Water can cause a jet of flame

You don’t always need a big laboratory for exciting experiments... Using just a few tealights and water you can create an impressive jet of flame.

What’s the probability of finding a burning chip pan in a lecture room? And what's the probability of a professor having a burning chip pan in his office? Almost nil. Nevertheless, Emergency Services (BHV) teams at the University of Groningen are trained in putting out burning pans of hot oil during their annual refresher course.

The most important part of the instructions on extinguishing burning oil is a demonstration of what not to do. The instructor demonstrates this by pouring a beaker of water over burning frying fat using a long pole from a safe distance.

This week, Cogito ergo boom! will show how you can simulate extinguishing a burning chip pan. Be warned: this is a dangerous experiment, and anything could go wrong. You could burn down your house, set fire to yourself, or suffer serious burns caused by splattering fat. If you’d rather not do the experiment yourself you can see what happens in the video on the left. If you are going to do the experiment then read the full text of the disclaimer first. If anything goes wrong it will be your own responsibility. You have been warned!

To do this experiment you need a small amount of boiling, burning fat. The original experiment described using five tealights, stacked in a pyramid. In theory, the heat produced by the tealights at the bottom should be strong enough to boil the fat in the topmost ones. In the original experiment, kiwifruit juice was then dripped onto the burning fat. Nobody was able to explain why kiwifruit juice, of all things, was supposed to produce a good jet of flame and we were not able to get the fat to ignite, whatever we did. Note: this does not mean that kiwifruit juice is a safe way to extinguish a burning chip pan!

To get the fat in one tealight burning we ended up needing a pyramid of fourteen tealights – and a lot of patience. A few drops of water dripped from a teaspoon were enough to produce a burst of flame.

A short refresher course in secondary school chemistry is sufficient to explain exactly what happened. Candle wax, frying fat and substances like petrol, fuel oil and diesel are apolar. This means that the centre of gravity of the molecule’s negative charge coincides with the centre of gravity of the positive charge. Water, however, is strongly polar, and polar and apolar compounds do not mix well at all.

When you pour water on burning boiling fat, the fat will float on the water. The heat makes the water vaporize with explosive force, causing the fat to splatter. A compound that is thoroughly mixed with air, and thus with oxygen, burns much faster than normal. And that’s your jet of flame.

Can you scale up this experiment? Yes and no. Last year, the UK (the university newspaper) described how the Groninger Studenten Alpenclub (Groningen Student Alpine Club – GSAC) turned fifty tealights on a campfire into a huge pillar of flame, several meters wide and high, during an introduction weekend. They call this ‘whooshing’. Don’t try this at home, and certainly not inside.

That leaves us here at Cogito ergo boom! with one question: why should kiwifruit juice, of all things, produce a jet of flame? Maybe it was just a trick that someone discovered during a candlelit Easter breakfast, not realizing that any other watery substance would have worked just as well? If you know the answer, please send us an e-mail!

Author: Ernst Arbouw