Pressure and Gases
Pressure
Force per unit areaP = F/ A
Atmospheric Pressure
Air is matter, so it has a massThe air’s mass is constantly pushing
down on us, so it is exerting a pressureThis pressure is very strong
Measuring PressureAn 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 = 101000 Pa
Converting Pressure UnitsHow many atm is 800 mm Hg? How
many Pa?How many mm Hg is 3.5 atm? How
many Pa?How many atm is 200000 Pa? How
many mm Hg?
Characteristics of GasesVapor – substance ordinarily a liquid or
solid but in its gaseous phaseGas expands to fill its containerHighly compressible when pressure is
applied – liquids and solids notForm homogeneous mixtures
Gas ParticlesProperties 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 moleculesAttractive 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 containerIn 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.