Maddie James
Burning a candle involves fascinating chemical processes that have captivated scientists for hundreds of years. When a candle burns, the heat of the flame melts the wax, which is drawn up through the wick and evaporates. This liquid wax then combusts, turning into water vapour and carbon dioxide, and releasing heat and light. The combustion process continues until the fuel is used up or the flame is extinguished. The byproducts of this combustion, such as unburned wax particles, can be seen as smoke when a candle is blown out.
| Characteristics | Values |
|---|---|
| What happens when a candle burns? | The wax turns into carbon dioxide, water vapour, and a little bit of ash. |
| What is wax made of? | Hydrogen and carbon. |
| What happens when a candle is lit? | The heat of the flame melts the wax near the wick. |
| What happens to the liquid wax? | It is drawn up the wick by capillary action and then vaporized by the heat of the flame. |
| What happens to the vaporized wax molecules? | They react with oxygen from the air to create heat, light, water vapour, and carbon dioxide. |
| What is the blue area at the base of the flame called? | The oxygen-rich blue zone, where the hydrocarbon molecules vaporize and start to break apart into hydrogen and carbon atoms. |
| What is the yellow region of the flame? | The yellow region is where the carbon ignites, giving off a yellowish hue. |
| What is the fourth zone of the candle flame? | The faint outside blue edge that extends from the blue zone, directly meeting with the oxygen in the air. |
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What You'll Learn
The chemical composition of candles
Candle Fuel
The candle fuel, commonly referred to as candle wax, is the primary component of a candle. Over the centuries, candle waxes have been derived from various sources, including fats, oils, and waxy substances obtained from animals, insects, plants, and even rocks. The specific type of wax used in candles has evolved based on the availability of raw materials, the ease and cost-effectiveness of processing, and the desirability of the final product.
- Paraffin Wax: This is the most frequently used candle fuel worldwide. It is a natural waxy substance derived from the process of refining petroleum or crude oil. Paraffin wax has a straight-chain hydrocarbon structure with a melting point ranging from 120 to 160 degrees Fahrenheit. Its chemical formula is often represented as C25H52.
- Stearin Wax: Stearin wax is derived from stearic acid, which is extracted from animal fatty acids. It was widely used in Europe and has a chemical formula of C57H110O6.
- Beeswax: Beeswax has been used for candles since ancient times, dating back to the Tang Dynasty in China (618-907 A.D.). It is produced by honey bees and was introduced to Europe during the Middle Ages. Beeswax candles are considered safer and are a popular choice today. The chemical formula for beeswax is C15H31CO2C30H61.
- Vegetable-Based Waxes: In the late 1990s, vegetable-based candle fuels were introduced for commercial use. These include soy wax, derived from soybean oil, and palm wax, derived from palm oil.
- Synthetic Waxes: During the latter half of the 20th century, synthetic and chemically synthesized fuels, such as gels, were developed for specialty candle uses.
Wicks
The wick is another crucial component of a candle. Traditionally, wicks were made from cotton, which could be twisted plant fiber or braided. Braided cotton wicks were designed to curl over and be completely consumed during burning. Modern wicks can also have a metal core, typically made of zinc, lead, or tin, surrounded by cotton.
Fragrances and Colourants
Many candles today incorporate fragrances and colourants to enhance their aesthetic and olfactory appeal. Fragrances can be derived from natural sources, such as essential oils, or synthetic aroma chemicals. Candle manufacturers have a wide range of aroma chemicals and essential oils to create unique and pleasing scents. These fragrance materials undergo health and safety tests to ensure their suitability for candle use. Colourants are also added to candles to create specific colour schemes or designs.
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How candles burn
When a candle burns, it produces light and heat energy. This energy comes from the combustion of wax.
The wick of a candle is usually made of cotton. When a candle is lit, the wick absorbs the liquid wax and draws it upwards. The heat of the flame then vaporises the wax, turning it into a hot gas. The vaporised wax molecules are drawn into the flame, where they react with oxygen from the air to create heat, light, water vapour, and carbon dioxide. The carbon dioxide and water vapour are invisible, and most of the matter in the candle ends up as these two gases.
The combustion only happens at the end of the wick, where the flame is visible. The rest of the wax is either in a liquid state or is melting. The liquid wax is heated by the flame, and this heat is what causes the wax to vaporise and burn. The hotter the flame, the faster the wax burns, and the faster the candle burns down.
The wick itself also burns, although the wax contributes most of the heat. The vaporising wax cools the exposed wick, protecting it from burning too quickly. The charring of the wick is mostly incidental.
