Fire is a chemical reaction that produces light and heat. The color of fire depends primarily on what is burning. For example, burning wood or paper generally appears yellow or orange, while burning propane or natural gas produces a blue flame.
However, various chemicals can be added to fires to produce different colored flames. Pyrotechnicians have long used metal salts to create colorful fireworks displays. With the right chemicals, it’s possible to change the color of an ordinary fire as well.
What Determines the Color of Fire?
The color of fire is primarily dependent on two factors:
1. Blackbody radiation
This is electromagnetic radiation emitted by hot objects due to their temperature. The hotter an object, the shorter wavelength (higher frequency and energy) the emitted radiation.
For example, an object at 500°C emits a faint red glow, while at 1000°C it glows orange-white. A fire’s blackbody radiation is responsible for its yellow/orange colors.
2. Excited electron emissions
When electrons in atoms gain energy, they jump to higher energy levels. As they drop back down, photons are emitted at specific wavelengths depending on the atom.
Metals and metal salts emit very visible colors this way. Sodium produces an intense yellow, copper makes a blue-green flame, etc. This is why fireworks use metal compounds.
How Chemicals Change the Color of Fire
By adding various chemicals to a fire, the emitted light changes based on their electron emissions. Here are some common examples:
Sodium (Na)
Table salt (sodium chloride) or sodium nitrate produce bright yellow flames. Sodium electrons emit photons at 589 nm wavelength when dropping to lower energy levels, corresponding to yellow light.
Potassium (K)
Potassium compounds like potassium chloride or potassium nitrate produce violet flames. Potassium electrons emit photons at 404 nm and 770 nm when dropping energy levels, corresponding to violet.
Copper (Cu)
Copper chloride or copper sulfate create blue-green flames. Copper emits photons at 521 nm and 582 nm wavelengths, corresponding to blue-green.
Barium (Ba)
Barium nitrate or barium chlorate produce bright green flames. Barium electrons emit photons at 493 nm and 553 nm wavelengths when dropping energy levels, corresponding to green.
Strontium (Sr)
Strontium nitrate or strontium carbonate create crimson red flames. Strontium electrons emit photons at 606 nm and 622 nm wavelengths, corresponding to deep red.
Calcium (Ca)
Calcium salts like calcium chloride produce orange flames. Calcium electrons emit photons at 612 nm wavelength, causing the orange color.
| Chemical | Flame Color |
|---|---|
| Sodium (Na) | Yellow |
| Potassium (K) | Violet |
| Copper (Cu) | Blue-green |
| Barium (Ba) | Green |
| Strontium (Sr) | Red |
| Calcium (Ca) | Orange |
How to Change the Color of Fire
To change the color of flames, simply add compounds of the desired metal to the fire. Some methods include:
Directly adding metal salts
Table salt (sodium chloride) or other powdered metal salts can be sprinkled directly into fires. The metal ions will vaporize and get excited, emitting colored light.
Coating materials
Soaking wood or paper in solutions of metal salts will coat the fuel with color-emitting compounds as it burns. This is a common technique in pyrotechnics.
Aerosol sprays
Spraying atomized solutions of metal salts into flames is an easy way to change colors. Potassium chloride for purple fire or copper sulfate for blue-green are common examples.
Packed containers
Placing metal compounds in packed containers with small holes allows colored flames to jet out. Calcium chloride for orange jets or strontium nitrate for red are typical choices.
Safety and Environmental Considerations
While flame colorants can produce impressive effects, there are some safety and environmental impacts to consider:
– Metal salts may be toxic or irritating if inhaled. Use caution and avoid breathing in any vapors.
– Colored flames may still be very hot and able to burn. Do not touch or get too close. Practice good fire safety.
– Some metal ions persist in the environment after release. Try to contain and properly dispose of any residual ash/salts.
– Perform demonstrations only in approved locations with proper fire safety equipment nearby. Do not light unauthorized or dangerous fires.
Conclusion
Various metal ions can dramatically change the color of fire when added to the flame. Compounds of sodium, potassium, copper, barium, strontium, and calcium are common choices for creating yellow, violet, blue-green, green, red, and orange fires respectively. While visually stunning, proper safety precautions need to be taken when altering flame colors. With care and responsibility, pyrotechnic chemicals provide a scientific and artistic way to appreciate fire in a new light.
