Brooke Martin
When a candle burns, it releases water vapour and carbon dioxide as a result of the hydrogen and carbon in the wax reacting with oxygen in the air. This water vapour is formed when hydrogen atoms in the wax react with oxygen to form H2O. The water vapour produced by a candle is hot and rises to the top of the glass, eventually filling it up. The water vapour and carbon dioxide produced by the candle will cool and mix into the air in the room, becoming indistinguishable from other molecules of water and carbon dioxide.
| Characteristics | Values |
|---|---|
| What happens when a candle burns | The wax vapor burns as the flame causes the wax to melt, flow up the wick, and evaporate |
| What is the flame | A continuous combustion reaction between oxygen and hydrocarbons (wax) |
| What is produced when a candle burns | Carbon dioxide and water vapour |
| What happens to the carbon dioxide and water vapour | They cool and mix into the air in the room, becoming indistinguishable from other molecules of CO2 or water |
| What happens to the molecules from the candle | They escape the room and begin to disperse into the atmosphere |
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What You'll Learn
The chemical composition of candles
Fuel
The most common type of candle fuel today is beeswax, which has been used since the Middle Ages. It is produced by honey bees and is considered safer than other waxes. Another popular candle fuel is paraffin wax, a natural waxy substance derived from refining petroleum or crude oil. It has a high melting point and is widely used in the United States.
Other types of candle fuels include stearin wax, which is derived from the fatty acids of animals and is prevalent in Europe, and plant-based waxes like soybean, carnauba, and palm wax. In the past, candles were also made from tallow, a substance rendered from animals, but this practice has become less common due to the availability of other sources.
Wick
The wick of a candle is typically made from braided cotton fibres. Cotton wicks are designed to curl over and be completely consumed by the flame. Non-cored wicks are usually made solely of cotton, while cored wicks have a metal core made of zinc, lead, or tin surrounded by cotton.
Colourant and Fragrance
Candle manufacturers can choose from a wide range of aroma chemicals and essential oils to create pleasing and compatible scents for their candles. These fragrance materials are also found in perfumes, soaps, and lotions, and they undergo health and safety tests to ensure their suitability for candle use.
Additionally, candles may include colourants to enhance their visual appeal.
Water Vapour
When a candle burns, the candle wax reacts with oxygen in the air. The hydrogen atoms separate first and react with oxygen to form water vapour. This hot water vapour rises and cools, condensing into tiny water droplets that can be observed as mist on the glass surrounding the candle.
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The combustion reaction
The hydrogen atoms react with oxygen to form water vapour, while some carbon burns to form carbon dioxide. As these gases rise, they are heated to extremely high temperatures, reaching approximately 1000 degrees Celsius in the yellow zone of the flame. Here, the remaining carbon particles continue to break down and form hardened soot particles. As these particles ignite, they emit a full spectrum of visible light, with the yellow portion dominating our perception, giving the flame its yellowish appearance.
The water vapour produced during combustion is initially hot and exists as steam or water vapour. When it comes into contact with a cold surface, such as a glass container, it condenses into tiny water droplets, visible as mist on the glass. This process demonstrates the transformation of gaseous water vapour back into liquid water.
The chemical process of combustion involves the depletion of oxygen, which is consumed in the reaction. As the candle burns, it converts oxygen into water vapour and carbon dioxide, releasing these gases into the surrounding environment. Over time, these molecules disperse into the atmosphere, eventually spreading around the globe.
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The water vapour produced
While candles do produce water vapour, it is important to note that they are not a significant source of indoor humidity. The amount of water vapour produced by a candle is relatively small and comparable to the amount of water vapour exhaled by an individual in the room. Therefore, burning candles are unlikely to have a noticeable impact on the humidity levels in a typical indoor environment.
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The effect of oxygen on the flame
The effect of oxygen on a candle's flame is integral to the candle's combustion. The flame of a candle is a continuous combustion reaction between oxygen and hydrocarbons (wax) that results in the production of water vapour, carbon dioxide, and heat. This combustion reaction is influenced by the presence of oxygen, which plays a crucial role in sustaining the flame.
When a candle burns, the wax melts and travels up the wick, where it evaporates and undergoes a chemical reaction with oxygen. Specifically, the hydrocarbon molecules in the wax vaporize and break apart into hydrogen and carbon atoms. The hydrogen atoms react with oxygen to form water vapour (H2O), while some of the carbon atoms combine with oxygen to form carbon dioxide (CO2). This process consumes oxygen from the surrounding air.
The presence of oxygen is essential for the candle's flame to continue burning. If oxygen is removed, such as by placing a lid on the candle jar or using a snuffers, the combustion reaction is halted, and the flame extinguishes. This method of smothering the flame is safer than blowing it out, as it produces less smoke and prevents smoke from circulating into the room.
The oxygen-rich region of the flame, known as the blue zone, is where the hydrocarbon molecules vaporize and initiate the combustion reaction. As the flame ascends, it encounters an area with relatively little oxygen, the dark orange or brown region. Here, the remaining carbon molecules continue to break down and form small, hardened carbon particles. These particles rise and are heated to extremely high temperatures, eventually igniting and emitting visible light. The yellow portion of the spectrum dominates, giving the flame its yellowish appearance.
Additionally, the presence of oxygen influences the shape of the candle flame. On Earth, the convection current created by the upward movement of warm air gives the flame its characteristic teardrop shape. However, in microgravity conditions, such as those studied by NASA scientists in space shuttle experiments, the flame assumes a spherical shape due to the absence of gravity's directional influence on warm air.
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The shape of the flame
A candle flame typically takes on a teardrop shape. This is due to the convection current created by the cycle of upward-moving air around the flame. However, when a candle burns in an environment with minimal gravity, such as in the experiments conducted by NASA in the late 1990s, the flame takes on a spherical shape. In microgravity, the absence of a distinct upward direction for warm air prevents the formation of a convection current, resulting in a spherical flame.
The shape of a candle flame can also be influenced by the presence of multiple candles in close proximity. When two or more candles are placed together and one is blown out, the wax vapour of the extinguished candle can reignite, causing the flame to jump from one candle to another. This phenomenon occurs because the wax vapour of the blown-out candle comes into contact with the flame of the burning candle, demonstrating the unique behaviour of candle flames in close arrangements.
The colour of a candle flame also plays a role in its overall appearance and shape. Upon close observation, a candle flame exhibits distinct colour zones. At the base is a blue area, followed by a small dark orange-brown section, and finally, the large yellow region commonly associated with candle flames. The blue zone, rich in oxygen, is where hydrocarbon molecules vaporize and break apart into hydrogen and carbon atoms. As carbon continues to break down in the orange-brown region with less oxygen, small, hardened carbon particles begin to form and rise with the water vapour and carbon dioxide.
The yellow zone of the flame is where the formation of carbon soot particles increases, resulting in the characteristic sooty flame of candles. The combustion of the wax vapour within the flame sustains this process, creating a stable and consistent teardrop-shaped flame. The heat generated by the flame melts the surrounding solid wax, providing additional fuel to maintain the combustion process until the fuel is depleted or the heat source is removed.
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Frequently asked questions
Yes, when a candle burns, the hydrogen and carbon in the wax combine with oxygen in the air to create carbon dioxide and water vapour.
The water vapour mixes with the air in the room and becomes indistinguishable from other water vapour molecules.
The wax in a candle is made up of hydrogen and carbon atoms. When the candle burns, the hydrogen atoms react with the oxygen in the air to form water vapour.
