Tyler Brooks
Lighting a candle involves a series of chemical and physical changes. The wick, made from absorbent fibres, draws up liquid wax, which is vaporized and combines with oxygen in the air to form a flame. This process, known as combustion, produces heat and light, melting more wax to sustain the flame. The hottest part of the flame is just above the blue base, reaching temperatures of 1400°C, though the average flame temperature is around 1000°C. The blue colour is due to chemiluminescence, while the yellow colour comes from radiative emission from hot soot particles. The thermal structure of a flame is complex, with steep temperature gradients. The combustion process stabilizes after a few minutes, burning cleanly with a quiet teardrop shape.
| Characteristics | Values |
|---|---|
| Wax melting | Physical change |
| Melted wax solidifying | Physical change |
| Candle burning | Chemical change |
| Soot (carbon) production | Chemical change |
| Candle changing shape | Physical change |
| Air heating up | Physical change |
| Wick burning | Chemical change |
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What You'll Learn
Wax melts
When a candle is lit, several changes occur, some physical and some chemical. One of the first physical changes that take place is the wax melting. The heat from the flame melts the solid wax, causing a change of state from solid to liquid. This is a physical change because the underlying chemical composition of the wax remains the same – the molecules are simply moving more freely.
The melting of the wax is a reversible process, as the wax can re-solidify when it cools down. This is another physical change, as the wax molecules arrange themselves back into a solid form. The conservation of mass is demonstrated here, as the wax's mass is conserved, it has just changed form.
The melting of the wax is an important part of the candle's combustion process. The heat created by the flame melts more wax, which keeps the combustion going until the fuel is used up or the heat source is removed. This is an example of how physical changes can influence the chemical processes that take place when a candle is lit.
The chemical changes that occur when a candle burns include the wick burning and the liquid wax reacting with oxygen in the air to produce carbon dioxide and water vapour. This combustion reaction releases energy in the form of heat and light. The presence of a flame indicates the release of energy.
In summary, when a candle is lit, the wax melting and re-solidifying are physical changes, while the wick burning and the wax combustion are chemical changes. These processes are interconnected and work together to create the unique characteristics of a burning candle, such as its light and warmth.
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Wick burns
The wick of a candle is a piece of string that absorbs the liquid wax and pulls it upward. The wick of a candle is lit, heating up and melting a small amount of solid fuel (the wax). The wick burns with oxygen from the air, producing new substances like carbon dioxide and water vapour. The flame then melts the top of the mass of solid fuel, which moves upward through the wick via capillary action to be continually burnt, thereby maintaining a constant flame. The candle shortens as the solid fuel is consumed, and so does the wick.
The wick of a candle is made from a material that is absorbent, like a towel, or has a strong capillary action, like a glass fibre wick. The absorbency of the wick is important as it absorbs the liquid wax and moves it upward while the candle is burning. The liquid wax is then absorbed by the wick and vaporized. The wax vapour burns and keeps the candle lit.
The wick of a candle is lit with a match or a lighter, which provides the heat source to melt and vaporize a small amount of fuel (the wax). The lighted wick begins to heat up the wax and turn it into liquid. The wick absorbs the liquid wax and pulls it upward. The wax then vaporizes and reaches its ignition temperature to keep the candle burning.
The wick of a candle is exposed to heat, which vaporizes the wax and cools the wick, protecting it from burning. The liquid wax acts as a cooling agent, similar to how water in a paper cup prevents the cup from burning. Only the tiny amount of wax on the wick is hot enough to vaporize and burn. The wick of a candle will continue to burn as long as there is enough heat to vaporize the wax.
The wick of a candle is an important component of the candle and works together with the wax to produce light and warmth. The heat from the flame melts the wax, which is then absorbed by the wick and vaporized. The vaporized wax combines with oxygen in the air to form a flame, releasing energy in the form of light and heat. The flame maintains a constant temperature, allowing the candle to burn steadily until the fuel is used up or the heat is eliminated.
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Candle changes shape
When a candle is lit, it undergoes several physical and chemical changes. One of the most noticeable changes is the melting of the solid wax, which transforms into a liquid state due to the heat of the flame. This is a physical change as the wax's chemical composition remains the same; its molecules are simply moving more freely within the liquid state. The liquid wax is then drawn up through the wick via capillary action, where it vaporizes and combines with oxygen in the air, sustaining the flame.
As the candle continues to burn, the solid wax near the flame melts and the candle gradually shortens. The top of the wax may also become uneven as the liquid wax moves through the wick, causing the candle to change shape. This change in shape is a direct result of the physical change from solid to liquid wax.
