Pressure and Gases

Pressure Force per unit area P = F/ A

Atmospheric Pressure Air is matter, so it has a mass The air’s mass is constantly pushing down on us, so it is exerting a pressure This pressure is very strong

Measuring Pressure An instrument called a barometer is used to measure pressure

Units of Pressure From a barometer, pressure is measured in mm Hg Another unit of pressure is the Pascal (Pa) ‏ The most modern pressure unit is the atmosphere (atm) ‏

Standard Pressure Normal pressure at sea level 760 mmHg = 1 atm = Pa

Converting Pressure Units How many atm is 800 mm Hg? How many Pa? How many mm Hg is 3.5 atm? How many Pa? How many atm is Pa? How many mm Hg?

Characteristics of Gases Vapor – substance ordinarily a liquid or solid but in its gaseous phase Gas expands to fill its container Highly compressible when pressure is applied – liquids and solids not Form homogeneous mixtures

Gas Particles Properties of gases are a result of the fact that the particles are very far apart and move very fast In air molecules make up only.1% of the volume In liquid 70% of volume is molecules Attractive force can exist only in liquids and solids – not in gases

Pressure and the number of molecules are directly related More molecules means more collisions. Fewer molecules means fewer collisions. Gases naturally move from areas of high pressure to low pressure because there is empty space to move in.

1 atm If you double the number of molecules

You double the pressure. 2 atm

As you remove molecules from a container the pressure decreases 4 atm

2 atm

Until the pressure inside equals the pressure outside Molecules naturally move from high to low pressure 1 atm

Changing the size of the container In a smaller container molecules have less room to move. Hit the sides of the container more often. As volume decreases pressure increases.

Temperature Raising the temperature of a gas increases the pressure if the volume is held constant. The molecules hit the walls harder and more often.