Brooke Martin
The burning candle and rising water experiment is a well-known demonstration that can be performed by anyone. It involves lighting a candle and submerging it in water, often beneath a glass container. This experiment showcases the wonders of heat energy transference and provides a visual representation of the amount of oxygen present in the air. As the candle burns, it combines oxygen from the air with carbon and hydrogen in the wax, forming carbon dioxide, water vapour, and heat. The heat produced causes the gases within the glass to expand, and when the flame eventually extinguishes due to oxygen depletion, the gases cool and contract, leading to water being pushed up into the glass to balance the pressure.
| Characteristics | Values |
|---|---|
| Can a candle burn underwater? | Yes, a candle can burn underwater. |
| How does it work? | The flame produces heat, causing the gases in the glass to expand. When the candle goes out, the heat stops being produced, and the gas in the glass cools and contracts, allowing water to enter the glass to balance the pressure. |
| What happens to the oxygen? | The oxygen gets used up, and the circular current within the jar ensures that oxygen from above is also used up. |
| What about carbon monoxide? | Very little is produced. |
| What about carbon dioxide? | The number of oxygen molecules is replaced by half the number of carbon dioxide molecules. |
| What about water vapour? | Water vapour can condense on the jar. |
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What You'll Learn
Candles can burn underwater
It is possible to keep a candle burning underwater. This is a classic experiment that can be performed at home, but it should be done with caution and preferably with the help of a parent or guardian. To try this experiment, you will need a candle, a bowl of water, and a glass.
First, light the candle and let it float on the surface of the water. Then, quickly place the glass over the candle, submerging it underwater. Surprisingly, the candle will continue to burn even when it is completely underwater.
So, what's going on here? This experiment demonstrates several scientific principles related to heat energy transference, gas laws, and chemical reactions. Firstly, it shows that air takes up space. When the candle is submerged, the air inside the glass allows the candle to continue burning. As the candle burns, it consumes oxygen and combines it with carbon and hydrogen in the wax to produce carbon dioxide, water vapour, and heat. Eventually, the oxygen inside the glass will be depleted, causing the flame to go out.
Additionally, this experiment illustrates the principles of gas laws. When the candle is burning, the gases in the glass expand due to the heat. Once the candle goes out, the gases cool and contract, creating a lower pressure inside the glass than outside. This pressure difference causes water to be pushed up into the glass, seemingly "sucking" the water up.
Overall, this simple experiment provides a fascinating glimpse into the wonders of science, showcasing the complex interactions between heat, gases, and chemical reactions in a visually captivating way.
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Water gets sucked into the glass
The burning candle and rising water experiment is a classic demonstration that you can try yourself at home. It is a simple experiment that can help visualise how much oxygen is present in the air.
To begin, you light a candle and place it in a bowl of water. Next, cover the candle with a glass and submerge it in the water. Interestingly, the candle will remain lit even when underwater. Eventually, the flame will go out as the candle uses up all the oxygen under the glass and cannot burn anymore.
As the flame goes out, the water will rise into the glass. This is because the pressure inside the glass is reduced while the atmospheric pressure outside the glass remains constant. Since the outside pressure is greater than the inside, the water is pushed up into the glass. This is not due to the water being sucked up, but rather due to the pressure difference.
The reduction in pressure inside the glass is caused by the chemical reaction between the wax and oxygen during burning. This reaction produces heat, which causes the gases in the glass to expand. However, once the candle goes out, the heat stops being produced, and the gases in the glass cool and contract. This decrease in volume leads to a pressure difference, causing water to enter the glass to balance the pressure.
The thickness of the candle and the number of candles used can also impact the level of water rise. Thinner candles produce less heat, resulting in less noticeable expansion and contraction of gases. Conversely, using multiple candles generates more heat, leading to a more noticeable volume change.
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Oxygen depletion and water rise
The "burning candle and rising water" experiment demonstrates the relationship between oxygen depletion and water rise. In this experiment, a lit candle is placed under a glass on a plate of water. As the candle burns, it consumes oxygen from the air trapped inside the glass, combining it with carbon and hydrogen in the wax to produce carbon dioxide, water vapour, and heat. This chemical reaction, commonly known as "burning," results in the depletion of oxygen within the enclosed space.
