Brooke Martin
A popular experiment involves placing a burning candle on a dish of water and covering it with an inverted glass. When the candle goes out, the water level inside the glass rises. The standard explanation for this phenomenon is that the candle consumes oxygen during combustion, and the volume of water that rises corresponds to the volume of oxygen consumed. However, this explanation has been disputed, and it is now understood that the primary cause of the rising water level is the change in air temperature and pressure. When the candle is lit, the air inside the container expands, and some of it escapes from under the container. As the candle goes out and the air inside the container cools, it contracts, creating a vacuum or lower pressure. The higher-pressure air outside the container then pushes water into the container until pressure equalization is achieved.
| Characteristics | Values |
|---|---|
| Air pressure | Decreases |
| Air temperature | Decreases |
| Number of air molecules | Decreases |
| Water level | Rises |
| Air density | Decreases initially, then increases |
| Air volume | Decreases |
| Air bubbles | May be observed escaping |
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What You'll Learn
The role of oxygen consumption
However, other sources argue that oxygen depletion is not the main cause of the water rising. They suggest that the chemical process of combustion and the physical process of temperature change are the primary factors at play. The water does not rise immediately but only after the candle dims and goes out, indicating that oxygen depletion alone cannot explain the rise in water level. Instead, it is the cooling of the air inside the jar that creates a vacuum or lower pressure, causing the water to be pushed into the jar from the outside to equalize the pressure.
While there is some disagreement about the extent to which oxygen consumption plays a role, it is clear that it is one of several factors influencing the pressure changes in the candle experiment. The consumption of oxygen by the candle flame does lead to a change in volume and pressure, but it is the interaction of this process with the temperature changes and the chemical process of combustion that creates the overall effect of the water rising up the jar.
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The effect of temperature change
As the air inside the jar cools down, its molecules slow down and exert less pressure on the water surface. Consequently, the air outside the jar, which is at a higher pressure, pushes water into the jar to equalize the pressure. This movement of water into the jar is what we observe as the water level rising. The formation of condensation at the top of the jar further contributes to the pressure changes.
It is important to note that the chemical process of combustion, where oxygen is consumed and carbon dioxide is produced, also plays a role in the overall pressure changes. However, the temperature change due to the candle going out has a more significant and immediate effect on the pressure inside the jar. The rapid rise in the water level occurs when the candle dims or goes out, indicating that temperature change is the primary driver of the pressure difference.
The size and shape of the jar used in the experiment can also influence the rate at which the flame goes out and the extent of water rise. Taller and narrower jars result in the flame going out faster and can lead to a more substantial rise in the water level. This is because the taller candle is closer to the top of the jar, reducing the available space for heat expansion and accelerating the cooling process once the candle goes out.
In summary, the temperature change caused by the candle going out leads to a decrease in air pressure inside the jar. This pressure difference draws water into the jar as the higher-pressure outside air pushes it in, equalizing the pressure. The interplay between temperature, pressure, and the chemical processes of combustion creates a dynamic and intriguing experiment that illustrates the complex behaviour of gases and their interactions with their surroundings.
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The creation of a vacuum
As the candle burns, it consumes oxygen from the air inside the jar, leading to a reduction in oxygen molecules. Simultaneously, the flame's heat warms the air, causing it to expand and creating higher air pressure within the jar. This expansion results in some air escaping from under the jar, equalizing the pressure. However, when the candle eventually burns out, the absence of its heat causes the air inside the jar to cool down and contract, leading to a decrease in air pressure.
This decrease in pressure within the jar relative to the outside air pressure sets the stage for the creation of a vacuum. The higher pressure outside the jar exerts a force on the water, pushing it upwards into the jar to equalize the pressure. As the water rises, it compresses the air inside the jar, leading to an increase in air density and pressure until equilibrium is reached between the inside and outside of the jar. This dynamic process results in the intriguing effect of rising water, demonstrating the power of pressure differentials.
The experiment showcases the complex interactions between heat, gases, and pressure. While the initial burning of the candle increases temperature and pressure, the subsequent cooling phase highlights the critical role of pressure changes in the creation of a vacuum. The condensation of water molecules and the formation of an imperfect vacuum further contribute to the overall effect.
