|MadSci Network: Chemistry|
Your question illustrates why you should never pour water on a grease or oil fire (or into hot oil or grease). To understand why you get a big ball of flame from your "demonstration" we have to examine what is taking place. So, let's take it one step at a time.
First, the grease is heated up on a stove or fire. When I fry chicken, I turn the oil temperature up to 365 degrees F, and I expect that your bacon grease is probably that hot.
Water boils at 212 degrees F (or less as altitude increases). When water turns to steam, it increases in volume about one thousand times. So when the water enters the hot grease, some of the water vaporizes as it cools the grease down to the boiling point of water. As the water turns to vapor, it blows the grease into a fine mist. So the second part of this process is the atomization of the grease.
The third thing that has to happen is that there must be an ignition source. When you are doing this demonstration, you are outside using a camp stove or campfire. That atomized grease is flying everywhere and some droplets will come close enough to the flame of the stove or fire to ignite. The surrounding droplets will be ignited by those first few droplets and soon the whole atomized mist of grease catches fire. This whole process happens fast; too fast for the eye to see where the whole ball of flame started initially burning.
Finally, the heat produced by the combustion of the grease causes the whole fireball to have a lower density than the surrounding air, so the fireball rises in the air - apparently up to 15 feet based upon your observations.
So, here is a test you could run to see if this explaination is correct, at least as far as the source of ignition is concerned. Heat up the grease hot, and then move it off the stove, away from the campfire and any other sources of flame and then dump water into the grease. You should get a cloud of white smoke and grease spattered all over the place but no fire.
Please keep in mind that your demonstration of burning grease is pretty hazardous not only because of the fireball but because grease burns can be very painful. And of course, not all the grease burns up; a lot may be scattered around producing a bit of a mess and contaminating the environment. So, be careful and try not to see how big a fireball you can produce.
Try the links in the MadSci Library for more information on Chemistry.