How a Candle Draws Water into a Glass: A Scientific Exploration
Written on
Understanding the Candle and Water Interaction
This classic experiment is one you can easily replicate at home. Start by lighting a candle and placing it in a dish of water (adding blue dye can make it visually striking). Next, cover the candle with a glass. As the flame extinguishes, you'll notice that water rises into the glass, which is quite impressive.
Let’s clarify the term "sucked." The water isn't genuinely being sucked upwards; rather, it's a result of lower pressure inside the glass compared to the constant atmospheric pressure outside. The external pressure is greater, hence it pushes the water upward into the glass.
Here's a visual representation of the setup:
The Mechanism Behind Pressure Change
You might wonder why the pressure inside the glass decreases. This reduction is primarily due to a chemical reaction occurring between the candle's wax and the oxygen in the air. This kind of reaction is commonly referred to as "combustion."
In essence, combustion reactions involve oxygen reacting with carbon-based materials. For simplicity, consider burning methane instead of exploring the complexities of wood and wax combustion. The fundamental reaction remains similar:
The equation indicates that one molecule of methane reacts with two molecules of oxygen to yield one molecule of carbon dioxide and two molecules of water. Initially, the gas in the glass consists of approximately 79% nitrogen and 21% oxygen. The candle consumes the oxygen, generating carbon dioxide and water vapor, until the oxygen supply is exhausted, and the flame extinguishes.
Understanding Gas Behavior with the Ideal Gas Law
To comprehend pressure changes, we must refer to the Ideal Gas Law. This law posits that gas particles are spaced apart enough to be treated as individual entities that collide with each other. There are four variables that can alter gas behavior:
- The volume of gas (V).
- The temperature of gas (T).
- The gas pressure (P).
- The quantity of gas particles (in moles, denoted as n).
The Ideal Gas Law is represented as follows:
The constant R is known as the "gas constant" and is valued at approximately 8.314 J/K·mol.
However, what happens to the total number of gas particles in the container? When two oxygen molecules are consumed, three particles (carbon dioxide and two water molecules) are produced, seemingly increasing pressure.
The intriguing factor here is that water vapor can condense into liquid water on surfaces, leading to a decrease in the number of gas particles in the glass, thereby lowering pressure.
Data Collection and Analysis
To further investigate, I conducted an experiment using a sealed container instead of just a glass over water. Here's the setup:
In the top stopper, I installed two sensors linked to a Vernier LabQuest data collection device, which allowed me to monitor both the pressure and temperature inside the container while the candle burned.
Here are the results:
Initially, temperature rises due to the fire. Once the flame extinguishes, the temperature drops as the gases cool. Pressure readings increase after sealing the container, followed by a decrease as water vapor condenses.
An interesting question arises: is the pressure drop linked to the temperature decrease, as per the Ideal Gas Law? To explore this, I calculated the ratio of pressure to temperature:
Upon plotting P/T against time, we can derive insights about the number of molecules within the container (assuming a constant volume). The graph indicates a decline in gas particles almost immediately, likely due to early condensation of water vapor. However, an unexpected increase in particle count after about 25 seconds could suggest a leak allowing air to enter.
Exploring Further: The Iron Experiment
To expand on this, I decided to experiment with burning steel wool, which reacts with oxygen to form rust:
In this case, burning iron produces no water vapor, leading to a quicker reduction in gas particles:
Interestingly, the decrease in gas volume is much more rapid, likely because there’s no water vapor to condense.
Lastly, some theories suggest that the water’s rise in the candle experiment results from cooling air after the flame extinguishes. The argument posits that heated air expands, some escapes the container, and upon cooling, the volume shrinks, causing the water to rise. However, I suspect this explanation is flawed, as it would imply an initial pressure spike followed by a return to baseline, which does not align with the observed results.