Jessie Taylor
The Candle Experiment is a classic introductory chemistry exercise that explores the chemical and physical properties of candles. By observing and experimenting with a burning candle, students can learn about basic chemical principles and develop their scientific reasoning skills. One of the key aspects of the experiment is investigating the role of oxygen in combustion. By placing a jar over a burning candle, students can observe that the candle eventually goes out due to limited oxygen, proving that oxygen is necessary for combustion. This simple experiment provides a tangible demonstration of the basic principles of chemistry and encourages students to think critically and make their own observations. Additionally, it offers a hands-on approach to learning, allowing students to explore the chemical reactions and energy transformations that occur when a candle burns.
| Characteristics | Values |
|---|---|
| Purpose | To introduce students to physical and chemical properties, as well as macro/micro/symbolic representations of chemical phenomena |
| Method | Students observe and make measurements of unlit candles, then receive matchbooks to light them |
| Observations | Changes in state of wax (solid to liquid to gas), different colours of the flame, smoke composition, energy transformations, chemical reactions |
| Experiments | Relighting a blown-out candle, using differently-sized jars to observe oxygen's role in combustion, proving the production of CO2 and H2O |
| Discussion Points | Conservation of mass, atomic theory, early development of chemistry, categorisation of changes as chemical vs. physical |
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What You'll Learn
The chemical reaction of a burning candle
The first step in the chemical reaction of a burning candle is the conversion of solid wax to liquid and then to gas. The heat of the flame melts the solid wax, and the wick draws the liquid wax upwards through capillary action. Once the wax vaporises, it can ignite and burn. This process is similar to that of fossil fuels, which are also hydrocarbons composed primarily of hydrogen and carbon atoms.
The flame of a candle is a mixture of hot gases, including carbon dioxide, water vapour, oxygen, and nitrogen. The yellow colour typically associated with candle flames is due to the presence of soot particles glowing from the heat. The blue base of the flame, known as the oxygen-rich zone, is where hydrocarbon molecules 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.
As the gases rise, they are heated to extremely high temperatures, causing the formation of carbon soot particles. These particles continue to heat up as they rise, eventually igniting and emitting a full spectrum of visible light. The yellow portion of the spectrum is the most dominant, giving the flame its characteristic yellowish hue. The blue outer edge of the flame, known as the veil, is the hottest part, reaching temperatures of around 1400°C.
In conclusion, the chemical reaction of a burning candle involves the conversion of solid wax to gas, the release of heat and light energy, and the formation of various hot gases that make up the flame. This simple experiment provides a wealth of learning opportunities and has played a significant role in the history of science.
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The colour of the flame
The colour of a candle's flame can be used to prove basic chemistry theories. A candle flame has three zones, each with a different temperature and colour. The outer zone is blue, the hottest of the three zones. This outer zone is non-luminous and is the result of complete combustion. The middle zone is yellow, the bright part of the flame, and is where partial combustion of fuel takes place. The innermost zone is the coolest and is black due to the presence of unburnt wax vapours.
The different colours of the flame are caused by the varying temperatures of each zone. Certain molecules of wax do not get completely reacted and glow a certain colour when they reach a particular temperature. As different parts of the flame have different temperatures, these wax molecules make those areas of the flame glow with different colours.
Students can observe these colours and draw the candle as it burns. They can also be encouraged to think about the chemical processes taking place. For example, the molecules in the candle wax combine and rearrange with oxygen from the air, releasing heat and light. This is called combustion.
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State changes in the wax
When a candle burns, the wax undergoes several state changes. Firstly, the solid wax melts due to the heat of the flame, turning into a liquid state. This process is called melting or fusion, and it occurs when the wax reaches its melting point, which typically ranges from 37°C to 68°C (99°F to 154°F), depending on the wax's composition. The liquid wax is then drawn up into the wick through capillary action, acting like a straw.
Once in the wick, the liquid wax is subjected to the intense heat of the flame, causing it to vaporize and turn into a gas. This gas is highly flammable and, in the presence of oxygen, combusts to produce a flame. The combustion of wax vapour is a chemical reaction that releases heat, light, water vapour, and carbon dioxide. The heat generated helps sustain the flame, creating a self-perpetuating cycle until all the wax is consumed or there is insufficient heat to melt the wax.