The shape of a candle's flame is also worth noting. On Earth, a candle flame typically has an elongated shape due to the pull of gravity, which influences the direction of warm air currents. However, in microgravity conditions, such as those studied by NASA scientists in space shuttle experiments, a candle flame takes on a spherical shape.
In summary, the physical changes of a burning candle include the melting of wax, the subsequent change in the candle's shape, and the potential solidification of excess wax on the candleholder. These changes occur without altering the chemical composition of the wax, demonstrating the unique properties and behaviour of candles when lit.
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Heat is released
When a candle is lit, one of the most prominent physical changes is the release of heat. This heat is generated through a process called combustion, where the candle's wax combines with oxygen in the air to produce carbon dioxide, water vapour, light, and heat. The heat released by the candle causes the wax molecules to gain energy and move more freely, resulting in the melting of the wax. This melting process is a physical change, as the wax changes from a solid to a liquid state without altering its chemical composition.
The heat from the flame is essential for sustaining the candle's burning process. Approximately one-fourth of the energy created by a candle's combustion is released as heat, radiating from the flame in all directions. This heat is sufficient to melt the top layer of solid wax, which is then drawn up through the wick via capillary action to fuel the flame. As the wax near the wick melts, it is absorbed by the wick, which pulls it upward. This liquid wax then vaporises and reaches its ignition temperature, allowing the candle to continue burning.
The temperature of a candle flame can reach up to 1,000 °C (1,800 °F) on average, with the hottest part of the flame just above the blue region, reaching temperatures of about 1,400 °C (2,550 °F). The blue colour observed in the flame is due to chemiluminescence, while the visible yellow colour is a result of radiative emission from hot soot particles. The soot particles are formed through complex chemical reactions, and their presence in the flame contributes to the heat output of the candle.
The heat released by a burning candle also affects the surrounding environment, causing the air around the candle to heat up. Additionally, the melted wax can solidify again on the candleholder or surrounding surfaces as it cools down. This solidification process is another example of a physical change, as the wax returns to its solid state without any change in its chemical composition. Overall, the heat generated by a lit candle plays a crucial role in the combustion process, sustaining the flame and driving the physical and chemical changes that occur during the burning of the candle.
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Carbon dioxide and water are produced
When a candle is lit, it undergoes several changes, some of which are physical and some are chemical. A physical change involves a change in the state of a substance without altering its chemical composition. One of the physical changes that occur when a candle is lit is the melting of the wax. The solid wax melts and turns into a liquid due to the heat produced by the flame. This is a physical change because the underlying chemical composition of the wax remains the same; its molecules are simply moving more freely. The liquid wax is then absorbed by the wick through capillary action and vaporized, contributing to the combustion process.
In contrast, a chemical change involves the formation of new substances through the transformation of the original substances' chemical composition. One of the chemical changes that occur when a candle is lit is the production of carbon dioxide and water. This happens through the combustion of the candle wax, which is often made of hydrocarbons. Hydrocarbons are composed of hydrogen and carbon atoms. When the candle is lit, the heat from the flame melts a small amount of wax, which then vaporizes and combines with oxygen in the air. This combustion process results in the formation of new substances: carbon dioxide and water vapour.
The blue area at the base of the candle flame is where the hydrocarbon molecules vaporize and start to 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 water vapour and carbon dioxide rise, they are heated to approximately 1000 degrees Centigrade in the dark orange-brown region of the flame. This is where the various forms of carbon continue to break down and form small, hardened carbon particles, known as soot.
The combustion process of a candle is self-sustaining. The heat from the flame melts the top of the solid wax, which moves upward through the wick and is continuously burned, maintaining a constant flame. As the candle burns, it emits light and warmth due to the energy released during the combustion process. The energy produced by the candle's combustion radiates in all directions, with approximately one-fourth of it given off as heat. This heat radiates back and melts more wax, fuelling the combustion process until the fuel is used up or the heat source is removed.
In summary, when a candle is lit, one of the physical changes that occur is the melting of the wax, while one of the chemical changes is the production of carbon dioxide and water through the combustion of the wax and its interaction with oxygen in the air.
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Frequently asked questions
When a candle is lit, the wax near the wick melts and the liquid wax is absorbed by the wick. The wick then pulls the liquid wax upward and the wax vapour burns, keeping the candle lit.
The physical changes that occur when a candle is lit include the melting of wax, the candle changing shape, and the air around the candle heating up.
The chemical changes that occur when a candle is lit include the burning of the wick, the production of soot, and the release of carbon dioxide and water vapour.
