When certain solids undergo combustion, the flame produced can exhibit different colors depending on the emission spectrum of the burning material. The color of a flame provides important information about the chemical composition and properties of the burning substance. While many common solid fuels burn with yellow/orange flames due to blackbody radiation, a few solids distinctly produce yellow flame coloration when burnt in air. Understanding the origin of yellow flame color in burning solids is important in fields like analytical chemistry, forensics, fire safety, and combustion engineering.
What causes the yellow color in flames?
The yellow color in flames is primarily caused by the emission of light by excited sodium atoms in the hot flame zone. As the solid fuel burns and reaches high temperatures, sodium impurities present within it get excited and emit a narrowband yellow light corresponding to the sodium doublet D lines at 589 nm wavelength. This masks other color emissions and gives the flame a distinct yellow appearance. Besides sodium, emission from atomic calcium and lithium can also contribute to yellow flames. The intensity of the yellow color depends on the sodium concentration in the solid.
Which chemicals produce yellow flames?
Some common chemicals known to burn with yellow flames include:
- Sodium chloride – Table salt produces bright yellow flames since sodium is a major constituent.
- Lithium chloride – Contains lithium which emits red-orange light, giving flames a yellowish tinge.
- Calcium chloride – Calcium emissions can add yellow color to the flame.
- Sodium nitrate – A common oxidizer used in pyrotechnics, a major source of yellow from sodium.
- Sodium carbonate – Also known as soda ash, produces yellow flame with sodium content.
- Lithium perchlorate – Used in flares and fireworks, burns yellow due to lithium.
- Potassium chlorate – Potassium emissions subtly add to the yellow flame color.
- Calcium sulfate – A major constituent of plaster which burns with a pale yellow flame.
- Magnesium ribbon – Though whitish in color, may burn with a tinge of yellow due to impurities.
Among these chemicals, sodium chloride and sodium nitrate produce the most vivid and bright yellow flames and are commonly used in colored flame chemistry experiments.
Common Solids That Burn with Yellow Flames
Many solid materials containing traces of sodium, potassium, lithium or calcium compounds can emit yellow light when burnt. Some notable examples include:
1. Sodium chloride
Table salt or sodium chloride is one of the most readily available chemicals that burn with a bright yellow flame. Heating sodium chloride releases sodium atoms that get excited in the flame and emit their characteristic yellow light. This is clearly visible when sprinkling table salt over a fire. Sodium chloride melts at 801°C and decomposes to form sodium vapor and chlorine gas.
2. Coal
Most types of coal burn with yellow flames since they contain mineral impurities like sodium, lithium, and potassium compounds. Bituminous and lignite coals produce yellow flames while anthracite coal burns with a blue flame due to the low impurity content. The mineral content in coal produces ash upon burning.
3. Paraffin wax
Paraffin wax used in candles contains traces of sodium, lithium, and calcium that color the flame yellow. Most candles produce yellow flames with soot due to incomplete wax combustion. The yellow color is more prominently seen in the inner flame region.
4. Natural gas
Though not very distinct, natural gas flames can exhibit a pale yellowish coloration along with the typical blue hues. This is attributed to the presence of small amounts of sodium and lithium compounds in natural gas composition arising from the purification process.
5. Paper and wood
Paper, cardboard, and untreated wood burn with yellow smoky flames owing to the presence of sodium, potassium, and calcium found naturally in plant matter. However, the yellow color is weaker compared to other fuels. Wood ash is known to contain these alkaline metal oxides.
6. Gunpowder
Gunpowder contains sulfur, charcoal, and potassium nitrate which burn rapidly with bright yellow flashes. Potassium impurities in charcoal and nitrate produce the yellow color. Flash powder used in firecrackers also burns with a yellow flame for similar reasons.
7. Butter and ghee
The combustion of butter or ghee yields moderately yellow flames due to the presence of traces of calcium, sodium, and potassium salts present naturally in milk fat and dairy products. These metallic constituents produce their corresponding metal oxides in the ash.
8. Cereal crops
Agricultural crops like wheat, rice, and maize contain potassium compounds that color the flames yellow when burnt. This is the reason for the yellowish burning of cereal straws. Ash derived from rice husk is rich in potassium carbonate and silicates.
9. Rubber and polymers
Incomplete combustion of rubber and plastic polymers can produce yellow flames along with lots of black smoke. Various organic additives as well as traces of sodium, calcium, and magnesium salts catalyze char formation and emission of yellow light.
10. Fireworks
Pyrotechnic mixtures in fireworks contain oxidizers rich in sodium or lithium that impart the yellow color to the brilliance. Sodium nitrate, lithium carbonate, and sodium oxalate are commonly used to create yellow fireworks.
Why Do These Substances Produce Yellow Flames?