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The role of oxygen in combustion
When a candle burns, it releases heat and light energy. This occurs through a process of combustion, which involves the combination of three key elements: wax, oxygen, and heat.
Wax, which is composed of hydrogen and carbon atoms, serves as the fuel source for the candle. When a candle is lit, the heat from the flame melts the wax near the wick. This liquid wax is then drawn up the wick, a process known as capillary action. As the liquid wax reaches the top of the wick, it vaporizes and turns into a hot gas.
At this stage, oxygen plays a crucial role in the combustion process. The vaporized wax molecules mix with atmospheric oxygen, igniting and sustaining the flame. The oxygen-rich zone at the base of the flame is where the hydrocarbon molecules of the wax vaporize and break apart into hydrogen and carbon atoms. The hydrogen atoms react with oxygen to form water vapour, while some of the carbon atoms combine with oxygen to form carbon dioxide.
The combustion process releases heat energy, which melts more wax, continuing the cycle until the wax or oxygen is depleted. The blue zone at the base of the flame, where oxygen is abundant, is the hottest part of the flame, reaching temperatures of up to 1400°C. This heat fuels the upward movement of air around the flame, known as a convection current, giving the flame its distinctive teardrop shape.
The role of oxygen in the combustion of a candle is essential. It is the oxidizing agent that combines with the hydrogen and carbon atoms from the wax to produce water vapour and carbon dioxide. When oxygen is restricted or depleted, as when blowing out a candle or placing a jar over it, the flame extinguishes due to the lack of oxygen required for the combustion reaction.
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The colour of candle flames
The colour of a candle flame is influenced by the combustion of wax and the presence of different particles. When a candle burns, the heat of the flame melts the wax near the wick, and this liquid wax is drawn upwards through capillary action. As the liquid wax reaches the top of the wick, it vaporises and combines with oxygen, creating heat, light, water vapour, and carbon dioxide.
The colour of the flame is primarily yellow due to the combustion of carbon, with a small dark orange-brown section above a blue base. The blue base, or oxygen-rich zone, is where the hydrocarbon molecules vaporise and break down into hydrogen and carbon atoms. The hydrogen reacts with oxygen to form water vapour, while some of the carbon burns to form carbon dioxide. The yellow region is a result of the carbon igniting and emitting a full spectrum of visible light, with the yellow portion being the most dominant colour perceived by the human eye.
As the flame rises, it continues to heat up, and the soot particles near the top of the flame's yellow region oxidise at approximately 1200°C. The outermost part of the flame, sometimes called the veil, is a faint blue edge that extends from the blue base and up the sides of the flame cone. This blue veil is the hottest part of the flame, typically reaching temperatures of 1400°C. It appears blue because it directly meets with the oxygen in the air.
The colour of the candle flame can also be influenced by impurities or additives in the wax. For example, candles with metallic salts or other colourants may produce flames of different colours. Additionally, the amount of oxygen available to the flame can affect its colour. If a flame receives too little or too much air, it may flicker or flare, and unburned carbon particles (soot) may escape, causing a wisp of smoke and altering the colour of the flame.
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The history of candle research
Faraday's work laid the foundation for future candle research, and in the late 1990s, NASA took this exploration to new heights. They conducted experiments on space shuttles to study the behaviour of candle flames in microgravity, adding a unique dimension to our understanding of combustion.
The fundamental chemistry of candle burning revolves around the transformation of wax. When a candle burns, the heat of the flame melts the wax near the wick, which is then drawn upwards through capillary action. This liquid wax is vaporized by the flame's heat, breaking down the hydrocarbons into molecules of hydrogen and carbon. These vapour molecules react with atmospheric oxygen, resulting in the production of heat, light, water vapour, and carbon dioxide.
While the basic understanding of candle combustion has been established, scientists in universities and research laboratories continue to delve deeper. They conduct experiments to unravel the intricacies of candle flames, emissions, and combustion, ensuring that the legacy of candle research continues to burn brightly.
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Frequently asked questions
When a candle burns, the heat of the flame melts the wax near the wick. This liquid wax is drawn up the wick and vaporized, turning into water vapour and carbon dioxide. These gases are invisible, so you don't see them after the wax burns.
Candles are made of wax, which is a hydrocarbon, largely composed of hydrogen and carbon atoms.
When the liquid wax is vaporized, it breaks down into molecules of hydrogen and carbon. These molecules react with the oxygen in the air, creating heat, light, water vapour and carbon dioxide.
While carbon dioxide and water vapour can be dangerous at high levels, the amount of gas produced by a candle is small and comparable to the amount that might be breathed out by another person in the room.
