In a familiar science demonstration, a lit candle placed in a shallow dish of water is covered by an inverted glass. Soon after, the candle flame diminishes and extinguishes, and then, surprisingly, the water level inside the glass begins to rise. This phenomenon prompts a common question: what forces cause the water to ascend into the seemingly empty space? Understanding this process reveals insights into the principles of air pressure and gas behavior.
Addressing the Common Misconception
Many believe the water rises because the burning candle consumes all the oxygen inside the glass, creating a vacuum that pulls the water upward. However, this explanation misunderstands the true mechanism. Air is about 78% nitrogen and 21% oxygen. If water rise were solely due to oxygen consumption, the volume drawn in would be limited to roughly 21% of the air’s original volume, which often doesn’t fully account for the observed water level change.
The candle flame does not consume all the oxygen; it extinguishes due to oxygen depletion, heat loss, and carbon dioxide buildup. The water also does not rise steadily as oxygen is consumed. Instead, it experiences a rapid increase in level only after the flame has gone out. This observation points to a different, more significant factor than oxygen depletion alone.
The Primary Scientific Explanation
The primary reason for the water rising inside the glass is the contraction of gases due to cooling. Initially, as the candle burns, its flame heats the gases within the enclosed space of the inverted glass. This heating causes the gases to expand, and some expanded hot air may escape from under the rim of the glass as small bubbles.
Once the candle’s flame extinguishes, the hot gases inside the glass begin to cool quickly. As gases cool, their molecules lose energy and move closer together, causing the overall volume of the gas to contract. This contraction leads to a decrease in pressure inside the glass.
At this point, the pressure inside the glass is lower than the atmospheric pressure outside it. The higher external atmospheric pressure then pushes the water in the dish up into the glass until the pressure difference equalizes. This change in pressure, driven by the cooling and contraction of the gases, accounts for the majority of the water’s ascent.
Other Contributing Factors
While the cooling and contraction of gases is the primary driver, other factors contribute to the water rising, albeit to a lesser extent. One factor is the condensation of water vapor. Candle combustion produces water vapor as a byproduct. As the hot air inside the glass cools after the flame goes out, this gaseous water vapor condenses into liquid water droplets. This change from gas to liquid further reduces the total gas volume within the glass, contributing to the pressure drop and water rise.
Another minor contributing factor is the solubility of carbon dioxide (CO2) in water. CO2 is also a product of the candle’s combustion. A small amount of this CO2 gas can dissolve into the water in the dish, slightly reducing the gas volume inside the inverted glass. However, CO2 solubility in water is minimal during the short experiment, making its contribution less significant than thermal contraction or water vapor condensation.