Plasma is a superheated matter that comprises over 99% of the visible universe. It is an ionized gas, meaning that some electrons have been ripped completely off the atoms that make up the gas. However, not every gas is a plasma, and there must be a cutoff point where there are enough ions in the gas that it begins acting like a plasma. A stricter definition of a plasma is a gas where there are enough freed electrons and ions that they act collectively. 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. 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.

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