Brooke Martin
Burning a candle involves both physical and chemical changes. The phenomena of candle burning involve the creation of new substances through chemical reactions, but also the physical transformation of the candle's components. The candle wick, for example, undergoes a physical change as the heat of the flame melts the wax near it, causing it to be drawn up the wick by capillary action and then vaporized. This physical change in the state of the wax is an important part of the combustion process, as the liquid wax is then broken down into molecules that react with oxygen to produce heat, light, water vapour, and carbon dioxide.
| Characteristics | Values |
|---|---|
| What happens when a candle is lit? | Heat from the flame melts the wax near the wick. |
| What happens to the liquid wax? | It is drawn up the wick by capillary action and vaporized by the heat of the flame. |
| What happens to the wax vapours? | They burn and get converted into carbon dioxide, soot, heat, light, and water vapour. |
| What happens when a candle flame is covered with a glass jar? | The flame goes out because the oxygen in the jar gets used up. |
| What happens when carbon dioxide is poured on a flame? | The flame goes out because carbon dioxide molecules are heavier than air and push the oxygen away from the wick. |
| What is a candle composed of? | Wax surrounding a wick. |
| What is wax made of? | Hydrogen and carbon atoms. |
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What You'll Learn
Heat melts wax near the wick
When a candle is lit, the heat from the flame melts the wax near the wick. This process is known as a physical change. The solid wax, which is a hydrocarbon, transforms into a liquid state due to the heat. This liquid wax is then drawn upwards through the wick by capillary action.
The heat of the flame vaporizes the liquid wax, turning it into a hot gas. This vaporization involves a breakdown of the hydrocarbon molecules into hydrogen and carbon atoms. These vapour molecules are then drawn into the flame, where they undergo a chemical reaction with oxygen from the surrounding air.
The chemical reaction between the vaporized wax and oxygen results in the production of carbon dioxide (CO2) and other substances. This combustion process continues as long as there is sufficient fuel in the form of wax to sustain it. The heat radiated from the flame contributes to melting more wax, ensuring a continuous supply of fuel for the reaction.
The melting of the wax is a reversible physical change, as the molten wax can be solidified and converted back into a candle. However, the subsequent combustion of the vaporized wax is a chemical change, as it involves the creation of new substances, such as carbon dioxide and water vapour, through the rearrangement of atoms.
The heat generated by the flame is crucial for sustaining the combustion process. Approximately one-fourth of the energy created by the candle's combustion is radiated as heat, which helps to melt more wax and maintain the cycle of fuel conversion and energy release.
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Liquid wax is drawn up the wick
When a candle is lit, the heat from the flame melts the wax near the wick. This liquid wax is then drawn up through the wick via capillary action. Capillary action is the process by which liquid moves up through the narrow spaces of a material—in this case, the small spaces between the fibres of the wick. The size of the wick is important, as it determines the amount of fuel (liquid wax) that is delivered to the flame. A wick that is too large will cause the flame to flare and soot, while a wick that is too small will cause the flame to sputter out.
The purpose of the wick is to act as a fuel pump, drawing the liquefied wax up into the flame to burn. The liquid wax is vaporised by the heat of the flame, turning into a hot gas. This gas mixes with oxygen from the air and burns, creating a flame. The heat from this flame then melts more wax, and the process repeats as long as there is wax to be melted and vaporised.
The wick plays a crucial role in this process by creating a steady supply of gas that can burn. Without the wick, the heat from a match or lighter would not be able to melt and vaporise enough wax to sustain a flame. The flame would quickly go out due to insufficient gas.
The type of wick used in a candle can vary depending on the wax type, size, shape, colour, and fragrance materials. Reputable candle manufacturers take care to select the proper wick to ensure the candle burns cleanly and properly. Most high-quality wicks are made from braided, plaited, or knitted fibres to encourage a slow and consistent burn. Some wicks are also cored, using materials such as cotton, paper, zinc, or tin to keep the wick straight or upright while burning.
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Heat vaporises liquid wax
When a candle is lit, the heat from the flame first melts the solid wax near the wick. This process is an example of a physical change, as the wax is transforming from a solid to a liquid state without forming any new substances. The liquid wax is then drawn up the wick by capillary action, where it undergoes another physical change. The intense heat of the flame vaporizes the liquid wax, turning it into a hot gas. This gas, composed of hydrogen and carbon molecules, reacts with the oxygen in the air to produce heat, light, water vapour, and carbon dioxide.
The combustion of a candle involves both physical and chemical changes. While the melting and vaporization of wax are physical changes, the reaction between the vaporized wax and oxygen constitutes a chemical change since new substances, namely heat, light, water vapour, and carbon dioxide, are formed. This chemical reaction releases energy in the form of heat and light, sustaining the flame's combustion until the wax is entirely consumed or the flame is extinguished.
