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Do flames contain plasma?
Category: Physics
Published: May 28, 2014
By: Christopher S. Baird, author of The Top 50 Science Questions with Surprising Answers and physics professor at West Texas A&M University
The answer to this question is more complicated than most people realize. Some flames do contain plasma and some flames do not. To properly answer this question, we really have to first strictly define what we mean by "plasma". A textbook definition of a plasma is an ionized gas. "Ionized gas" means that some electrons have been ripped completely off the atoms that make up the gas. The effectively-free electrons are negatively charged and the resulting ionized atoms end up positively charged. An "ion" is an atom with an unequal number of electrons and protons. This definition is a good starting point, but it is not exact enough. Every gas contains a few ions and freed electrons, and yet not every gas is a plasma. There must by some cutoff point where there are enough ions in the gas that it begins acting like a plasma.
What does it mean to act like a plasma? A plasma is an ionized gas that is reflective to low-frequency electromagnetic waves like radio waves. Described on a more basic level, a plasma shields out electric fields. A plasma is able to do this because enough negatively-charged electrons and positively-charged ions are locally free and are able bind to each other in a long-range, collective way. The collective behavior of ions and electrons means that they are able to respond strongly to incident electric fields and move to cancel out these fields. Therefore, a stricter definition of a plasma is a gas where there are enough freed electrons and ions that they act collectively. The distance that an external electric field can reach into a cloud of charged particles is characterized by the "Debye length". The more atoms that are ionized, the stronger the collective oscillations of the charges, and the smaller the Debye length. The strictest definition of a plasma is therefore an ionized gas with enough ionization that the Debye length is significantly smaller than the width of the gas cloud.
In a flame, ionization of the air atoms occurs because the temperature is high enough to cause the atoms to knock into each other and rip off electrons. Therefore, in a flame, the amount of ionization depends on the temperature. (Other mechanisms can lead to ionization. For instance, in lightning, strong electric currents cause the ionization. In the ionosphere, sunlight causes the ionization.) The bottom line is that a flame only becomes a plasma if it gets hot enough. Flames at lower temperatures do not contain enough ionization to become a plasma. On the other hand, a higher-temperature flame does indeed contain enough freed electrons and ions to act as a plasma.
For example, an everyday wax candle has a flame that burns at a maximum temperature of 1,500 degrees Celsius, which is too low to create very many ions. A candle flame is therefore not a plasma. Note that the vibrant red-orange-yellow colors that we see in a flame are not created from the flame being a plasma. Rather, these colors are emitted by incompletely-burnt particles of fuel ("soot") that are so hot that they are glowing like an electric toaster element. If you pump enough oxygen into a flame, the combustion becomes complete and the red-orange-yellow flame goes away. With this in mind, it should be clear that a candle flame gives off light even though it is not a plasma. In contrast to candle flames, certain burning mixtures of acetylene can reach 3,100 degrees Celsius, with an associated Debye length of 0.01 millimeters, according to the Coalition for Plasma Science. Such flames are therefore plasmas (as long as the flame is much larger than 0.01 millimeters, which is usually the case). Other flames, including flames from campfires, propane stoves, and cigarette lighters, have temperatures that lie somewhere between these two extremes, and therefore may or may not be plasma. Everyday flames such as from the burning of wood, charcoal, gasoline, propane, or natural gas are typically not hot enough to act like a plasma.