Chemistry Blog

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Jan 24

23 Million Times Slower than Molasses

by Kenneth Hanson | Categories: fun, general chemistry | (163986 Views)

I have the pleasure of teaching general chemistry II for the first time ever this semester. It is fun to go back and revisit concepts that I have not spent time with since taking general chemistry ~13 years ago. Our first few classes will focus on intermolecular forces (dipole-dipole, London Dispersion, etc.) and some of their macroscopic manifestations. Some examples covered in the book include surface tension, capillary action and viscosity. Searching these topics online led me to my new favorite experiment: the Pitch Drop Experiment.
Pitch Drop

In 1927, Professor Thomas Parnell started the Pitch Drop experiment in which he sought to measure the viscosity of pitch. Pitch is a general term used to describe a highly viscous solid polymer, but this material is often a complex mixture of phenols, aromatic and long chain hydrocarbons. Unfortunately, I could not find the exact composition of the pitch used in this specific experiment, but needless to say this sample does meet the description of a highly viscous material.

The experiment was initiated when Prof. Parnell heated up a sample of pitch and poured it into a conical piece of glass. The pitch was then left to sit for three years, presumably the length of time it needed to cool and completely settle into the cone shape. Immediately after the bottom of the funnel was cut open the pitch came rushing out. Just kidding. The pitch ever so slowly began dripping out of the funnel. How slowly? At a rate of approximately one drop every 10 years. In fact, the most recent drop—the 9th drop ever– fell on April 17th 2014.

The first report on this experiment was published in the European Journal of Physics in 1984. In that manuscript they calculated the pitch (2.3 x 108 Pa s) to be 230 billion times more viscous than water (1.0 x 10-3 Pa s). That means it’s more than 23 million times slower than molasses (5-10 Pa s). The longevity and creativity of this experiment won Thomas Parnell and John Mainstone (the caretaker of the experiment for more than 50 years) an Ig Nobel Prize in 2005. The experiment has also been officially included in the Guinness World Records as the world’s longest continuously running laboratory experiment.

Despite this experiment’s epicness, or maybe because of it, no one has ever been in the room to watch one of the pitch drops fall. The closest anyone has come is a time-lapse video below.


The video is unfortunately anticlimactic. It shows the 9th drop making contact with the 8th drop in the beaker. It isn’t the spectacle of a full ‘drop’ event, but don’t worry. The next occurrence is right around the corner: about 14 years away. In anticipation of this event the University of Queensland has set up three webcams and a continuously streaming live feed on a website called The Tenth Watch. Regardless of where you find yourself, you can keep a constant eye on the experiment as it progresses. And even if you miss the 10th drop, don’t worry, it’s estimated that there is enough pitch in the funnel to produce several more drops over the next 100 years.

2 comments

1 ping

  1. anon

    Cute Cute but you should probably just direct your readers and students to Dr. Spevacek’s posts on http://www.rheothing.com/

  2. Samad

    I read this post. It is a hilarious experiment to measure the viscosity of pitch having an Ig Nobel Prize in 2005. In Life time of a man, 5-6 will drip out of the funnel.

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