The origin of the yellow flame color in the above solid fuels can be attributed to the following chemical reasons:
- Presence of sodium impurities – Most critical reason as sodium emissions give the most intense yellow color.
- Existence of lithium and potassium compounds – Their emissions subtly contribute to the yellow hue.
- Traces of calcium salts – Calcium ions emit pale yellow light when excited.
- Incomplete combustion generating soot – Soot particles glow yellow when heated due to blackbody radiation.
- Organic radicals and hydrocarbon fragments – Decomposition of polymers forms excited carbon clusters emitting yellow light.
The intensity and shade of the yellow color depend on the relative contribution from these emitting species. Sodium provides the most distinctive and bright yellow signature. In most cases, the yellow arises from naturally occurring inorganic salts containing these elements.
Importance of Yellow Flame Color
Identifying solids that burn with yellow flame is important for the following reasons:
- Reveals the presence of alkali and alkaline earth metals in the sample.
- Aids in qualitative analytical analysis and detection of specific elements.
- Understand and control color characteristics in pyrotechnics.
- Diagnostic of the composition and impurities in solid fuels.
- Potential safety hazard as yellow color indicates sodium compounds that can catalyze char formation.
- Can help in Fingerprinting the fuel source based on the color signature.
- Monitor mixing and substitution of yellow-flame coal with other grades.
- Identify additives used for yellow coloring in fireworks.
The yellow flame test is widely used as a simple analytical technique to detect the presence of sodium, lithium, and potassium in inorganic salts and solutions. It has diverse applications across many fields involving combustion and fire.
Some Common Solids That Do Not Burn with Yellow Flames
While many solids burn with yellow flames, some exceptions that produce flames of different colors include:
- Magnesium – Burns with a brilliant white flame.
- Copper – Imparts a green or blue-green flame color.
- Calcium – Burns with an orange-red flame.
- Barium – Produces an apple green colored flame.
- Strontium – Emits a deep red flame.
- Boron – Burns with a vivid green flame.
- Methanol – Has a pale blue non-luminous flame.
- Graphite – Combusts with a blue flame since it is very pure carbon.
- Phosphorus – Oxidizes with a pale blue flame.
- Zinc – Burns with a bluish-green flame.
These elements or compounds do not generally contain sodium or the other alkali metals. Their flame colors arise from entirely different emitting species. This demonstrates the importance of sodium and lithium in producing yellow flames.
How to Test if a Solid Sample Burns with Yellow Flame?
A simple experimental procedure to test if a solid sample emits yellow light when burnt is as follows:
Requirements
- Sample of the solid chemical or material
- A wire loop or spatula to hold the sample
- A Bunsen burner or alcohol lamp
- Tongs to hold the wire loop
- A dark background
- Lighter to ignite the burner
- Safety equipment like gloves and goggles
Procedure
- Take a small amount of the solid sample on a wire loop or spatula.
- Ignite the Bunsen burner and adjust to get a blue flame.
- Hold the loop with the sample in the hottest region of the flame using tongs.
- Observe the color of the flame produced by the burning sample.
- Compare the color against a dark background to cancel out other extraneous light.
- A lemon yellow or golden yellow flame indicates the presence of sodium or lithium in the sample.
- Repeat the test 2-3 times to verify the yellow color.
- Perform a blank test without any sample to compare.
Precautions
- Conduct the experiment in a well-ventilated area or fume hood.
- Wear suitable hand and eye protection.
- Hold the sample away from your face while burning.
- Keep combustible materials away from the flame.
- Extinguish the flame before discarding the sample.
This simple activity demonstrates the yellow flame test for detecting sodium content qualitatively in solid salt samples.
Some Common Examples of Solids and Their Flame Colors
| Solid Sample | Flame Color |
|---|---|
| Sodium chloride | Bright yellow |
| Lithium chloride | Reddish-yellow |
| Potassium chloride | Violet |
| Calcium chloride | Orange-red |
| Copper chloride | Green |
| Strontium nitrate | Red |
| Barium chloride | Pale green |
| Magnesium ribbon | White |
| Graphite | Blue |
This table summarizes the characteristic flame test colors for some common metallic salts, demonstrating the distinctive yellow color given by alkali metal compounds.
Conclusion
The yellow coloration in flames is primarily caused due to the emission from hot excited sodium atoms generated during the combustion of sodium-containing compounds. Solid fuels like sodium chloride, coal, and natural gas impart a bright yellow hue to the flame due to traces of sodium impurities. Compounds of lithium and potassium also subtly contribute to the yellow color. Understanding the origin of yellow flames helps in chemical analysis and identification of the composition of burning solids. A simple experimental flame test can qualitatively detect the presence of sodium or lithium in a solid sample based on the resultant yellow color.
