This video investigates the maximum theoretical and practical height a siphon can achieve. Through a series of experiments using a scissor lift to gradually raise the siphon's apex, the video demonstrates that a water siphon breaks at approximately 10.07 meters at room temperature. This breaking point is attributed not just to the atmospheric pressure limit, but specifically to the water boiling at a reduced pressure (effectively a vacuum) at the siphon's highest point, due to its vapor pressure at ambient temperature. The video then delves into a more nuanced explanation, initially disregarding atmospheric pressure to highlight gravity's role in driving the siphon (like a chain) and the crucial, often overlooked, concept of water's tensile strength. It explains that in perfectly clean conditions, water can be "super-sucked" (placed under tension without boiling), allowing a siphon to theoretically exceed the atmospheric pressure limit, as observed in historical experiments reaching 15 meters. The conclusion emphasizes that while gravity drives siphons, atmospheric pressure typically prevents water from "falling apart" at the top, unless its tensile strength can be leveraged under extremely pure conditions.
Exploring the Limits of Siphon Height
0:00 Introduction to Siphons: The video opens with an appreciation for siphons and poses the central question: how high can a siphon go before it breaks?
0:43 Initial Hypothesis: The narrator references previous explanations, like a Veritasium video on drinking straws, suggesting a known limit related to atmospheric pressure.
1:00 The Scissor Lift Experiment: A practical experiment is set up using a scissor lift to continuously raise a working siphon, measuring the height at which it eventually fails.
1:34 Increasing Height: The siphon is shown successfully working at various heights: 5.4m, 7.2m, 8.7m, 9.8m, and 9.95m.
2:20 Siphon Breaks at 10.07m: The siphon ceases to function at an exact height of 10.07 meters, with the flow reducing to a dribble before stopping.
2:27 Why Did It Break? - Initial Explanation: Bubbles started appearing at 9m, indicating dissolved gases coming out of solution due to reduced pressure. However, these don't break the siphon if flow is sufficient. The siphon breaks when water vapor appears (boils) at the top.
3:36 Boiling Without Heat (Boiling in a Vacuum): Water boils at the top because the weight of the water columns perfectly counteracts atmospheric pressure, creating a vacuum at the apex, causing the water to vaporize.
4:36 Theoretical vs. Experimental Height: The theoretical atmospheric pressure limit for pushing water upwards is 10.33m. The experimental result of 10.07m is lower due to the vapor pressure of water at room temperature (around 20Β°C).
5:44 The Vapor Pressure Correction: Derek from Veritasium confirms that the 10.33m calculation doesn't account for water's vapor pressure. The correct calculation, subtracting water's vapor pressure from atmospheric pressure, yields approximately 10.07m.
6:39 "World's Tallest Siphon" (Contextual): The narrator claims this is the tallest possible siphon under normal conditions, acknowledging it would be slightly taller at sea level due to higher atmospheric pressure.
7:10 Exceeding the Limit (15m Siphon): The video introduces historical experiments where siphons reached 15m, hinting at a more complex explanation related to water's properties and "super-sucking" (under tension).
7:46 The Full Explanation - Gravity and Water's Tensile Strength: Siphons are primarily driven by gravity (like a chain). Atmospheric pressure's role is to prevent the water from falling apart (boiling/flying apart) at the low-pressure top, not to "pull" the water up.
10:08 Siphon in a Vacuum: Footage of a siphon working in a vacuum is shown, prompting the question of why the water isn't boiling.
10:22 Superheating and Surface Tension: Water can be superheated beyond its boiling point without boiling in the absence of nucleation sites (impurities/scratches), due to surface tension resisting bubble formation. This is analogous to "super-sucking" or water being under tension.
12:51 Conclusion - Dual Mechanisms: Siphons are driven by gravity. For practical siphons with water, atmospheric pressure is necessary to hold the water together at the top. However, if conditions are extremely clean (no dissolved gases, no impurities), water's tensile strength can hold it together, allowing siphons to exceed the atmospheric pressure limit.
13:28 Practical Limit: For common scenarios, the maximum height for siphoning water is about 10.07m, unless the water is incredibly pure and the tube perfectly clean.
13:47 Sponsor Segment: The video concludes with a sponsorship message for Jane Street.