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Properties of Gases:

  • Compressible
  • Expands or shrinks as the temperature increases or decreases
  • Low density
  • Miscible with other gases

Pressure in general is defined as force per unit area.

Gas pressure: Gas pressure in a container is the amount of force exerted per unit area by the moving gas molecules due to their collision with the surface wall. Pressure increases or decreases depending on the gas temperature, the number of molecules present, and the volume of the container. 

Pressure Units

pascal (Pa). 1 Pa = 1 N/m2 (recommended IUPAC unit)

kilopascal (kPa). 1 kPa = 1000 Pa

pounds per square inch (psi).

air pressure at sea level is ~14.7 psi

atmosphere (atm). 1 atm = 101,325 Pa = 760 torr

air pressure at sea level is ~1 atm

bar (bar, or b) 1 bar = 100,000 Pa (exactly)

millibar (mbar, or mb). 1000 mbar = 1 bar

inches of mercury (in. Hg). 1 in. Hg = 3386 Pa

torr 1 torr = 1/760 atm

millimeters of mercury (mm Hg). 1 mm Hg ~1 torr

Relating Pressure, Volume, Amount, and Temperature: The Ideal Gas Law

The relationships between the various macroscopic physical properties of gases - pressure, volume, number of molecules, and temperature - are established using the ideal gas law. 

The ideal gas law is expressed using the following formula:

PV = nRT

Where :

P = pressure

V = volume

n = number of molecules

R = gas constant

T = temperature

Boyle's Law

This law demonstrates that the pressure of a gas is inversely proportional to the volume when other variables (such as the number of molecules and temperature) are kept constant. In other words, as the volume increases, the pressure decreases - and vice versa - as shown in the animation below (the green blocks in the animation represent pressure). 


An animated version of Boyle's law.
 Pressure times volume equals a constant.

Figure 1: Animated Boyle's Law. From Tom Benson (2015, May 5) for the Glenn Research Center, NASA.

Boyle's Law Formula

PiVi = PfVf

Where :

Pi = initial pressure

Vi = initial volume

Pf = final pressure

Vf = final volume

Charles' Law

Charles' law demonstrates that when the pressure of a dry gas sample is kept constant, its volume is directly proportional to its temperature (in Kelvin) as shown in the animation below (the green blocks in the animation represent the pressure). 


An animated version of Charles and Gay-Lussac's law.
 Volume equals a constant times the temperature.

Figure 2: Animated Charles and Gay-Lussac's Law. From Tom Benson (2015, May 5) for the Glenn Research Center, NASA.

Charles' Law Formula

Where :

Ti = initial temperature

Vi = initial volume

Tf = final temperature

Vf = final volume

Combined Gas Law

Boyle's law and Charles' law are combined into form combined gas law as shown below.

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