Jessie Taylor
Candles have been used for over two millennia for functional, symbolic, and aesthetic purposes. The combustion of a candle refers to the chemical reaction that occurs when a candle burns, producing heat, light, carbon dioxide, and water vapour. The combustion process begins when the heat of the flame melts the wax near the wick, which is then drawn up the wick by capillary action. The liquid wax is vaporised and the hydrocarbons in the wax break down into molecules of hydrogen and carbon. These vapour molecules react with oxygen from the air, creating heat, light, water vapour, and carbon dioxide. The by-products of candle combustion are released into the air, contributing to indoor pollution, especially if burned frequently.
| Characteristics | Values |
|---|---|
| Chemical reaction | Combustion |
| Heat | 25% of the energy created by a candle's combustion is given off as heat |
| Light | Light |
| Water vapour | Water vapour |
| Carbon dioxide | Carbon dioxide |
| Soot particles | Soot particles |
| Colour | Blue, orange-brown, Yellow |
| Shape | Elongated or teardrop |
| Fuel | Wax, tallow |
| Wick material | Cotton, zinc, zinc alloy, paper |
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What You'll Learn
Heat, light, water vapour, and carbon dioxide are created during combustion
The combustion of a candle produces heat, light, water vapour, and carbon dioxide. This occurs through a chemical reaction between the candle's wax and oxygen from the surrounding air.
The wax, which is composed of hydrogen and carbon atoms, is drawn up the candle's wick towards the flame via capillary action. The heat of the flame melts the wax, which then vaporises and combines with oxygen to create the aforementioned products. This process releases energy in the form of light and heat, which gives the flame its characteristic teardrop shape.
The flame of a candle can be divided into several zones, each with distinct characteristics. Zone I is the non-luminous, coolest part of the flame, located at the base of the wick, where there is insufficient oxygen for combustion. Zone II, the blue zone, surrounds Zone I and is where oxygen is plentiful, causing the fuel to burn clean and blue. The heat from this zone melts the wax. Zone III, the dark zone, is directly above the wick and contains unburnt wax, where pyrolysis occurs at temperatures of around 1000°C.
Zone IV is the middle, luminous zone, which is yellow or white, and is the brightest but not the hottest part of the flame. It is oxygen-depleted, resulting in incomplete combustion and the formation of soot particles, which give off light through black body radiation. Zone V, the outermost zone, is non-luminous and light blue in colour, although mostly invisible. Here, complete combustion occurs at temperatures of around 1400°C, the hottest part of the flame.
The combustion process releases heat, light, carbon dioxide, and water vapour, with approximately one-fourth of the energy given off as heat radiating from the flame. The water vapour and carbon dioxide produced rise with the heated air, creating an upward convection current that gives the flame its elongated shape.
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The wax melts and is drawn up the wick to the flame
When a candle is lit, the heat of the flame melts the wax near the wick. This is because wax is a hydrocarbon, composed of hydrogen and carbon atoms. The liquid wax is then drawn up the wick by capillary action.
The flame's heat vaporizes the liquid wax, turning it into a hot gas. This is known as vaporization. The gaseous wax then feeds the flame, which in turn heats more wax, creating a continuous cycle. As the wax vapour rises, it is broken down into hydrogen and carbon molecules. This process is known as combustion.
The wax vapour molecules are then drawn into the flame, where they react with oxygen from the air. This reaction produces heat, light, water vapour, and carbon dioxide. The water vapour and carbon dioxide mix into the surrounding air, while the heat sustains the flame and the light creates the candle's warm glow.
The wax of a candle does not disappear but is converted into heat and light energy. The process of burning a candle involves melting the wax, drawing it up into the wick, vaporizing it, and then combusting it. The end products are heat, light, water vapour, and carbon dioxide.
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The flame's shape is influenced by convection currents
The combustion of a candle is a self-sustaining process. When a candle is lit, the heat of the flame melts the wax near the wick, which is drawn up by capillary action. The liquid wax is then vaporized and combines with oxygen in the air to form a flame. This creates a continuous cycle of upward-moving air around the flame, known as a convection current.
The convection current gives the flame its characteristic teardrop shape. The cycle begins when the flame heats the air, causing it to rise. As the warm air moves up, cooler air and oxygen rush in at the bottom of the flame to replace it. This cooler air is then heated and rises, creating a continuous cycle. The direction of the warm air is influenced by the Earth's gravity, which pulls it upward, creating the elongated shape of the flame.
