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 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 = 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 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.