The combustion of candle wax involves the interaction of wax molecules, primarily composed of hydrocarbons, with oxygen molecules in the air. This process transforms the solid wax into invisible carbon dioxide gas and water vapour. The water vapour produced can be observed as condensation on the inside of a glass container placed over a burning candle. The specific equation for wax combustion depends on the type of wax used, but all equations follow a general pattern of converting solid wax into gaseous products.
The journey of candle wax, from its solid state to gas, is a captivating process that combines chemistry and physics. The disappearing act of the wax is not merely an illusion but a testament to the intricate chemical reactions occurring during combustion, releasing energy in the form of heat and light. The efficiency of this process is evident in the steady burning of a candle with a teardrop-shaped flame, where combustion is optimized, and only carbon dioxide and water vapour are released into the air.
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Energy transformations
The "Candle Experiment" is a well-known introductory exercise in chemistry education. It was popularised by Michael Faraday in 1860 in his lecture "The Chemical History of a Candle". The experiment is a great way to introduce students to physical and chemical properties, as well as the application of macro, micro, and symbolic representations of chemical phenomena.
The Candle Experiment typically begins with the distribution of unlit candles to students, who are then asked to make observations and note possible measurements they could take with equipment such as a thermometer or balance. After some discussion and instructor guidance, matchbooks are distributed, and the candles are lit.
The burning of a candle is a classic example of energy transformation. The chemical energy of the candle wax is converted into heat and light energy. The heat from the flame melts the wax, which is then drawn up through the wick by capillary action. This liquid wax, or fuel, is burned in a process of oxidation, releasing heat and light energy. The molecules in the wax combine and rearrange with oxygen from the air, producing carbon dioxide, water vapour, and heat and light energy. This process is called combustion.
The colour of the flame is also indicative of energy transformations. The yellow colour is due to hot soot particles emitting black body radiation. Other colours, such as blue and green, are produced by excited molecules during combustion, which then emit visible light through spectral band emission.
The Candle Experiment also allows for further exploration of concepts. For example, placing a jar over a burning candle demonstrates the role of oxygen in combustion, as the flame will eventually go out due to a lack of oxygen. This experiment also shows the production of water as a combustion product, as water condensation builds up on the inside of the jar.
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The smoke and soot
The yellow colour of the flame is due to these soot particles glowing because they are hot (blackbody radiation). The blue zone at the base of the flame is where the wax vapour and oxygen combine and rearrange, releasing heat and light. As the gases rise, they are heated to approximately 1000 degrees Centigrade. At the bottom of the yellow zone, the formation of carbon (soot) particles increases. As they continue to rise, they heat up further until they ignite and emit a full spectrum of visible light. The yellow portion of the spectrum is the most dominant when the carbon ignites, which is why candle flames appear yellowish.
The smoke that sometimes appears when a candle flickers is caused by unburned soot particles that have escaped from the flame due to incomplete combustion. Horizontal air movements in the vicinity of the flame, such as from an open window, can cause a flickering flame and allow some soot to escape without being oxidised. This is referred to as "sooting burn mode".
The chemical composition of candle smoke particles has been studied, revealing differences in organic composition between paraffin and beewax candle smoke due to variations in fuel composition. During the "efficient burning mode", ultrafine particles dominated, but their chemical composition could not be determined. During the sooting burn mode, larger particles above 100 nm appeared, and organic carbon emissions were associated with smouldering burning upon candle extinction.
Candle soot has traditionally been considered an unwanted source of air pollution. However, a 2007 study by Liu and colleagues discovered the presence of fluorescent carbon nanoparticles (CNPs) in untreated candle soot. Subsequent studies have found that these CNPs can be refined and extracted for various applications, including humidity sensing, trace element detection, biomedical uses, and energy storage.
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Frequently asked questions
A candle flame is the result of a chemical reaction between wax gas and oxygen in the air. The molecules in the candle wax and oxygen from the air combine and rearrange, releasing heat and light.
Place a jar over a burning candle. The candle will burn for a while, and then it will go out as it has only a limited amount of oxygen in the jar. This proves that a candle needs oxygen to burn.
Hold a glass beaker over the candle with the opening at the level of the flame. Water condensation will build up on the inside of the glass, proving that water vapour is produced as a by-product of combustion.
Use a burning candle and a jar of hydrogen. Push the hydrogen into the jar containing the candle. The candle will be extinguished, proving that hydrogen does not support combustion.
