Brooke Martin
Candles are a leading cause of residential fires in the United States, with almost 10% of civilian injuries and 6% of fire-related deaths attributed to candles. The combustion of a candle occurs when the wick is lit, and the heat melts and ignites the wax, which vaporizes and combines with oxygen in the air to form a flame. This multi-step process results in a flame that produces light and heat, with the wax acting as the combustible substance or fuel.
| Characteristics | Values |
|---|---|
| Definition | A candle is an ignitable wick embedded in wax or another flammable solid substance such as tallow. |
| Composition | All waxes are essentially hydrocarbons, which means they are largely composed of hydrogen and carbon atoms. |
| Combustion | The combustion of the candle proceeds in a self-sustaining manner. The heat of the flame melts the wax near the wick. This liquid wax is then drawn up the wick by capillary action. The heat of the flame vaporizes the liquid wax, and starts to break down the hydrocarbons into molecules of hydrogen and carbon. These vaporized molecules are drawn up into the flame, where they react with oxygen from the air to create heat, light, water vapour, and carbon dioxide. |
| Temperature | The temperature of a candle flame varies across its zones, from around 600 °C (1,112 °F) in Zone I to around 1,400 °C (2,550 °F) in Zone V, the hottest part of the flame. |
| Colour | The colour of a candle flame varies across its zones, from blue in Zone II where there is a plentiful supply of oxygen and the fuel burns clean and blue, to yellow/white in Zone IV where there is insufficient oxygen for complete combustion, resulting in partial combustion and a yellow/white flame. |
| Use | Candles provide light, heat, and can be used for functional, symbolic, and aesthetic purposes, as well as for keeping time and in specific cultural and religious settings. |
| Safety | Candles are a leading source of residential fires, with almost 10% of civilian injuries and 6% of fatalities from fires attributed to candles. |
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What You'll Learn
The combustion process of a candle
A candle is a combustible substance, and its combustion process is fascinating. Firstly, candles are made of wax, which is a solid fuel, and a wick, usually made of braided cotton. When the wick of a candle is lit, the heat melts a small amount of the solid wax, which is drawn up the wick through capillary action. This liquid wax then reaches the flame and vaporizes, turning into a hot gas.
The hot gas molecules are broken down into hydrogen and carbon, which react with oxygen from the air to create heat, light, water vapour, and carbon dioxide. This combustion process is self-sustaining, as the heat generated is enough to melt more wax, which is then drawn up the wick to continue the cycle. The flame's shape is teardrop-like due to the convection current created by the rising warm air, which pulls cooler air and oxygen to the bottom of the flame.
The candle flame has several zones, each with distinct characteristics. The first zone is non-luminous and the coolest part, with insufficient oxygen for the fuel to burn. The second zone, known as the blue zone, has an abundant oxygen supply, causing the fuel to burn clean and blue. The heat from this zone melts the wax. The third zone, the dark zone, is where pyrolysis occurs, and it contains unburnt wax.
The fourth zone is the luminous yellow or white zone, where the flame is brightest but not the hottest. Here, there is insufficient oxygen for complete combustion, resulting in partial combustion. The outermost zone, or veil, is the hottest part of the flame, reaching temperatures of around 1400°C. This zone is light blue in colour and is where complete combustion occurs.
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The chemical reaction of wax and oxygen
A candle is a combustible substance, and its combustion proceeds in a self-sustaining manner. When a candle is lit, the heat of the flame melts the wax near the wick. The liquid wax is then drawn up through the wick via capillary action. The heat of the flame vaporises the liquid wax, turning it into a hot gas. This vapour combines with oxygen in the air to form a flame.
The flame of a candle is made up of hot gases, primarily carbon dioxide, water vapour, oxygen, and nitrogen. The yellow colour of the flame is due to soot particles glowing because they are hot. The blue zone, which surrounds the base of the flame, is where the supply of oxygen is plentiful, and the fuel burns clean and blue. It is the hottest part of the flame, typically reaching 1400°C (2552°F). The heat from this zone causes the wax to melt.
As the wax melts, it is drawn up the wick and continues to vaporise. The vaporised molecules are drawn up into the flame, where they react with oxygen from the air. This reaction produces heat, light, water vapour, and carbon dioxide. The chemical equation for this reaction is:
${C_{25}}{H_{52}} + {O_2} \to 26{H_2}O + 25C{O_2}$
This equation represents the complete oxidation of hydrocarbons in the wax, yielding water and carbon dioxide. The combustion process continues until the fuel is used up or the heat is eliminated.
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How candles produce heat and light
When a candle burns, it undergoes a process of combustion, releasing heat and light energy. This is due to the chemical reaction between the candle's fuel (usually wax) and an oxidizer, typically the oxygen in the air.
The wick of the candle plays a crucial role in this process. It acts as a fuel delivery system, transporting the melted wax fuel upwards through capillary action. As the wick draws the liquid wax, it also delivers the wax vapour to the flame's base, providing a consistent fuel source to sustain combustion.
