Re: Why isn't water flammable?

That question is one that a lot of people have problems with. It is not a silly question. I am going to sidetrack a little to put before you one of my favourite quotations. It comes from an English chemist called William Ford Stephenson, around 1780. Referring to the French chemist Lavoisier, he said

This arch-magician has so far imposed on our credulity as to persuade us that water, the most powerful natural anti-phlogistic we possess, is a compound of two gases, one of which surpasses all others in its inflammability.

I need to explain some of this. The English chemists of that time believed that flames were made of a substance called phlogiston. Phlogiston was what gave metals and plant and animal tissue the properties of being strong and shiny and elastic, and when these materials burned, they lost their phlogiston (visible as the flame) and turned into heaps of powdery ash, dust, or oxide.

When Stephenson referred to an "anti-phlogistic" he meant something that would not itself burn, and that was pretty good at putting out fires. Lavoisier pioneered our modern understanding that burning involved materials combining with a component of the air (oxygen). It was not until much later that chemists could recognise that flames were a manifestation of energy rather than of matter.

So you are not the first to find this aspect of chemistry puzzling. You are in good company!

Hydrogen gas and oxygen gas are, in a sense, high energy chemicals. They have a lot of stored chemical energy. Water is a low energy chemical. When you combine hydrogen and oxygen together to get water, you also get a lot of energy released. If you simply burn hydrogen in oxygen, it is released as heat and light, but it is also possible to make a hydrogen/oxygen fuel cell and get the energy released as electricity.

However, water cannot burn. The only way you could imagine getting energy out of water burning would be if it could combine with even more oxygen to produce an even lower energy chemical. That cannot happen. H2O2 hydrogen peroxide has a much higher chemical energy than water, and H2O3, H2O4, etc. simply do not exist as stable compounds.

The whole of chemistry raises similar problems. It is the nature of chemistry that the various compounds you encounter usually do not reflect the properties of the simple substances they are made of. Ordinary salt is made of white crystals that dissolve in water, make food taste nice, and don't do much else. But sodium is a light metal which tarnishes rapidly in air, and reacts violently with water, while chlorine is a very poisonous yellow/green gas.