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Gases, Atmospheric Pressure, and Pressure Unit Conversions

SECTION 5 - Gases


Although very few substances exist as gases under typical conditions, they're very important.  Our atmosphere for example:

➞  supports life.

➞  serves as a waste receptacle for exhaust gases.

➞  shields us from harmful radiation


All Gases Exert Pressure on Their Surroundings

Pressure  =  the force exerted by gas molecules (or atoms) as they strike the surfaces around them.

The gases most familiar to us are the ones that make up our atmosphere:

N2  ,  O2  ,  Ar  ,  CO2  ,  Ne  ,  He  ,  CH4  ,   others...

Together, they exert atmospheric pressure on us, and on the earth.

A device used to measure atmospheric pressure is a barometer (invented by the Italian physicist Torriceli).

Here's a rough sketch of a typical barometer:

Standard Pressure and the Barometer

As the image illustrates, at sea-level the atmosphere "pushes" the mercury (Hg) up the barometer tube to a height of 760 mm.

That gives us a standard pressure of 760 mmHg.


Two Factors that can Change Atmospheric Pressure

Atmospheric pressure is affected by both altitude and the weather.  See here...

1.  Altitude

At higher elevations there is a column (amount) of "air" above you, so the atmospheric pressure would be less.

The barometer would read less than 760 mmHg.

At lower elevations there's an even greater amount of atmosphere above you (longer column of air), so the atmospheric pressure would be greater.

The barometer would read higher than 760 mmHg.


2.  Weather (Humidity)

In the tropics, there's more H2O molecules in the air.  So, atmospheric pressure is greater.

In dry regions and deserts, the humidity is very low.  Relatively few H2O molecules in the air leads to lower atmospheric pressures.


Units of Pressure

760 mmHg = 1 atm = 760 torr = 101,325 Pa = 14.7 psi  = 1.01325 bar = 101.325 kPa

Most common pressure conversion factors:

Pressure Unit Conversions

These are often used as conversion factors. Check out this example below...


Pressure Unit Conversions

ex:  Represent a pressure of 49 torr in atmospheres, mm Hg, and pascals.

Gas Pressure Unit Conversions

NOTE - All of these pressure values represent the same amount of pressure, just expressed in different units.


Next up in SECTION 5 - Gases,

We'll watch a video and talk about: 

Boyle's Law, Charles' Law, Avogadro's Law, Gay-Lussac's Law, the Combined Gas Law, and the Ideal Gas Law.