Gases

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Kinetic Molecular Theory

All atoms and molecules are in constant random motion (even seemingly "stationary" solid objects!)

The Kinetic Molecular Theory states:

Gases are so spread apart that gas molecules take up practically zero space.
Gases move in constant random motion and often collide. Energy is perfectly conserved (no energy is lost) in these collisions.
Gases are neither attracted or repelled from each other.
Temperature is proportional to the kinetic energy of the molecules. Therefore, molecules in colder objects move more slowly because they have less energy. The molecules of hotter objects move quickly because they have more energy.

Pressure

Pressure is the force exerted over an area.

The Earth's gravity pulls the air down. Thus the air is more dense closer to the surface of the earth. All of the air above us is pressing down on us. We don't often think about air pressure because we cannot feel it, but it is substantial. There are about 14 lbs of pressure on every square inch of our bodies.

Measuring Pressure

A barometer is a tool for measuring atmospheric pressure. The classic barometer uses an open dish and a tube filled with a dense liquid, originally mercury. The height of the mercury would rise or fall with the atmospheric pressure. Hence, another unit for pressure is millimeters of mercury or mmHg.

This device was originally designed by Evangelista Torricelli in 1643. Therefore the original unit of mmHg is the Torr.

Common SI units for pressure include:

Newton per square meter (N/m² ) or pascal (Pa)
Standard atmospheric pressure at sea level, or simply an atmosphere (atm)
Millimeters of mercury (mmHg) or torr

Units conversions:

1 atm = 14.7 psi (pounds per square inch) = 101,325 Pa = 101.3 kPa (kilopascals) = 760 mmHg = 760 torr

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