei would fuse giving off more energy. The ultimate nuclear ash is iron which is the lowest energy nucleus. Iron and nickel are formed in very high amounts when a giant star blows up forming a supernova. That is why so many meteorites are nickel-iron in nature. The whole reason for bringing this up is that when the universe was created, only very light atoms were formed. With the exception of hydrogen, practically every atom seen on Earth was formed in the furnace of a supernova.