What are the zones of a candle flame and how does combustion occur
Have you ever observed a flame? Or have a close view at it? If you have seen a candle flame you must have noticed the different colors in it, the distinct shape it obtains, its alluring movement. So let us have a deeper look at candle flames and their various types.
The term flame is derived from the Latin word “Flamma”. There exist myriads of physics and chemistry that are responsible for the light and beauty of candle flames. Candle flames, in fact, have been getting a lot of attention from various scientists and researchers for quite a long time.
Michael Faraday, in the nineteenth century,conveyed his famous lecture series on the Chemical History of a Candle, elaborating loads of scientific theories with the help of his thorough analysis of a burning candle.
NASA, during the end of the 20th century,upgraded their research approach about candle flames by performing space shuttle experiments to understand the nature of candle flames in microgravity.
A lot of researchers and scientists in institutions and research centers all over the globe keep on conducting experiments with candle flames to know more about them, as well as their combustion and emissions.And Last but not the least, a lot of students each year examines the theories of light, heat, and combustion in the form of school science projects that involve candles.
How Do Candles Burn?
Every wax typically consists of hydrocarbons, meaning that they are majorly comprised of carbon (C) and hydrogen (H) atoms.
First of all you light a candle. The wax is gradually melted by the heat of the flame,close to the wick. Then, the molten wax, due to the capillary action, is drawn up the wick.
The heat of the flame evaporates the liquid wax, turns it into a hot gas and begins to decompose the hydrocarbons into carbon and hydrogen molecules. These evaporated molecules are pulled up into the flame, where they combine with atmospheric oxygen (from the air) to produce light, heat,carbon dioxide (CO2) and water vapor (H2O).
About 25% of the energy produced by the combustion of a candle is dissipated as heat releases from the flame in every direction.The heat generated is enough to radiate back and melt more amount of wax to ensure that the combustion keeps going unless the entire fuel is consumed or the heat is expelled.
When you first light a candle, it generally takes a couple of minutes for the combustion become stable. The flame mightsmoke or flutter little at the initial phase, yet the moment the process becomes stable, the flame would burn in a clean and steady manner, taking the shape of a quiet teardrop, releasing water vapor and carbon dioxide.
A candle flame burning quietly proves to be a highly effective combustion medium. However, if the flame receives either a lot of or very little fuel or air, it can flutter. Also, this leads to partially burned carbon particles or soot escaping from the flame before they are completely combusted.
The hint of smoke that you observe sometimes when a candle flutters is actually because of the partially burned soot particles that have went off from the flame due to partial combustion.
Reason Why Candle Flame Always Points Up
When a candle starts to burn, the flame warms up the air in its vicinity and begins to rise. Because this hot air goes up, the colder air and oxygen moves in at the lower region of the flame to substitute it.
When that colder air is warmed up, it also goes up and is substituted by the colder air at the bottom of the flame.This gives rise to a continuous process of upward going air about the flame (convection current) that offers the flame its teardrop or elongated shape.
As we know that the up and down are a feature of the gravity of the earth, scientists had a thought about the appearance of a candle flame in outer space, where the gravitational pull is minimum and there actually is no up or down.
Towards the end of the 20th century, scientists at NASA performed numerous space shuttle experiments to observe the nature of candle flames in microgravity. As you can figure out from the photos below taken by NASA, a candle flame in the microgravity setting takes a spherical shape instead of its usual teardrop shape on the earth. There is no upward direction for the warm air to go up and generate a convection current without gravity.
Structure of candle flame
If you take a close look at a candle flame, you will observe a black region at the bottom part of the flame. Above that black region is a tiny dark orange-brown/yellow region, and over that is the blue region.
In general terms, the colors of the flame rely on the following factors:
Temperature
Nature of material that goes through combustion
Amount of oxygen
This brings us to take a deep dive into various parts of the flame which are as follows:
The Inner part
The innermost region is black in color due to the presence of unburnt wax vapors. This is the zone where the molecules of hydrocarbon evaporate and begin to dissociate into carbon and hydrogen atoms. At first, the hydrogen atoms get separated here and they react with the oxygen to produce water vapor. A small portion of the carbon burns in this region to create carbon dioxide.
