This post covers two separate aspects of fire; color and shape. The flame color is determined by the fuel used, available oxygen, and the rate of combustion. The colors that we are most familiar with are generally blue, associated with propane, butane, and other gases, and yellow as with candles and other petrochemical sources. This is of course, a generalization, and we can see red and combinations of the base colors depending for example, on the composition of a candle wick. But, typically, we would not expect to see green. However, the color of flames can be changed by the simple addition of some salts of metals. Examples of this modification can be found in materials for homeowners to make their own colored flame logs for fireplace use. And, safety flares and fireworks also make use of the incorporation of metal salts to provide many different and spectacular colors. In chemistry, the well-known flame test can indicate the presence of many metals from the metal salts.
The shape of a flame is typically controlled by air flow and how the air interacts with the emerging process of combustion. For example, if air is added to a flame under pressure as in a propane torch or Bunsen burner, we see a more focused and hotter flame. But normally, it is not typical to see flames assume the shape of a vortex. But, this does occur in nature in a fire tornado or fire whirl. Similar to a standard tornado, the air around a flame is circular and causes a vortex to form and move laterally. In this video this phenomena is demonstrated and is interesting to watch.
In this experiment I am using boric acid (hydrogen borate) to provide the green color. Boric acid is used as an antiseptic, eye wash, and ant killer. It is available at drug stores and garden centers. The fuel is methyl alcohol, available as the gas line antifreeze, Heet. This is sold at auto stores and Walmart and is a clean burning fuel. Other alcohols may work as well and are available at drug stores as Isopropyl, and Ethyl alcohol. In this video I have used 1/4 cup Heet, and 1 teaspoon of boric acid for a 5 minute burn time.
Neither the green fire or tornado experiments are original, but, I have not seen the two ideas combined and I think that the green fire is more visually interesting when viewed as a vortex. A search for "green fire", "fire tornado", and "flame test", will yield results showing other chemicals and their colors, and video of fire tornadoes. Please be safe when using fire and burning chemicals. If you cannot be safe, don;t do it! Enjoy the video: