The wind got stronger and colder. The darkened sky flashed intermittently as the thunderstorm raged on. As an avid photographer, Nikil Sreenivas knew that he was on the cusp of a spectacular show by nature. He opened the camera app on his iPhone and waited, until he got the perfect footage of a lightning bolt striking the lightning conductor of Bishan Stadium.
He was awestruck by the ghostly glow of neon purple from the bolt, which painted the sky pink. But he also sensed danger, as he posted the video on the Facebook page for Singaporean hikers to warn them of the grave danger.
After all, each bolt contains up to one billion volts of electricity and heats the surrounding air to around 20000 °C. At that temperature, the air turns into a plasma, which is the fourth state of matter that emits light and other electromagnetic waves.
Plasma: the fourth state of matter
We are all too familiar with the three states of matter. With enough heat, solids melt into liquids and then evaporate to form gases. This happens as heat energy overcomes the attraction between tiny particles, separating them from each other until they escape into the air.
But the story usually stops here and we rarely ask for the sequel. What if we continue to heat a gas to around 20000 °C?
As the temperature goes way past the boiling point, heat energy accumulates dangerously, ready to tear more things apart. Since it has already separated particles from one another, it zeroes in on their subatomic particles. It pulls some or all of the electrons away from the positively charged nucleus of an atom. This disrupts the balance of electrons and protons in the atom. Since there are now fewer electrons, the atom becomes an ion with a positive charge. We call this process ionisation.
When enough particles have been ionised, a gas officially becomes a plasma. This happens when lightning strikes and the temperature spikes.
The electrons that are yanked out during ionisation are free-moving, allowing lightning to conduct electricity from the clouds to structures on the ground, like the poor Bishan Stadium.
Glow, plasma, glow
The plasma in lightning glows when free-moving electrons bump into positively charged ions. The bump transfers energy from the free-moving electron to a bound electron in the ion. This extra energy promotes the bound electron from its original electron shell to a shell at a higher energy level.
However, in the new shell, the bound electron is edgy and unsettled. It does not want to remain in there! And so it loses the energy as light, emitting a glow in a specific colour.
Every substance gives off a different, characteristic glow in the plasma state. The purplish glow of lightning is due to the ionisation of nitrogen, which makes up 78% of the air.
Many ways of forming plasma, just not chemistry
Extreme heating is not the only way to form plasma.
Neon signs use electrical energy to eject electrons from neon atoms. This forms plasma at close to room temperature. The neon plasma gives an iconic orange-red glow that is different from the purplish glow of nitrogen plasma. Furthermore, scientists have used laser to produce plasma, whereby light energy strips electrons from atoms.
What about chemical reactions? Since metallic atoms tend to lose their outermost electrons as they become oxidised, it seems reasonable to suspect that this can be a method to form plasma.
However, we cannot forget that the electrons lost are immediately gained by non-metallic atoms to complete their octet configuration. In other words, the transfer of electron from one reactant to another in a chemical reaction produces oppositely charged ions instead of free-moving electrons. Plasma is not formed.
Even though chemical reactions cannot form plasma, there is still so much plasma that it makes up 99% of the universe. Kudos to the lightnings and other physical processes making them!
