Ben Carter
When you blow out a candle, you might notice that it produces smoke for a few seconds before stopping. This smoke is not actually smoke, but rather vaporized paraffin wax. When a candle is burning, the heat of the flame vaporizes the wax, which then reacts with oxygen to produce the flame. When you blow out the candle, you are simultaneously diluting the wax vapour and cooling it below the point where the wax and air molecules can react. The flame is blown away from the candle because the air you blow moves faster than the speed of the flame, moving the flame away from its fuel source. The residual heat from the wick continues to vaporize the wax for a few seconds, producing a trail of what appears to be smoke.
| Characteristics | Values |
|---|---|
| What is seen when a candle is blown out | Smoke |
| What is smoke | Vaporized paraffin wax |
| Why does smoke occur when a candle is blown out | The flame is blown away from the candle due to the air blown being faster than the speed of the flame front |
| Why does smoke smell | Pyrolysis continues for a few seconds after the candle is blown out, producing solid particles and smelly gases |
| How to prevent smoke | Burn candles in a well-ventilated room, away from drafts, vents or strong air currents |
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What You'll Learn
The smoke is actually vapourised paraffin wax
When you blow out a candle, the smoke you see is not actually smoke but vapourised paraffin wax. This happens because, when a candle is burning, the heat of the flame vapourises the wax, and the hot wax vapour reacts with oxygen to produce the flame. When you blow on a candle, you are simultaneously diluting the wax vapour and cooling it below the point where the wax and air molecules can react. However, the heat remains for a few seconds and continues to vapourise the candle wax, producing a trail of what appears to be smoke.
The smoke is more noticeable when a candle is blown out because the particles produced are larger than those produced during flaming. The particles produced during flaming are very small and invisible, whereas those produced when a candle is blown out are larger and visible. This is due to the larger mass of material turned directly into particles.
The smoke from a candle is also caused by pyrolysis continuing for a few seconds after the flame has been extinguished. The wick and the wax are still hot, so solid particles and smelly gases are produced for a moment, but with no flame to burn them, they rise like smoke. This is why the smoke smells stronger when a candle is blown out.
The smoke from a candle is also a result of incomplete combustion. This can be caused by constant flickering, which allows small amounts of unburned carbon particles, or soot, to escape from the flame as wisps of smoke. Incomplete combustion can also be caused by too much or too little air reaching the flame, creating microscopic incomplete combustion points in the wick. This can be avoided by burning candles in a well-ventilated room and trimming the wick regularly.
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The wick continues to burn after blowing out
When you blow out a candle, the flame is blown away from its fuel source, causing it to go out. However, the wick and the wax are still hot, so pyrolysis continues for a few seconds. This results in the production of solid particles and gases, which rise like smoke. This smoke is not actually smoke but vaporized paraffin wax. The wax vapour continues to be produced after the candle is extinguished, but without sufficient heat, the reaction cannot continue.
The smoke produced when a candle is blown out is a result of incomplete combustion. When a candle burns, the heat of the flame vaporizes the wax, which then reacts with oxygen to produce the flame. Blowing on the flame simultaneously dilutes and cools the wax vapour, preventing it from reacting with the oxygen. However, the wick remains hot, and combustion continues, but it is not complete. This results in the production of larger smoke particles that can be seen and are potentially harmful to health.
To prevent a candle from smoking when blown out, it is important to ensure complete combustion. This can be achieved by burning the candle in a well-ventilated room, away from drafts and strong air currents. Additionally, regular wick trimming can help keep the soot from appearing. A long wick can cause the wax to be drawn up into the cooler part of the flame, where there is insufficient oxygen for complete combustion, resulting in black carbon smoke.
It is worth noting that candles produce particles (smoke) while they are burning, but these particles are minuscule and not easily noticeable. The smoke produced when a candle is blown out is more visible due to the larger particle size. The plume visible after extinguishing a candle is a stream of unburned fuel particles rather than smoke, and it is powered by the residual heat of the wick.
The smoke from a blown-out candle can be quite intriguing as candles do not produce smoke while they are still lit. This is because the heat from the flame vaporizes the wax, and there is sufficient heat for the reaction to continue. However, when the candle is blown out, the heat remains for a few seconds, continuing to vaporize the wax and producing what appears to be smoke.
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Incomplete combustion causes smoke
When a candle is burning, the energy from the flame heats up the wax, and while the wick does burn, it is actually the wax that fuels the flame and keeps it burning. The melted wax rises through the wick and is drawn up into the flame. The wax vapour then burns to produce carbon dioxide and water. These products are invisible.