The depletion of oxygen plays a crucial role in the water rise phenomenon observed in the experiment. As the oxygen inside the glass is consumed, its volume decreases. This reduction in volume creates a partial vacuum, causing the pressure inside the glass to decrease. Simultaneously, the pressure outside the glass, due to the surrounding atmosphere, remains constant. The resulting pressure differential leads to the water being pushed upwards into the glass to equalize the pressure.
The rate of oxygen depletion directly influences the rate at which the water rises. A thinner candle, for example, produces less heat and consumes oxygen at a slower rate, resulting in a less noticeable expansion and contraction of gases within the glass. In contrast, using two candles produces more heat and depletes oxygen at a faster rate, leading to a more rapid and noticeable volume change.
While oxygen depletion is a critical factor, it is not the sole explanation for the rising water. The physics of gas laws, such as the ideal gas law and the van der Waals equation, also come into play. These laws describe the relationship between gas pressure, volume, temperature, and the number of gas molecules. As the candle burns, it converts oxygen molecules into carbon dioxide molecules, which occupy a smaller volume due to their higher molecular weight. This change in the number and type of gas molecules contributes to the decrease in volume and pressure inside the glass, further facilitating the rise of water.
In summary, the "burning candle and rising water" experiment illustrates the complex interplay between oxygen depletion and water rise. The consumption of oxygen by the burning candle creates a partial vacuum, reducing the pressure inside the glass. This pressure differential, in conjunction with the principles of gas laws, results in the water being pushed upwards into the glass to equalize the pressure. The rate of oxygen depletion and the subsequent volume change of gases influence the rate and extent of water rise.
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Heat and gas expansion
The famous "lit candle under a glass of water" experiment demonstrates the principles of heat transfer and gas expansion. This experiment showcases how a candle can continue burning underwater for a short period.
When a lit candle is placed underwater in a glass, it continues to burn due to the presence of air in the glass. This air contains oxygen, which is essential for the candle's combustion. However, as the candle burns, it consumes the limited oxygen supply within the glass. As a result, the flame eventually extinguishes once the oxygen is depleted.
The heat produced by the flame plays a crucial role in this experiment. The heat causes the gases in the glass to expand, following the basic principle that hot gases expand while cold gases contract. Once the candle goes out and heat production ceases, the gases in the glass begin to cool and contract. This contraction leads to a decrease in pressure inside the glass.
Consequently, the higher atmospheric pressure outside the glass pushes the water up into the glass to balance the pressure difference. This phenomenon gives the appearance of water being "sucked" up into the glass, even though it is actually being pushed in by the external atmospheric pressure.
The chemical reaction between the candle wax and oxygen during combustion is also significant. This reaction typically involves the combination of carbon and hydrogen in the wax with oxygen to form carbon dioxide, water vapour, and heat. The production of carbon dioxide contributes to the depletion of oxygen and the increase in gas volume within the glass.
The experiment highlights the role of heat and gas expansion in the underwater candle experiment. By observing the expansion and contraction of gases, we can gain insights into the principles of heat transfer and pressure dynamics.
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Chemical reaction between wax and oxygen
The chemical reaction between wax and oxygen is what we commonly refer to as ["burning"]. When a candle burns, the flame heats the nearby air, and the warmer, less dense air rises. This movement of air creates a convection current, causing cooler air and oxygen to rush in at the bottom of the flame to replace the warm air.
The solid wax, composed of hydrocarbon molecules, melts into a liquid through the heat of the flame. The wick then draws the liquid wax up by capillary action. The flame vaporizes the wax molecules, turning them into hot gases, and breaking them down into molecules of hydrogen and carbon. These vaporized molecules are drawn up into the flame, where they react with oxygen from the air.
This reaction produces heat, light, water vapour, and carbon dioxide. The carbon dioxide molecule contains one more carbon atom than the original oxygen molecule, making it heavier. However, it turns out that only the number of molecules matters, not their weight. As the wax is consumed, more liquid wax is drawn up the wick to continue the combustion process until the fuel is used up or the heat source is eliminated.
The colour of the flame is also influenced by the chemical reaction between wax and oxygen. The yellow colour is due to soot particles, or carbon, glowing because of the heat. The blue colour at the base of the flame is where the oxygen-rich hydrocarbon molecules vaporize and start to break apart into hydrogen and carbon atoms.
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Frequently asked questions
The candle will burn out as it uses up all the oxygen in the air under the water.
The pressure inside the glass decreases as the oxygen is used up, and this pulls the water up into the glass.
The water rises higher as there is more heat produced, which causes gases in the glass to expand.
