Modern Alchemy: Pouring Water on an Oil Fire

with Dr Peter Wothers

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Up on the roof...

Most people know the advice about never putting water on an oil fire. We joined Dr Peter Wothers on the roof of the Ri building in London to show exactly why.

Above the famous Lecture Theatre, the team set up a clear protective shield around a beaker filled with 150ml of oil. This was heated over a Bunsen until a small fire was burning. Wearing a fire protective suit, Dr Wothers then poured water from a small cup directly onto the fire.

The reaction is so violent because water and oil don’t mix. When the water is poured into the beaker of burning oil it sinks to the bottom and, due to the intense heat, vaporizes into steam almost instantaneously. With this phase change from a liquid to a gaseous state the water expands by up to 1700 times and forces the fire above it upwards. This oxygenates the oil and creates the huge flame you can see in the video.       

Peter Wothers explains more:

"This is a great reaction to illustrate the topic of our second Christmas Lecture (Water), but it would also have done for Fire too if we had been presenting a fourth lecture. It is one of my favourite demos since it really rams home how dangerous it is to add water to an oil fire. I was never really aware of how large the flames are around me since I was busy concentrating on what I was doing!"

Devised to promote the 2012 Christmas Lectures, this is one of three large-scale, chemistry demos that were too big (or too dangerous!) to perform in the Ri Lecture Theatre in front of 400 young people.

This experiment was undertaken under the supervision of professionals and should not be replicated. 




Dr Peter Wothers, Andrew Marmery
Royal Institution, London
Filmed in:
The Theatre

The Refinery

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CHRISTMAS LECTURES 2012 - The Modern Alchemist

cc_by-nc-sa License: Creative Commons



Of all of the chemical compounds-- and there are many millions-- I think water has to be one of the most amazing. And it's one of my favourite. What we're going to do today is see how just a tiny amount of water can cause an enormous fire ball.

This is a giant safety screen. A giant cylindrical safety screen, as it will be, to protect Peter from the effects of pouring a small amount of water into some burning, boiling oil, which hopefully will produce a giant fire ball. Thanks to the screen it will go up, rather than out. We're going to take you up to the roof because no ordinary room could contain the fire ball that we are hoping to produce.

As you may know, oil and water just don't mix. And I can show this. So we'll put some water in here and put some oil in here. The oil molecules are really actually quite large. They contain a lot of hydrogen, a lot of carbon atoms, a bit of oxygen, and maybe a few other elements. But this is really quite a large molecule. The water? Just three atoms-- which one's going to come out on top?

The amazing thing is the water is actually more dense than the oil. And this again is because of the interactions between the water molecules really holding them in closely together. The oil molecules are not polarised in the same way. They don't have that same separation of charge that the water does, and that's why we've got these separate layers. But what would happen if the oil was really hot?

Here we've got around 100 millilitres of oil. We're going to heat this up, and eventually it will catch fire. Then we're going to add some water to the burning oil. Now we've seen that the water sinks underneath the oil, and at this high temperature it's going to instantly turn into steam. And then we'll see what happens.

One millilitre of water will produce enough steam to fill this entire flask, which takes up around 2000 times the volume that the water did.

OK, I think we've got ignition here. If we can have the gas off.

Gas off?

Gas off.

Gas off.

OK? Ready?


Well, now you can see why you should never add water to burning oil. How did it look?

It was awesome.

Did it come out the top, or--

Really good. Yes it did, for several feet, a foot and a half--

I'm sorry, I have no idea. Absolutely none. But I did feel this. It was good.

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