Jessie Taylor
Lighting a candle involves a series of chemical and physical changes. The wax in a candle is a hydrocarbon, composed of hydrogen and carbon atoms. When a candle is lit, the wax melts, and the wick draws the liquid wax up by capillary action. The wax then vaporizes and reacts with oxygen from the air, creating heat, light, water vapour, and carbon dioxide. This process, known as combustion, involves a change in the substance's chemical composition, making it a chemical change. The flame of a candle is a mixture of hot gases, including carbon dioxide, water vapour, oxygen, and nitrogen. The colour of the flame is influenced by the presence of soot particles and other transient reaction intermediates. The burning of a candle also results in physical changes, such as the melting of wax, which can be reversed by allowing it to solidify.
| Characteristics | Values |
|---|---|
| Chemical reaction | Candle wax combines with oxygen in the air to produce carbon dioxide, water, and heat |
| State changes | Solid to liquid to gas |
| Light energy | Produced as a result of the chemical reaction |
| Heat energy | Produced as a result of the chemical reaction |
| Soot | Produced as a result of incomplete combustion |
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What You'll Learn
The wick burning
The wick of a candle is usually made from braided cotton, which is a natural fibre derived from plants. When a candle is lit, the wick is the first to undergo a chemical change. The wick burns and reacts with oxygen in the air, producing new substances: carbon dioxide and water vapour. This combustion reaction transforms the original materials of the wick into entirely different substances.
The wick's role is to provide a pathway for the liquid wax to travel up to the flame, where it too can burn. Capillary action draws the liquid wax up through the wick. This process is essential for the candle's combustion to continue. The liquid wax acts as fuel for the flame, and the heat of the flame melts more wax, creating a continuous cycle.
The wax, typically made of hydrocarbons, is primarily composed of hydrogen and carbon atoms. When the wax reaches the flame, it vaporises and breaks down into its constituent hydrogen and carbon atoms. The hydrogen atoms react with oxygen in the air to form water vapour, while some of the carbon burns to form carbon dioxide.
The colour of the candle flame provides insight into the chemical reactions occurring. The base of the flame is blue, indicating an oxygen-rich environment where hydrocarbon molecules vaporise and disassociate into hydrogen and carbon atoms. Above this is a small dark orange-brown section, where carbon particles begin to form. As these particles rise, they are heated to approximately 1000 degrees Celsius, causing them to glow and produce the characteristic yellow flame.
The chemical reactions involved in candle combustion have long fascinated scientists, dating back to Michael Faraday's 1860 lecture series on the Chemical History of a Candle. NASA also contributed to our understanding of candle flames by conducting experiments in microgravity conditions, revealing that a candle flame takes on a spherical shape in the absence of gravity's influence.
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Wax melting is a physical change
When a candle is lit, several chemical and physical changes occur. One of the most noticeable changes is the melting of the solid wax, which is primarily a physical change.
Wax melting is a physical transformation where the wax changes from a solid to a liquid state. This is a reversible process, and the liquid wax can be turned back into a solid without any chemical reactions or the creation of new substances. The substance involved in a physical change remains structurally identical before and after the change. In the case of wax melting, the chemical composition of the wax does not change; only its physical state does.
The process of wax melting involves the application of heat from the flame, which causes the solid wax near the wick to melt and turn into a liquid. This liquid wax is then drawn up by the wick through capillary action. Once the wax reaches the flame, it vaporizes and combines with oxygen from the air to produce heat, light, water vapour, and carbon dioxide.
The chemical reaction of the wax combustion can be represented by the formula: C25H52 + 38 O2 → 25 CO2 + 26 H2O. This reaction releases energy in the form of heat and light, sustaining the candle's flame. The generic chemical formula of wax combustion is C(n) H(2n+2) + O2 → CO2 + H2O, where n represents the number of carbon atoms in the wax molecule.
While wax melting is a physical change, the subsequent combustion of the wax is a chemical change. This chemical reaction involves the breakdown of hydrocarbon molecules in the wax and their reaction with oxygen to produce heat, light, and by-products such as carbon dioxide and water vapour. The blue region at the base of the flame is where the 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.
In summary, wax melting is a physical change that involves the transformation of solid wax into a liquid state without altering its chemical composition. This physical change is crucial for the subsequent chemical reaction of combustion, where the liquid wax vaporizes and reacts with oxygen to produce heat, light, and by-products.
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Chemical reaction with oxygen
When a candle is lit, it undergoes several changes, some of which are chemical reactions with oxygen.
Firstly, the wax in the candle combines with oxygen in the air to produce carbon dioxide, water, heat, and light. This is a process known as combustion. The wax, which is often made of hydrocarbons, melts with the heat of the flame, and the wick draws the liquid wax upwards through capillary action. Once the wax becomes a gas, it can burn. This is a chemical change as the substance's chemical composition has changed, and new substances have been formed.
