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MCAT Content / Gas Phase / Gas Phase Gc Phy Pressure Simple Mercury Barometer

### Pressure, Simple Mercury Barometer

Topic: Gas Phase

A barometer is used to measure atmospheric pressure, the pressure exerted by gas particles in Earth’s atmosphere as those particles collide with objects.

Atmospheric pressure is the pressure exerted by gas particles in Earth’s atmosphere as those particles collide with objects. A barometer is an instrument used to measure atmospheric pressure. A traditional mercury barometer consists of an evacuated tube immersed in a container of mercury. Air molecules push down on the surface of the mercury. Because the inside of the tube is a vacuum, the mercury rises inside the tube. The height to which the mercury rises is dependent on the external air pressure.

At sea level, a mercury column will rise a distance of 760 mm. This atmospheric pressure is reported as 760 mmHg (millimeters of mercury). At higher altitudes, the atmospheric pressure is decreased and so the column of mercury will not rise as high. On the summit of Mt. Everest (elevation of 8848 m), the air pressure is 253 mmHg. Atmospheric pressure is slightly dependent on weather conditions. From the graph we can see the decrease in atmospheric pressure as the altitude increases. At sea level, the atmospheric pressure would be a little over 100 kPa (one atmosphere or 760 mm Hg). If we climb to the top of Mount Everest (the highest mountain in the world at 29,029 feet or 8848 meters), the atmospheric pressure will drop to slightly over 30 kPa (about 0.30 atmospheres or 228 mm Hg). This marked decrease in atmospheric pressure results in much lower levels of oxygen. Teams that climb this mountain must bring supplies of oxygen with them in order to breathe at these high altitudes.

Practice Questions

The tubular system in an aquarium

MCAT Official Prep (AAMC)

Key Points

• Atmospheric pressure is the pressure exerted by gas particles in Earth’s atmosphere as those particles collide with objects.

• A barometer measures atmospheric pressure.

• Atmospheric pressure decreases as the altitude increases.

Key Terms

aneroid barometer: Measures pressure by the expansion and contraction of a small spring within an evacuated metal capsule.

atmospheric pressure: The pressure exerted by gas particles in Earth’s atmosphere as those particles collide with objects.

barometer: Measures atmospheric pressure.

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