At the flame's base, the vaporized wax mixes with oxygen and combusts, producing carbon dioxide, water vapour, and heat. This heat then causes more wax near the wick to melt and be drawn up, continuing the cycle. The flame's tip, which appears brightest, is where the hottest reaction occurs, with temperatures reaching up to 1400°C. Here, the wax vapours and oxygen undergo complete combustion, releasing the most significant amount of energy in the form of heat and light.
The light produced by a candle results from the flame's hot base emitting thermal radiation, including visible light, in all directions. This light is a product of the flame's temperature and the combustion process, with the brightness and colour of the light depending on the type of wax and any added colourants or fragrances.
Additionally, the flickering nature of a candle flame is due to the convection currents created by the heated air rising above the flame, causing a continuous cycle of air movement around the wick, which influences the combustion process and creates the characteristic flickering effect. Overall, the simple act of a burning candle showcases the fascinating transformation of chemical energy into heat and light through the process of combustion.
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The different zones of a candle flame
A candle is a combustible substance, and its flame has several distinct zones, each with its own unique characteristics. Understanding these zones provides insight into the chemical processes that occur during combustion.
Firstly, Zone I, also known as the blue zone, is the non-luminous, coolest part of the flame, with temperatures around 600-800°C. This zone surrounds the base of the wick, where there is insufficient oxygen for complete combustion. The oxidation of vaporized wax in this zone produces carbon monoxide, which burns to create a bluish flame.
Secondly, Zone II, or the dark zone, is located directly above the wick and contains unburnt wax. This zone has a temperature of around 1000°C. Pyrolysis occurs in this region, where the breakdown of hydrocarbons results in the formation of molecules such as hydrogen and carbon.
Thirdly, Zone III, also referred to as the middle or luminous zone, is yellow or white and is located above the dark zone. It is the brightest zone due to the incomplete combustion of wax, but it is not the hottest. The depletion of oxygen in this zone results in partial combustion, leading to the generation of ashes and soot.
Finally, Zone IV, or the outer zone, is the hottest part of the flame, reaching temperatures of approximately 1400°C. This zone is non-luminous and light blue in colour, although most of it is invisible. Here, complete combustion occurs as the wax vapours fully react with oxygen from the air, creating heat, light, water vapour, and carbon dioxide.
Each zone within a candle flame plays a specific role in the combustion process, contributing to the overall characteristics of the flame, such as its colour, intensity, and heat output. The zones are differentiated by their distinct colours, temperatures, and the chemical reactions that occur within them.
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The history of candle usage
The use of candles dates back thousands of years, with the earliest known use attributed to the Ancient Egyptians. They made rushlights by soaking the dried pith or core of the rush plant or reeds in melted animal fat or grease. However, these early candles lacked a wick, a key component of the candles we know today.
The Ancient Romans are generally credited with developing the first true wicked candles. They created these by wrapping papyrus around a length of twine and repeatedly dipping it in tallow, a rendered form of beef or mutton fat. This process was also used by other ancient civilisations, who utilised waxes derived from locally available plants and insects. For example, the Chinese moulded their candles in paper tubes, using rolled rice paper as a wick, and wax from indigenous insects. In India, candle wax was made by boiling the fruit of the cinnamon tree, while in Japan, wax was extracted from trees.
Candles played a significant role in ancient times as a source of light and in religious ceremonies. They were widely used in early Western cultures, particularly those rendered from animal fat (tallow). During the Middle Ages, beeswax candles were introduced in Europe, offering a purer, cleaner-burning alternative to tallow, with a pleasant scent. However, beeswax candles were more expensive and typically only the wealthy or churches could afford them.
The 18th and 19th centuries saw major changes in candlemaking due to the growth of the whaling industry. Spermaceti, a wax derived from sperm whale oil, became available in large quantities. Spermaceti candles burned brighter, had no unpleasant odour, and were harder, making them more resistant to heat. During this period, French chemist Michel Eugene Chevreul discovered how to extract stearic acid from animal fatty acids, leading to the development of stearin wax, which burned cleanly and was harder and more durable.
The invention of mechanised production methods in the 19th century, such as Joseph Morgan's machine for producing moulded candles, made candles affordable and accessible to the masses. The introduction of paraffin wax, derived from petroleum, further contributed to the widespread use of candles due to its economical production and clean-burning properties. However, with the advent of electric lighting, such as the light bulb, the demand for candles began to decline.
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Frequently asked questions
A candle is an ignitable wick embedded in wax or another flammable solid substance, such as tallow.
The heat of the flame melts the wax near the wick. This liquid wax is then drawn up the wick by capillary action. The heat of the flame then vaporizes the liquid wax, turning it into a hot gas that reacts with oxygen from the air to create heat, light, water vapour, and carbon dioxide.
Yes, a candle is a combustible substance. The combustible material in a candle, or its fuel, is the wax.
