Gas is one of the three phases and its properties include the ability to be easily compressed, and theability to fill containers with odd shapes. A Flemish physician named Jan Baptista Van Helmont was the first scientist to realize that "air" was not an element, but actually a mixture of different gases. This set the pace for an Italian physicist named Evangelista Torricelli who determined that air exerts a pressure. This discovery led him to the creation of the first barometer.
The first barometer was a long tube that was closed at one end and filled with mercury and then inverted into a pool of mercury. From the barometer, Torricelli notied that the mercury was always in the tube at around 760 mm. He determined that this was due to the atomospheric pressure.
Torricelli determined that the standard atmospheric pressure was 760 mm Hg or 760 torr (named for Torricelli). Also 760 mm Hg is equal to 1 atm (standard atmospheric pressure)
Pressure is defined as force / area. In the metric system there is another unit for pressure and it is the pasca (Pa). There are 101325 Pa is every atm, so:
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Unit 2 - Section 3
Unit 2 - Section 5