The blue area at the base of the flame is oxygen-rich, and it is here that the hydrocarbon molecules vaporize and break apart into hydrogen and carbon atoms. The hydrogen separates first and reacts with the oxygen to form water vapour. Some of the carbon burns here to form carbon dioxide. The dark orange-brown region above has relatively little oxygen. This is where the various forms of carbon continue to break down, and small, hardened carbon particles (soot) begin to form.
The generic chemical formula of wax is: C(n) H(2n+2). The wax combines with oxygen to release carbon dioxide and water vapour as by-products. This chemical reaction can be seen when a lit candle is placed under a glass jar, as the flame will eventually go out due to a lack of oxygen.
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Carbon dioxide and water vapour produced
When a candle is lit, it undergoes several changes, some of which are chemical. One of the most notable chemical changes is the production of carbon dioxide and water vapour.
The wax in a candle is made up of hydrocarbon molecules, which are composed of hydrogen and carbon atoms. When a candle is lit, these hydrocarbon molecules vaporize and start to break apart into hydrogen and carbon atoms. The hydrogen atoms react with oxygen in the air to form water vapour (H2O). Simultaneously, some of the carbon atoms burn and react with oxygen to form carbon dioxide (CO2).
The blue area at the base of the flame is where the hydrocarbon molecules vaporize and begin to break down. The orange or brown area above it has relatively little oxygen, and this is where the various forms of carbon continue to break down and form small, hardened carbon particles (soot). These particles, along with the water vapour and carbon dioxide created in the blue zone, are heated to approximately 1000 degrees Centigrade as they rise.
The chemical reaction that occurs when a candle burns can be represented by the formula: C25H52 + 38 O2 → 25 CO2 + 26 H2O. This formula shows that for every 25 molecules of hydrocarbon (C25H52) and 38 molecules of oxygen (O2), 25 molecules of carbon dioxide (CO2) and 26 molecules of water (H2O) are produced, along with energy in the form of heat and light.
The combustion of candle wax is a chemical reaction that involves the rearrangement of atoms to form new substances. In this case, the original wax molecules are transformed into entirely new products, namely carbon dioxide and water. This change in the chemical composition of the substances involved is what defines a chemical change.
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Heat and light energy released
When a candle is lit, it undergoes a chemical process known as a combustion reaction. This reaction involves the candle's fuel—in this case, the wax—and oxygen from the air. The wax, which is a hydrocarbon, serves as the fuel for the candle, while the wick is the conduit through which the fuel is burned. The heat of the flame causes the wax to melt and become a liquid, which is then drawn up the wick through capillary action. This liquid wax is exposed to the heat of the flame, causing the wax molecules to undergo combustion.
Combustion is an exothermic reaction, meaning it releases energy. In the case of a burning candle, the combustion reaction releases energy in the form of heat and light. Approximately one-fourth of the energy created by a candle's combustion is given off as heat that radiates from the flame in all directions. This heat is what we perceive as a candle's warmth, and it is also what sustains the candle flame. The heat radiates back down and melts more wax, keeping the combustion process going until the fuel is used up or the heat source is eliminated.
The light from a candle is also a result of the combustion reaction. As the wax undergoes combustion, energy stored in the chemical bonds is released. This energy is absorbed by the surrounding molecules, causing them to vibrate faster and emit more heat and light. The emitted light is what we observe as the candle's flame. The yellow colour of the flame is due to soot particles glowing because they are hot (black body radiation). When the soot particles oxidize near the top of the flame's yellow region, the temperature is approximately 1200° C.
The chemical equation representing the combustion of a candle can be written as:
C_nH_{2n+2} + (3n+1/2) O_2 -> nCO_2 + (n+1) H_2O
In this equation, C_nH_{2n+2} represents the hydrocarbon (wax) molecule, and O_2 represents oxygen. The products of the reaction are carbon dioxide (CO_2) and water (H_2O). The generic chemical formula of wax is C(n) H(2n+2). The wax combines with oxygen (O2) molecules to release the same products.
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Frequently asked questions
When a candle is lit, the wax near the wick melts and is drawn up by the wick through capillary action. The wax then turns into gas and burns.
The candle wax, which is made of hydrocarbons, reacts with oxygen in the air to produce carbon dioxide, water, and heat. This process is known as combustion.
There are physical and chemical changes. The wax melting and the candle changing shape are physical changes as there is no change in the chemical composition of the wax. The candle burning and soot being produced are chemical changes as new substances are formed.
In microgravity, a candle flame takes on a spherical shape instead of its elongated shape on Earth. This is because there is no "up" direction for warm air to rise and create a convection current.
