Ben Carter
Candles have fascinated scientists for hundreds of years, with many experiments conducted to understand the behaviour of candle flames. The process of burning a candle involves several chemical reactions. When a candle burns, the heat of the flame vaporises the liquid wax, turning it into hot gas. The vapour then rises into the flame, reacting with oxygen from the air to create heat, light, water vapour, and carbon dioxide. This combustion process continues until the fuel is used up or the heat source is removed. While the chemical processes behind candle burning are complex, the physical act of shortening a candle can be achieved through mechanical means, such as cutting the wax with a heated knife or using a bandsaw.
| Characteristics | Values |
|---|---|
| Method | Heat a knife and cut/melt the candle |
| Use a band saw or a sharp knife/wick trimmer to cut the desired amount of wax from the top of the candle | |
| Heat the cut edge of the wax with a lighter or matches to smooth it out | |
| Materials | Candle |
| Knife or wick trimmer | |
| Band saw | |
| Lighter or matches | |
| Chemical Reaction | Wax vapour burns and forms gases that mix with the air |
| The heat of the flame vaporizes the liquid wax | |
| Hydrocarbons break down into molecules of hydrogen and carbon | |
| Vaporized molecules are drawn into the flame and react with oxygen to create heat, light, water vapour, and carbon dioxide | |
| Water vapour and carbon dioxide rise and continue to heat until they ignite and emit visible light |
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What You'll Learn
The chemical reaction of burning a candle
Burning a candle involves several chemical reactions and energy transformations. Firstly, the solid wax melts due to the heat of the flame, and this liquid wax is drawn up through the wick by capillary action. The wax then evaporates, turning into a gas, and it is at this point that it can burn. The chemical energy of the candle wax is converted into light energy and heat energy.
The molecules in the wax combine and rearrange with oxygen from the air, releasing heat and light. This reaction is known as combustion, and it can be represented by the chemical equation: CH4 + 2O2 → CO2 + 2H2O. In this equation, CH4 represents the wax, which is made up of chains of carbon and hydrogen atoms, and 2O2 represents the oxygen from the air. The products of the reaction are carbon dioxide (CO2) and water (H2O).
The flame of a candle is a mixture of hot gases, primarily carbon dioxide, water vapour, oxygen, and nitrogen. The yellow colour of the flame is due to the presence of soot particles, which are unburned carbon particles that glow when heated, a phenomenon known as black body radiation. The blue area at the base of the flame is the oxygen-rich zone, where the wax molecules vaporize and break apart into hydrogen and carbon atoms. The hydrogen reacts with oxygen to form water vapour, and some of the carbon burns to form carbon dioxide.
As the gases rise, they are heated to high temperatures, and the formation of soot particles increases. These particles continue to heat up as they rise, eventually igniting and emitting visible light. The yellow colour perceived by the human eye is due to the dominance of the yellow portion of the spectrum when the soot particles ignite. The outside edge of the flame, sometimes called the veil, is the hottest part, reaching temperatures of around 1400°C. It appears blue because it directly meets with the oxygen in the air.
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The role of the wick
The wick is a critical component of a candle, playing a central role in delivering fuel (wax) to the flame. It acts as a fuel pump, drawing the liquefied wax upwards through capillary action, a process known as "wicking". The wick's design and size are crucial in determining the amount of fuel that reaches the flame. If too much fuel is delivered, the flame may flare and produce soot, while insufficient fuel can cause the flame to sputter out. Therefore, selecting the appropriate wick is essential for ensuring the candle burns cleanly and efficiently.
Wick sizes vary, and this variation influences the burning characteristics of a candle. For instance, larger diameter wicks generally result in a more substantial flame, a larger pool of melted wax, and a faster-burning candle. The stiffness of the wick is another important characteristic, as it helps the wick remain upright and conduct heat downward, aiding in the melting of the wax. To achieve the desired stiffness, different core materials are used, with cotton, paper, zinc, and tin being common choices.
The type of wick used depends on the specific application of the candle. For example, flat wicks, which are typically braided or knitted, are often found in taper and pillar candles. These wicks burn consistently and curl back into the flame, making them self-consuming. Square wicks, on the other hand, are preferred for beeswax candles as they help prevent wick clogging, which can occur with certain wax formulations. Twisted wicks, though generally lower quality, are useful for birthday candles as they burn faster due to their loose construction.
Additionally, specialty wicks are designed for specific purposes, such as oil lamps and insect-repelling candles. Wooden wicks have gained popularity for their aesthetic appeal and the soft crackling sound they produce. They are available in various shapes and materials, including 100% wood, semi-wood, fibrous material, or cotton and wood combinations. While wicks are traditionally made from braided cotton, other materials such as string, cord, and even historical examples of wood and asbestos have been used.