When a candle is extinguished, the smoke comprises wax vapour, intermediate breakdown products, and unburned carbon. This is due to incomplete combustion, which occurs when there is insufficient oxygen for the wax to fully combust. The residual heat from the wick continues to vaporize the wax for a few seconds, producing a trail of what appears to be smoke. This smoke is actually a stream of unburned fuel particles, which can be reignited by a flame.
Incomplete combustion can be caused by several factors, including an insufficient amount of oxygen reaching the flame, a build-up of debris on the wick, or constant flickering of the flame. To avoid incomplete combustion, candles should be burned in a well-ventilated room, away from drafts and strong air currents. Additionally, regular wick trimming can help prevent the build-up of debris, which can add more fuel to the candle and disrupt the combustion process.
The smoke produced by incomplete combustion can have a strong smell, which is due to the pyrolysis products that did not burn properly in the flame. These solid particles and gases rise like smoke, and their smell is more noticeable in the absence of a flame.
Overall, incomplete combustion of the candle wax and wick results in the production of smoke when a candle is blown out. This smoke consists of unburned fuel particles, which can be visible and odorous, depending on the size of the particles and the presence of a flame.
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Pyrolysis continues after blowing out
When a candle is extinguished, the smoke that rises from the wick is not actually smoke but vapourised paraffin wax. This phenomenon occurs because, when you blow out a candle, the flame stops immediately, but the wick and the wax are still hot. Thus, pyrolysis continues for a few seconds, producing solid particles and smelly gases. However, without a flame to burn them, these particles and gases rise like smoke.
Pyrolysis is the chemical decomposition of compounds by heating in the absence of oxygen. In the context of a burning candle, the wick draws up liquid wax through capillary action. The flame then vapourises the wax, and the hot wax vapour reacts with oxygen to produce the flame.
When a candle is extinguished, the heat from the wick continues to vapourise the wax for a few seconds. This vapourised wax is what we perceive as smoke. Importantly, the vapourised wax is still combustible. In fact, you can relight the candle by lighting the smoke instead of the wick.
The smoke produced when a candle is extinguished differs from the smoke produced while a candle is burning. The smoke from a burning candle is composed of minuscule particles. In contrast, the smoke from a blown-out candle is made up of larger particles, which are more visible and have a larger mass of material that can be harmful to the lungs.
To summarise, pyrolysis continues after a candle is blown out because the wick and wax remain hot, vapourising the wax and producing solid particles and gases that resemble smoke.
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Blowing introduces a large mass that cools the system
When you blow out a candle, you introduce a large mass of air that cools the system, causing the flame to burn out. This is because a candle flame relies on a continuous cycle of upward-moving air, known as a convection current, to stay alight.
The flame of a candle produces heat, which causes the air around 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 creates a convection current, which gives the flame its elongated or teardrop shape.
When you blow on a candle, you disrupt this cycle by introducing a large mass of air that moves faster than the speed of the flame front. This air current pushes the flame away from its fuel source, causing it to burn out due to a lack of fuel. Additionally, the air you blow onto the candle dilutes and cools the wax vapour, lowering its temperature below the point where the wax and air molecules can react to sustain the fire.
The introduction of a large mass of air that cools the system is similar to the effect of using water to extinguish a fire. Water is effective at putting out fires because it is extremely dense and can absorb a vast amount of heat in a short time, depriving the fire of the energy it needs to sustain itself.
It is worth noting that while blowing on a candle introduces a large mass that cools the system, it does not extinguish the flame by simply lowering the temperature. Our breath is actually warmer than the coolness required to extinguish a flame. Instead, the flame is blown away from its fuel source and starved of fuel, causing it to go out.
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Frequently asked questions
When a candle is burning, the heat from the flame vaporises the wax, which then reacts with oxygen to produce the flame. When you blow out a candle, you are removing the flame from its fuel source and cooling the system. However, the residual heat from the wick continues to vaporise the wax for a few seconds, producing what appears to be smoke.
The smoke is not actually smoke, but vaporised wax. It also contains intermediate breakdown products and unburned carbon particles, which are the result of incomplete combustion.
The particles produced during burning are very small and invisible. When the candle is blown out, the particles become larger and more visible.
The smell is due to the pyrolysis products that didn't burn properly in the flame. These solid particles and gases rise like smoke when there is no flame to burn them.
To prevent smoking, always burn your candles in a well-ventilated room, away from drafts and strong air currents. Regularly trim the wick to remove debris and keep the candle debris-free.
