Why doesn't water burn?

05 December 2016
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Water contains hydrogen and oxygen, both of which are important in combustion, so why is it used to put out fires? Why doesn't water burn? Graihagh Jackson put this to chemist Peter Wothers from the University of Cambridge...

In this episode

00:00 - Why doesn't water burn?

Water contains hydrogen and oxygen, both of which are important in combustion, so why is it used to put out fires?

Why doesn't water burn?

Water contains hydrogen and oxygen, both of which are important in combustion, so why is it used to put out fires? Graihagh Jackson put this to chemist Peter Wothers from the University of Cambridge...

Peter - Well, this is actually a chemical reaction that's taking place between the fuel and the gases in the atmosphere, the oxygen. So, for instance, if hydrogen gas, which we know is very, very flammable, this burns because it will chemically combine with the oxygen that's in the air and this will form water.

Graihagh - Wait, a fire makes water? Chemistry never fails to surprise me. But what about liquid fuels?

Peter - Now other things burn such as petrol, a nice wet liquid, because this also contains things that can react with the oxygen from the air. So the petrol is made up of the elements carbon and hydrogen combined together. And each of those, combined with the oxygen to form carbon dioxide and, again, more water when the hydrogen part of the petrol combines with the oxygen. So this is what's happening when things burn. Energy is released in a very violent reaction as energy comes out as the elements, hydrogen or carbon, combine with the oxygen from the air. Graihagh - I'm with you - but if water is made of hydrogen, very flammable, and oxygen, which sustains a fire, then why doesn't water burn?

Peter - Well, in a sense, the answer is because it already has carried out this reaction. The hydrogen is already chemically combined with the oxygen the air. So there's no way it can do that again. Graihagh - That's because the bond between two hydrogen atoms and an oxygen is so strong, you need lots of energy to pull them apart. Much more energy than, say, a wood-fire creates and that's why firemen use water to put out fires. The water cools the combustible material and also helps stop the fuel come into contact with the air, the oxygen, which is the thing that sustains the fire... But, as you well know, we don't just use water to put out fires...

Peter - Similarly, carbon dioxide is another good fire extinguisher, especially for things such as wood burning, or petrol and so on because again, this is already combined. The carbon has combined with the oxygen to form a very stable compound, carbon dioxide.

Graihagh - However, if you tried to put water on a magnesium fire, that would be hot enough the break H2O bond, and will actually make your fire worse. So, let's leave any fire extinguishing to the experts.

Chris - We also heard from Brendan on Facebook also wrote to us with the correct answer and James also wrote in to add:

"A very important thing to remember is that compounds do not necessarily take on the properties of the elements from which they are made. Chlorine is a toxic gas, sodium is a metal that explodes on contact with water. But sodium chloride, that's otherwise known as table salt, is fine to put on your chips, of course ignoring the slightly elevated risk of heart failure and high blood pressure that goes with it!"

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