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State changes in the wax
Firstly, the solid wax near the wick melts due to the heat of the flame. This is a critical phase change, as it transitions the wax from a solid to a liquid state. The liquid wax is then drawn up by the wick through capillary action, which is the ability of a substance to flow in narrow spaces against gravity.
As the liquid wax rises through the wick, it vaporizes and turns into a hot gas. This process involves breaking down the wax's hydrocarbon molecules into hydrogen and carbon atoms. The vaporized wax molecules are drawn into the flame, where they combine with oxygen from the surrounding air.
The combustion of wax vapour results in the release of heat and light energy. Approximately one-fourth of the energy produced by the candle is radiated as heat, contributing to the sustained combustion process. The heat radiates in all directions, melting more solid wax at the base of the wick and perpetuating the cycle.
The flame of a candle undergoes distinct colour changes due to the varying temperatures and chemical reactions within the flame. The blue zone at the base of the flame is oxygen-rich and extremely hot, reaching temperatures of approximately 1400° C. Here, the wax vapour burns and breaks down into hydrogen and carbon atoms, forming water vapour and carbon dioxide. As these gases rise, they continue to heat up, eventually igniting and emitting visible light. The yellow colour typically associated with candle flames is due to the dominant yellow portion of the spectrum when the carbon ignites.
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The chemical reaction of combustion
The process of burning a candle involves a series of chemical reactions. Firstly, the solid wax melts due to the heat of the flame, and the wick draws the liquid wax upwards through capillary action. The wax then evaporates and turns into wax vapour, which burns and forms gases that mix with the air. This process is known as combustion, a chemical reaction that involves the conversion of chemical energy into light energy and heat energy.
Combustion is a high-temperature exothermic redox chemical reaction that occurs between a fuel (the reductant) and an oxidant, typically atmospheric oxygen. This reaction releases oxidized products, often in the form of smoke. While combustion does not always result in fire, the presence of a flame indicates that the substances undergoing combustion have vaporized.
The chemical equation for the combustion of a candle made of paraffin wax can be represented as CH4 + 2O2 → CO2 + 2H2O. In this reaction, the paraffin wax, which is composed of long chains of carbon and hydrogen atoms (alkanes), combines with oxygen to produce carbon dioxide and water vapour.
The colour of the flame in a candle is influenced by the presence of soot particles, which emit a yellow glow due to black body radiation. Additionally, other colours in the flame, such as blue and green, arise from transient reaction intermediates during combustion, such as the Methylidyne radical (CH) and Diatomic carbon (C2).
The rate of combustion, or the amount of material that undergoes combustion per unit of time, can be influenced by various factors. For example, the speed at which the reactants combine is determined by the nature of the chemical reaction and the amount of energy generated. Incomplete combustion can also occur, leading to the presence of unburnt products such as carbon monoxide, hydrogen, or carbon (soot or ash).
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The colour of the flame
The colour of a candle's flame is a result of a chemical reaction in action. The yellow colour of the flame is due to soot particles glowing because they are hot, a phenomenon known as black-body radiation. The blue area at the base of the flame is the oxygen-rich zone, where hydrocarbon molecules vaporize and break apart into hydrogen and carbon atoms. The hydrogen reacts with oxygen to form water vapour, while some of the carbon burns to form carbon dioxide.
As the carbon particles rise, they continue to heat up until they ignite and emit a full spectrum of visible light. The yellow portion of the spectrum is the most dominant when the carbon ignites, which is why the human eye perceives the flame as yellowish. When the soot particles oxidize near the top of the flame's yellow region, the temperature rises to approximately 1200°C.
The fourth zone of the candle, sometimes called the veil, is the faint outside blue edge that extends from the base of the flame up the sides of the flame cone. This part of the flame directly meets the oxygen in the air and is the hottest, reaching around 1400°C.
Overall, the colour of a candle's flame is a complex interplay of temperature, soot particles, and the chemical composition of the wax and its combustion products.
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Frequently asked questions
A candle is made of wax, which is a hydrocarbon, largely composed of hydrogen (H) and carbon (C) atoms.
When a candle burns, the heat of the flame vaporizes the liquid wax, turning it into hot gas. The gas is drawn into the flame and reacts with oxygen from the air to create heat, light, water vapour (H2O), and carbon dioxide (CO2).
To shorten a candle physically, you can use a sharp knife or a wick trimmer to cut off the desired amount of wax from the top. If the candle is too wide, cut the wax in sections. Then, use a lighter to gently heat the cut edge to smooth it out.
You can place a glass jar over the burning candle. The flame will go out once the oxygen inside the jar is used up. This is because the candle wax reacts with oxygen to burn, and without oxygen, the flame cannot sustain itself.
The reaction between vinegar and baking soda produces carbon dioxide (CO2), which can be used to put out a flame. The carbon dioxide molecules are heavier than air, so they push the oxygen and other molecules away, smothering the flame.