The process of candle combustion begins with the melting of solid wax, which is a physical change as it only alters the wax's state without changing its chemical composition. The liquid wax then moves up the wick, where it undergoes vaporization due to the flame's heat. This transformation from a liquid to a gaseous state is another physical change.
The heat of the flame causes the liquid wax to vaporize, breaking down the wax's hydrocarbon molecules into hydrogen and carbon constituents. This vaporization process is crucial for the subsequent chemical reaction that defines candle combustion. The vaporized molecules, rich in hydrogen and carbon, react with the surrounding oxygen to produce heat, light, and by-products such as water vapour and carbon dioxide.
The physical changes a candle undergoes, particularly the melting and vaporization of wax, are essential prerequisites for the chemical reaction that sustains the flame. The heat generated by this reaction then initiates the cycle anew, melting more solid wax and perpetuating the candle's combustion until it exhausts its fuel or is deliberately extinguished.
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Hydrocarbons break down into hydrogen and carbon
When a candle wick burns, it undergoes a physical change. The heat of the flame melts the wax near the wick, which is then drawn up the wick by capillary action. This liquid wax is vaporized and starts to break down into molecules of hydrogen and carbon. These vaporized molecules are drawn into the flame, where they react with oxygen from the air to create heat, light, water vapour, and carbon dioxide.
Hydrocarbons are molecules consisting of hydrogen and carbon atoms. They are the primary constituents of fossil fuels, such as natural gas, petroleum, and coal. The combustion of hydrocarbons produces energy, carbon dioxide, water, and heat. This process is a significant contributor to anthropogenic global warming due to the release of carbon dioxide, a greenhouse gas.
The general formula for acyclic saturated hydrocarbons (alkanes) is given by the expression CnH2n+2, where 'n' represents the number of carbon atoms in the hydrocarbon. These hydrocarbons have only single bonds between their carbon atoms. The first three members of this series are methane (CH4), ethane (C2H6), and propane (C3H8).
Unsaturated hydrocarbons, on the other hand, have one or more double or triple bonds between carbon atoms. Those with one or more double bonds are called alkenes and follow the formula CnH2n, assuming non-cyclic structures. Aromatic hydrocarbons, obtained from certain plant extracts, are another type of hydrocarbon. They are classified as arenes or nonbenzenoid aromatic hydrocarbons, depending on whether they contain a benzene ring.
Different hydrocarbons have different ratios of hydrogen to carbon, resulting in varying ratios of water to carbon dioxide when combusted. For example, burning coal releases more carbon dioxide than burning the same mass of oil or natural gas because coal contains the longest and most complex hydrocarbon molecules.
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Carbon dioxide is produced
When a candle is lit, several changes occur, and the process of combustion begins. The heat of the flame melts the wax near the wick, which is then drawn up the wick by capillary action. This liquid wax is vaporized by the heat of the flame and begins to break down into molecules of hydrogen and carbon. As the vaporized molecules are drawn into the flame, they react with oxygen from the air, creating heat, light, water vapour, and carbon dioxide. This reaction produces a new substance, carbon dioxide (CO2), through the combustion of hydrogen and carbon molecules with oxygen.
The combustion process involves the breakdown of hydrocarbons, which are the primary components of wax. The oxygen-rich region, known as the blue zone, is where hydrocarbon molecules vaporize and separate into hydrogen and carbon atoms. The hydrogen reacts with oxygen first, forming water vapour. Some of the carbon atoms also burn in this region, producing carbon dioxide.
As the combustion process continues, the carbon particles move into the orange or brown region of the flame, which has relatively less oxygen. Here, the carbon continues to break down and form small, hardened particles of soot. These particles, along with the water vapour and carbon dioxide produced in the blue zone, are heated to high temperatures of approximately 1000 degrees Celsius.
The formation of carbon soot particles increases as they rise into the yellow zone of the flame. As they continue to heat up, they ignite and emit a full spectrum of visible light, with the yellow portion being the most dominant, giving the candle flame its characteristic yellowish appearance.
The amount of carbon dioxide produced during candle combustion can vary depending on the completeness of the combustion process. Incomplete combustion may result in the formation of soot or other byproducts, affecting the amount of carbon dioxide released. Additionally, the type of wax used in the candle can also impact the production of carbon dioxide.
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Frequently asked questions
A physical change involves a change in the state or appearance of a substance without changing its chemical composition.
When a candle burns, the heat from the flame melts the solid wax into liquid wax. This phase change from solid to liquid is a physical change as the wax remains chemically the same.
The liquid wax is drawn up through the wick by capillary action.
Capillary action is the process by which liquid wax is absorbed by the candle's wick.
The heat of the flame vaporizes the liquid wax and starts to break down the hydrocarbons into molecules of hydrogen and carbon. These vapour molecules then react with oxygen in the air to produce light and heat.