It is important to note that the experiment has sparked discussions and debates among curious minds. Some sources suggest that the consumption of oxygen during combustion may play a role in the rising water level, while others emphasize the dominant influence of temperature and pressure changes. These conversations showcase the intricacies of scientific exploration and our ongoing pursuit of understanding.
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The equalization of pressure
The candle and rising water experiment is a popular experiment that demonstrates the effects of heat on air pressure. It involves placing a burning candle on a dish filled with water and covering it with an inverted glass. As the candle burns, it heats the air inside the container, causing it to expand and some of it to escape from under the glass, sometimes forming bubbles.
When the candle eventually goes out, the air inside the container cools down and contracts, creating a vacuum or lower pressure. This lower pressure inside the container is compared to the higher pressure outside, which is exerted onto the water in the dish. The outside air then pushes water into the container to equalize the pressure. This movement of water continues until equilibrium is reached, and the pressure is the same on both sides.
The rise in water level is often incorrectly attributed solely to oxygen depletion by the burning candle. While oxygen consumption does play a role, it is minor compared to the physical process of temperature change. As the air cools down, its pressure decreases, allowing more water to be pushed into the container. The chemical process of burning, where two oxygen molecules are replaced by one carbon dioxide molecule, also contributes to the change in pressure. However, the overall effect is initially cancelled out by the temperature increase caused by the burning candle.
The experiment can be varied by using taller candles, narrower jars, and adding food colouring to the water to better observe the movement of water. These variations can result in more dramatic rises in water level and provide further insights into the relationship between air pressure and temperature.
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The chemical process of burning
The process of burning is a chemical reaction, a chemical change. When a candle burns, it releases energy through the same kind of reaction that our body uses to obtain energy. The wax molecules undergo a chemical change; they react with a substance in the air to change into different molecules. Candle wax is a hydrocarbon, largely composed of hydrogen (H) and carbon (C) atoms.
The flame of a candle is a chemical reaction in action. The oxygen-rich blue zone is where the hydrocarbon molecules vaporize and start to break apart into hydrogen and carbon atoms. The hydrogen is the first to separate and reacts with the oxygen to form water vapour. Some of the carbon burns here to form carbon dioxide. The dark or orange/brown region has relatively little oxygen. This is where the various forms of carbon continue to break down and small, hardened carbon particles (soot) start to form. As they rise, along with the water vapour and carbon dioxide, they are heated to approximately 1000 degrees Centigrade. At the bottom of the yellow zone, the formation of the carbon (soot) particles increases. As they rise, they continue to heat until they ignite to incandescence and emit the full spectrum of visible light. The yellow portion of the spectrum is the most dominant when the carbon ignites, so the human eye perceives the flame as yellowish.
The flame heats the nearby air and starts to rise. As this warm air moves up, cooler air and oxygen rush in at the bottom of the flame to replace it. When that cooler air is heated, it too rises and is replaced by cooler air at the base of the flame. This creates a continual cycle of upward-moving air around the flame (a convection current), which gives the flame its elongated or teardrop shape.
When a candle burns in a closed container, the pressure increases due to the increase in temperature. The hot air expands, which leads to higher air pressure. This causes tiny air bubbles to escape under the edge of the jar to equalize the pressure. When the candle goes out, the air in the jar cools and the pressure drops, causing water to enter the jar to equalize the pressure.
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Frequently asked questions
The candle eventually goes out due to a lack of oxygen, and the air in the jar cools down and creates a vacuum.
The pressure inside the jar decreases as the air in the jar cools down.
The candle flame heats the air inside the jar, and this hot air expands and escapes from the jar. When the flame goes out, the air in the jar cools and takes up less space, leading to a decrease in pressure.
The water level in the jar may rise due to the increased temperature and pressure created by the burning candle. However, this rise in water level is not solely due to oxygen depletion but also the chemical and physical processes involved in the experiment.
Yes, adding a few drops of food coloring to the water before placing the jar over the candle can help visualize the movement of water more effectively.
