A green flame in fire is caused by the presence of certain elements or compounds that emit green light when heated. The most common sources of green flames are boron compounds (such as boric acid or borax), copper compounds, and zinc. When these elements or their compounds are heated in the flame, their electrons get excited and then release energy as light at a wavelength corresponding to green in the visible spectrum.

Common Causes of Green Flames

• Boron: Boron compounds like boric acid or borax produce a bright green flame when burned. This is why adding boric acid to alcohol or methanol creates green fire.
• Copper: Copper or copper salts in the flame cause a green color due to the emission spectrum of copper atoms.
• Zinc: Zinc compounds can also produce a greenish flame when they oxidize.

How It Works Physically

When these elements are heated, their electrons jump to higher energy levels and when they return to lower levels, photons of light are emitted. The energy of these photons corresponds to particular colors, green in this case.

Summary of Practical Applications

• Boric acid or borax-based green flames are used in experiments, pyrotechnics, and sometimes in artistic fire displays.
• Copper-containing metals or salts are also known to color flames green during metal welding or flame tests.

Thus, green fire is essentially a flame that contains excited atoms or ions of elements like boron or copper that emit green light during combustion.