Water flows effortlessly, but honey pours slowly. Liquids move at different rates because of their viscosity: the resistance to flow. Though you might feel it takes a long time to get ketchup onto your burger, the viscosity of some liquids can be measured in years, not minutes. Long-term experiments have shown that tar pitch, once thought to be a solid, is actually an extraordinarily viscous liquid at room temperature.
The Language of Liquids
One of the reasons it took so long to identify tar pitch as the slowest-moving liquid on the planet is because it looks like a solid at room temperature. Liquids share specific properties whether they flow quickly or agonizingly slowly. The particles of all liquids are fairly close together but lack a definite arrangement. They vibrate, shift position and even slip past each other. Level of viscosity is also a property. It depends upon the force of attraction between particles and the temperature of the liquid. As temperature increases, the kinetic, or motion, energy increases. The more kinetic energy a substance has, the easier it is for particles to break the force of attraction that holds them together. This makes it easier for the substance to flow.
Tar pitch, a carbon-based substance, feels hard to the touch and can be broken into pieces with a hammer blow. The tar pitch used in the long-term experiments comes from coal. Its common names are bitumen and asphalt. Outside the laboratory, tar pitch is used in building roads, waterproofing buildings and producing electrodes. The Centers for Disease Control considers tar pitch vapors carcinogenic.
The Australian Trial
The original Pitch Drop Experiment began at the University of Queensland in 1927. A physics professor, Thomas Parnell, set it up to illustrate that some substances have unexpected traits. Parnell intended to show that contrary to its appearance, tar pitch is actually a viscous liquid. Pitch was heated and poured into a sealed funnel. The sample rested for three years, settling. In 1930, the funnel was opened, and seemingly solid pitch began to flow –- very slowly. Drops typically form in seven to 13 years. The first drop fell after eight years; the second one took nine years. The third drop came in 1954. Parnell was no longer alive to run the experiment, so the school largely ignored the test. The experiment received renewed interest in 1975. In 2013, 83 years after the funnel was opened, the ninth drop was released, with a video camera capturing the occasion.
The Dublin Drop
In 1944, a similar tar pitch test was set up at Trinity College in Dublin, Ireland. Funnel, resting time, waiting period, loss of interest -- all were the same as in the Australian experiment. In the 21st century, some of the school’s physicists began following the drip again. Web cams were installed to allow any interested party to monitor the progress. The broadcast showed a drop finally letting go on July 11, 2013, at about 5 in the afternoon.
- Purdue University: Department of Chemistry: Viscosity
- Purdue University: Department of Chemistry: Gases, Liquids, and Solids
- UPI: Pitch Is a Liquid, Finally Drips After 69 Years
- University of Queensland:School of Mathematics and Physics: The Pitch Drop Experiment
- Popular Science: FYI: How Long-Running Is the Longest-Running Lab Experiment?
- Encyclopedia Britannica: Pitch
- CDC: Coal Tar Pitch Volatiles
About the Author
Living in upstate New York, Susan Sherwood is a researcher who has been writing within educational settings for more than 10 years. She has co-authored papers for Horizons Research, Inc. and the Capital Region Science Education Partnership. Sherwood has a Ph.D. in curriculum and instruction from the University at Albany.
George Doyle/Stockbyte/Getty Images