The Middle Part
This yellow region consists of comparatively lesser amount of oxygen so partial burning of carbon particles take place. That’s why it’s yellow in colour. This is the zone where the different forms of carbon keep on breaking down and begin to produce tiny, toughened carbon particles. As the carbon particles move up, along with the carbon dioxide and water vapor formed in the dark region, they are warmed up at a temperature of about 1000 degrees Celsius.
The creation of the carbon or soot particles gains speed below the yellow region. As those soot particles move up, they keeping on getting heated unless they burn into illumination and release the entire spectrum of visible light. As partial combustion takes place in this region, the human eye sees the flame as yellowish. When the soot particles oxidize close to the top of the yellow region of the flame, the temperature then is around 1200 degrees Celsius.
The Outer Part
The outer zone of the candle, which is also called the veil, is blue in colour. Its color is blue because of complete combustion, and is the warmest area of the flame, usually clicking 1400 degrees Celsius (2552 degrees Fahrenheit).
Types of Flames
Typically, there are two kinds of flames:
Luminous flames
Non-luminous flames
FAQs on Candle Flame and Its Structure in Chemistry
1. What is a candle flame in chemistry?
A candle flame is the visible result of a combustion reaction between vaporized wax and oxygen in the air. Candle wax, usually made of hydrocarbons like paraffin, melts and vaporizes near the wick. The wax vapor reacts with oxygen in an exothermic reaction, releasing heat and light. This makes a candle flame a practical example of:
- Oxidation reaction
- Exothermic chemical change
- Hydrocarbon combustion
2. What is the chemical reaction of a candle flame?
The chemical reaction of a candle flame is the combustion of hydrocarbons in wax with oxygen to form carbon dioxide and water. A simplified balanced equation for paraffin wax (approximated as C25H52) is:
- C25H52(s) + 38O2(g) → 25CO2(g) + 26H2O(g)
3. What are the different zones of a candle flame?
A candle flame has three main zones: the inner dark zone, the luminous yellow zone, and the outer blue zone. These zones differ in temperature and type of reaction:
- Inner dark zone: Contains unburned wax vapors; lowest temperature.
- Luminous zone: Yellow region where partial combustion forms glowing carbon particles (soot).
- Outer non-luminous zone: Blue region with complete combustion; hottest part of the flame.
4. Why is the candle flame yellow?
A candle flame is yellow because of glowing carbon particles formed during incomplete combustion. In the luminous zone, limited oxygen causes partial burning of hydrocarbons, producing tiny hot carbon (soot) particles. These particles become incandescent and emit yellow light. This explains why:
- Good oxygen supply → blue flame (complete combustion)
- Limited oxygen → yellow luminous flame
5. Why is the outer part of the candle flame blue?
The outer part of a candle flame is blue because it undergoes complete combustion with sufficient oxygen. In this region:
- Wax vapors react fully with O2
- Excited molecular species like CH* and C2* emit blue light
- The temperature is highest
6. What products are formed when a candle burns?
When a candle burns completely, the main products formed are carbon dioxide (CO2) and water vapor (H2O). The balanced reaction (example for C25H52) is:
- C25H52(s) + 38O2(g) → 25CO2(g) + 26H2O(g)
7. Is a candle flame a physical or chemical change?
A candle flame involves both a physical change and a chemical change. The melting of wax (solid → liquid) and vaporization are physical changes, while the combustion of wax vapor with oxygen is a chemical change. During combustion:
- New substances (CO2 and H2O) are formed
- Heat and light are released (exothermic reaction)
8. How does a candle flame stay lit?
A candle flame stays lit due to a self-sustaining combustion cycle involving heat, fuel, and oxygen. The process works as follows:
- Heat melts the wax near the wick.
- Liquid wax travels up the wick by capillary action.
- Wax vaporizes and reacts with oxygen.
- The exothermic reaction releases heat to continue melting more wax.
9. What type of reaction is burning of a candle?
The burning of a candle is an exothermic oxidation (combustion) reaction. In this reaction:
- Hydrocarbon wax acts as the fuel.
- Oxygen acts as the oxidizing agent.
- Heat and light energy are released.
10. Why does a candle flame produce soot?
A candle flame produces soot due to incomplete combustion when there is insufficient oxygen. Under limited oxygen supply:
- Hydrocarbons break down into carbon particles.
- These particles do not fully oxidize to CO2.
- Solid carbon (soot) escapes as black smoke.
