Big Barometer
This is a demonstration of an eleven foot high barometer made with a concentrated solution of potassium tetraiodomercurate(II). A second movie compares the densities of mercury, potassium tetraiodomercurate(II) solution, and water. It illustrates that barometers made from the three liquids have different heights.
Keywords
atmospheric pressure, barometric pressure, barometer, hydrostatic pressure
Multimedia
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movie (1 minute 42 seconds, 5.9 MB)
This is a demonstration of a barometer [made from an aqueous solution of potassium tetraiodomercurate(II)] . To make a potassium mercury iodide barometer, the liquid is poured into a long glass tube. It will take approximately 1 liter of solution to fill the barometer. Air bubbles are forced to the top by tapping the tube. The tube is topped off with a dropper. A rubber stopper is placed at the end of the filled tube and the tube is stood up while holding the stopper in place.
Additional potassium mercury iodide is added to a dish at the bottom until the opening of the tube is under the level of the solution. The stopper is removed and the solution flows out until the pressure of the surrounding air is equal to the pressure exerted by the solution at the bottom of the column. This barometer is about eleven feet tall.
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movie (55 seconds, 3.1 MB)
A mercury barometer is about 2.2 feet tall. This container of mercury weighs about 300 grams. A different volume of potassium mercury iodide also weighs 300 grams. It has a different density and can can be used to make an eleven or twelve foot barometer. All these liquids weigh approximately the same but differ in volume and therefore differ in density. Water, the least dense of the three, would need a tube of about 40 feet tall to make a barometer.
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Discussion
The correct IUPAC name for K2HgI4 is potassium tetraiodomercurate(II). The voice over refers to this compound as potassium mercury iodide and does not make explicit the fact that an aqueous solution of a solid compound is being used.
Additional still images for this topic