The size and shape of the flame are also influenced by the diameter of the wick. A thicker wick leads to a larger flame, while a thinner wick results in a smaller flame. The type of wax used also affects the flame's size, with beeswax and coconut wax burning longer and producing a larger flame compared to paraffin or soy wax.
The colour of the flame provides insight into the combustion process. The base of the flame is blue, indicating an oxygen-rich environment where hydrocarbon molecules vaporize and break down into hydrogen and carbon atoms. Above this is a small dark orange-brown section where carbon particles continue to break down and form soot. The uppermost region is the familiar yellow flame, produced by hot soot particles glowing due to blackbody radiation.
The study of candle flames has intrigued scientists for centuries, with Michael Faraday delivering a renowned lecture series on the Chemical History of a Candle in 1860. More recently, NASA scientists have conducted experiments in microgravity environments, revealing that candle flames take on a spherical shape in the absence of gravity's upward pull on warm air.
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The colour of the flame indicates combustion completeness
The colour of a candle's flame can indicate the completeness of its combustion. The blue colour of the outer zone of a candle flame signifies complete combustion, which occurs when there is an unlimited supply of oxygen. This zone is the hottest part of the flame, reaching temperatures of around 1,400°C (2,550°F). The blue colour indicates that the combustion process is producing minimal soot, a black carbonaceous substance that is a byproduct of incomplete combustion.
The orange flame, observed in the intermediate zone of a candle flame, indicates partial combustion of the fuel. This occurs when there is a limited supply of oxygen, resulting in the production of carbon monoxide and other partially burned fuel particles. The orange flame is relatively cooler compared to the blue zone, emitting a moderate amount of light.
The yellow colour of the flame is due to the presence of unburned carbon particles or other impurities in the flame, indicating incomplete combustion. This occurs when there is insufficient oxygen to completely burn all of the wax vapour rising from below, resulting in only partial combustion. The yellow zone is the brightest zone but not the hottest, with temperatures reaching approximately 1,200°C.
The colour of a candle flame can also be influenced by the presence of other substances or impurities in the wax or fuel. For example, the addition of fragrance oils, essential oils, or aniline-based dyes can alter the colour of the flame. Additionally, the height of a candle flame is primarily determined by the diameter of the wick, with thicker wicks producing larger flames.
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Candles emit volatile organic compounds and indoor pollution
Candles are made of wax, a hydrocarbon largely composed of hydrogen and carbon atoms. When a candle is lit, the heat of the flame melts the wax near the wick, which is then drawn up the wick by capillary action. The liquid wax is vaporized and combusted, and the hydrocarbons break down into molecules of hydrogen and carbon. These vaporized molecules react with oxygen in the air to create heat, light, water vapour, and carbon dioxide.
However, candles also emit volatile organic compounds (VOCs) that can negatively impact indoor air quality and human health. VOCs are chemicals that easily become vapours or gases at room temperature. They can originate from the wax, fragrance, and wicks of candles. Common VOCs released by burning candles include benzene, toluene, formaldehyde, and acetaldehyde, which are linked to various health issues.
The US Environmental Protection Agency (EPA) has found that levels of indoor air pollutants are typically more than three times higher than outdoors. Sources of indoor pollution include cooking, heating, scented cleaning products, and deodorizers such as candles, diffusers, and room sprays. While candles enhance our living spaces by creating a cozy atmosphere, offering soothing light, and filling rooms with pleasant fragrances, they may also pose hidden health risks due to the emission of VOCs.
To minimize potential harm, it is recommended to choose candles made from natural waxes such as soy, beeswax, or coconut wax, and to avoid candles with synthetic fragrances, opting instead for those scented with natural essential oils. Using candles with cotton or wood wicks instead of metal-cored wicks can also help reduce the release of heavy metals. Ensuring proper ventilation in the home by opening windows and using exhaust fans can further help reduce the concentration of indoor pollutants.
While candles may emit VOCs and contribute to indoor air pollution, there are ways to minimize their impact and still enjoy their benefits.
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Frequently asked questions
The combustion of a candle produces heat, light, carbon dioxide, and water vapour.
Candle combustion is a chemical reaction that occurs when wax reacts with oxygen in the air to produce a colourless gas called carbon dioxide.
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 hot soot particles.
